# Troubleshooting Discordance Between Rapid AST Results and Conventional Methods ## Quick Answer - Discordance between rapid antimicrobial susceptibility testing (AST) and conventional methods requires a systematic investigation of pre-analytical, analytical, and post-analytical factors before reporting results. - Begin by verifying inoculum preparation, instrument calibration, and quality control strains using standards-based protocols from the Clinical and Laboratory Standards Institute. - Important limitation: rapid AST platforms measure growth kinetics differently than reference broth microdilution, so minor categorical differences may reflect methodological variation instead of true errors. ## Understanding Rapid AST Discordance Rapid antimicrobial susceptibility testing platforms have transformed clinical microbiology by reducing the time to actionable results from 24 to 48 hours down to several hours. These systems use a variety of detection principles including automated microscopy, flow cytometry, and molecular markers to assess bacterial growth in the presence of antimicrobial agents. The speed advantage is substantial for patient care, particularly in cases of sepsis or severe infections where early appropriate therapy improves outcomes. However, the speed of these platforms introduces a distinct set of challenges. Discordance between rapid AST results and conventional methods such as broth microdilution, disk diffusion, or gradient diffusion occurs at rates that vary by organism, antimicrobial agent, and platform. Understanding the sources of these discrepancies is essential for laboratory professionals who must decide whether to release rapid results to clinicians or wait for confirmation. The stakes are high. A rapid AST result that incorrectly reports susceptibility could lead to undertreatment and clinical failure. A result that incorrectly reports resistance could lead to unnecessary use of broad-spectrum agents, contributing to antimicrobial resistance. The World Health Organization identifies antimicrobial resistance as a global health threat and emphasizes the importance of accurate susceptibility testing for surveillance and stewardship programs. Laboratories therefore need a structured approach to identify, investigate, and resolve discordant results. ## At a Glance | Discordance Category | Common Causes | Initial Investigation Steps | Escalation Criteria | | --- | --- | --- | --- | | Pre-analytical | Mixed culture, wrong colony selection, delayed processing, improper storage | Review specimen handling records, assess culture purity, verify colony morphology | Repeat testing from isolated colonies, request new specimen if contamination suspected | | Analytical | Method-specific breakpoint differences, inoculum density variation, instrument calibration drift | Check quality control results, verify inoculum preparation, review instrument maintenance logs | Contact manufacturer technical support, repeat with reference method | | Post-analytical | Data entry errors, result interpretation mistakes, software or LIS transmission issues | Review manual entry, verify result transmission, check interpretation rules | Correct the record, notify clinical staff, document the discrepancy | ## Understanding the Methodological Landscape ### Rapid AST Platforms Rapid AST systems are designed to provide susceptibility results within hours instead of days. These platforms typically use a combination of bacterial identification and susceptibility testing in a single workflow. The most common approaches include: - **Molecular-based methods** that detect resistance genes directly from clinical samples or positive blood cultures - **Phenotypic methods** that measure bacterial growth in the presence of antimicrobials using optical or electrochemical detection - **Flow cytometry-based methods** that assess cell viability after antimicrobial exposure Each approach has distinct limitations. Molecular methods detect the presence of resistance genes but cannot always predict phenotypic expression. Phenotypic rapid methods measure growth over a shorter period than conventional methods, which can lead to differences in minimum inhibitory concentration (MIC) readings. ### Conventional AST Methods Conventional AST methods remain the reference standard for susceptibility testing. The CLSI publishes standardized protocols for broth microdilution, agar dilution, and disk diffusion methods. These methods are calibrated against clinical outcomes and are used to establish breakpoints that define susceptible, intermediate, and resistant categories. The key difference between rapid and conventional methods is the duration of incubation. Conventional methods typically require 16 to 24 hours of incubation to allow for sufficient bacterial growth. Rapid methods attempt to shorten this period by using more sensitive detection systems or by measuring early growth kinetics. This fundamental difference can produce discordant results when the growth characteristics of the organism do not align with the detection threshold of the rapid system. ## Pre-Analytical Sources of Discordance ### Specimen Collection and Transport The quality of the specimen is the foundation of accurate AST. A specimen that is contaminated with normal flora, collected from the wrong site, or transported under improper conditions can produce misleading results. For example, a blood culture that is collected through a contaminated line may yield a mixture of organisms, and the rapid AST system may detect the growth of the contaminant instead of the pathogen. Laboratory professionals should review the specimen collection procedure when discordance is observed. Key questions include: - Was the specimen collected from the correct anatomical site? - Was the specimen collected before antimicrobial therapy was initiated? - Was the specimen transported to the laboratory within the appropriate time frame? - Was the specimen collected using sterile technique? The CDC and NIH provide biosafety guidance for specimen handling and processing in the BMBL, which emphasizes the importance of proper containment and handling procedures to prevent contamination and ensure accurate results. ### Inoculum Preparation The inoculum density is a critical variable in AST testing. Both rapid and conventional methods require a standardized inoculum, typically adjusted to a 0.5 McFarland standard. If the inoculum is too heavy or too light, the MIC readings will be affected. Rapid methods may be particularly sensitive to inoculum variations because they measure growth over a shorter period, and small differences in starting cell density can produce larger relative differences in growth detection. When discordance is observed, the laboratory should verify that the inoculum was prepared according to the manufacturer's instructions. This includes checking the turbidity of the suspension, the time between preparation and testing, and the use of appropriate diluent. ### Purity and Identification A pure culture is essential for accurate AST. If the culture contains more than one organism, the rapid AST system may detect the growth of the organism that grows fastest under the test conditions, which may not be the organism of interest. The laboratory should verify the purity of the culture by examining the colony morphology on the primary isolation plate and by performing Gram stain. The identification of the organism is also important. Different organisms have different growth characteristics and resistance mechanisms. If the identification is incorrect, the rapid AST system may apply the wrong interpretation criteria. The American Type Culture Collection provides authenticated reference strains that can be used to verify the performance of identification and susceptibility testing systems. ## Analytical Sources of Discord ### Method-Specific Breakpoints Breakpoints are the concentrations of an antimicrobial that define susceptible, intermediate, and resistant categories. These breakpoints are established by organizations such as the CLSI and are based on a combination of pharmacokinetic and pharmacodynamic data, clinical outcomes, and microbiological data. Rapid AST systems may use breakpoints that differ from those used in conventional methods. Some rapid systems use the same breakpoints as the CLSI, while others use breakpoints that are specific to the system. When the breakpoints differ, the categorical interpretation of the MIC can differ even when the MIC value is the same. Laboratory professionals should verify that the rapid AST system is using the current breakpoints and that the breakpoints are appropriate for the organism and antimicrobial being tested. The CLSI provides standards for antimicrobial susceptibility testing that include breakpoint tables, but these are paywalled and should not be reproduced. ### Instrument Calibration and Maintenance Rapid AST systems rely on optical or electronic detection to measure bacterial growth. These systems require regular calibration and maintenance to ensure accurate readings. If the instrument is not calibrated correctly, the readings can be systematically biased, leading to discordant results. The laboratory should maintain a log of instrument calibration and maintenance activities. When discordance is observed, the laboratory should review this log to determine whether the instrument was calibrated within the appropriate time frame and whether any maintenance issues were noted. ### Reagent Quality The quality of the reagents used in rapid AST testing can affect the results. Antimicrobial solutions that are expired, improperly stored, or prepared at the wrong concentration can produce inaccurate results. The laboratory should verify the expiration dates of all reagents and ensure that they are stored according to the manufacturer's instructions. The American Type Culture Collection provides authenticated biological materials that can be used for quality control testing. These materials include reference strains with known susceptibility profiles that can be used to verify the performance of the AST system. ## Post-Analytical Sources of Discord ### Result Interpretation and Reporting The interpretation of AST results requires careful attention to the MIC value, the breakpoint, and the clinical context. A result that is reported as susceptible or resistant may be misinterpreted if the breakpoint is not applied correctly or if the MIC value is close to the breakpoint. Laboratory professionals should verify the interpretation of the result before reporting it to the clinician. This includes checking the MIC value against the breakpoint table and considering the clinical context of the patient. ### Laboratory Information System Transmission The transmission of results from the AST instrument to the laboratory information system (LIS) can introduce errors. A result that is transmitted incorrectly can be reported to the clinician with the wrong interpretation. The laboratory should verify the transmission of results by comparing the instrument output with the LIS record. ### Documentation and Traceability Accurate documentation is essential for troubleshooting discordant results. The laboratory should maintain records of the specimen, the culture, the AST method, the instrument, the reagents, and the results. This documentation allows the laboratory to trace the source of the discordance and to identify patterns that may indicate a systematic problem. ## A Systematic Troubleshooting Workflow ### Step 1: Verify the Specimen and Culture The first step in troubleshooting discordance is to verify the specimen and culture. The laboratory should: - Review the specimen collection and transport records - Examine the primary isolation plate for colony morphology and purity - Perform a Gram stain to verify the organism morphology - Confirm the identification of the organism If the culture is mixed or the identification is incorrect, the rapid AST result should be considered invalid and the test should be repeated. ### Step 2: Verify the Inoculum and Test Conditions The next step is to verify the inoculum and test conditions. The laboratory should: - Confirm that the inoculum was prepared to the correct density - Verify that the inoculum was used within the appropriate time frame - Check the incubation conditions, including temperature and atmosphere - Verify that the correct antimicrobial concentrations were used ### Step 3: Review Quality Control Data Quality control is essential for verifying the performance of the AST system. The laboratory should: - Review the quality control results for the day of the test - Verify that the quality control strains produced the expected results - Check the instrument calibration and maintenance records ### Step 4: Repeat the Test If the initial investigation does not identify a clear cause, the test should be repeated. The repeat test should be performed using the same method and the same conditions. If the repeat test produces the same result, the discordance is likely due to a methodological difference. If the repeat test produces a different result, the initial result was likely due to a technical error. ### Step 5: Confirm with a Reference Method If the discordance persists, the result should be confirmed with a reference method. The CLSI provides standards for reference methods such as broth microdilution. The reference method should be performed in parallel with the rapid method to determine which result is correct. ### Step 6: Document and Report The final step is to document the investigation and report the result. The laboratory should document the steps taken to investigate the discordance, the findings, and the final result. The result should be reported to the clinician with a note explaining the discordance and the method used to resolve it. ## Mermaid Flowchart ```mermaid flowchart TD A[Discordant Result Observed] --> B[Verify Specimen and Culture] B --> C[Verify Inoculum and Test Conditions] C --> D[Review Quality Control] D --> E{Repeat Test} E -->|Same Result| F[Confirm with Reference Method] E -->|Different Result| G[Report Corrected Result] F --> H{Reference Confirms Rapid Result} H -->|Yes| I[Report Rapid Result] H -->|No| J[Report Reference Result] I --> K[Document Investigation] J --> K G --> K ``` ## Records and Measurements ### Essential Records for Troubleshooting The laboratory should maintain the following records to support troubleshooting of discordant AST results: - Specimen collection and transport records - Culture and isolation records - Inoculum preparation records - Quality control records - Instrument calibration and maintenance records - Reagent lot numbers and expiration dates - The final result and interpretation ### Measurements to Track The laboratory should track the following measurements to identify patterns of discordance: - The rate of discordance for each rapid AST system - The rate of discordance for each organism - The rate of discordance for each antimicrobial - The time to result for rapid AST compared to conventional methods Tracking these measurements can help the laboratory identify systematic problems with the rapid AST system and implement corrective actions. ## Common Failure Patterns ### Pattern 1: Mixed Cultures The most common cause of discordance is a mixed culture. When the specimen contains more than one organism, the rapid AST system may report the susceptibility profile of the organism that grows fastest under the test conditions. This can lead to a result that does not match the conventional method, which may test a pure culture of the pathogen. **Resolution:** Verify the purity of the culture before testing. If the culture is mixed, isolate the organism of interest and repeat the test. ### Pattern 2: Inoculum Density Errors Inoculum density errors can produce discordant results, particularly for rapid methods that are sensitive to the initial cell density. A heavy inoculum can produce a false-resistant result, while a light inoculum can produce a false-susceptible result. **Resolution**: Verify the inoculum density using a 0.5 McFarland standard and adjust the inoculum if necessary. ### Pattern 3: Breakpoint Differences Breakpoint differences between the rapid AST system and the conventional method can produce discordant results. This is particularly common for antimicrobials where the breakpoints have been recently updated. **Resolution**: Verify the breakpoints used by the rapid AST system and compare them with the current CLSI breakpoints. If the breakpoints differ, the result should be interpreted using the breakpoints that are appropriate for the clinical context. ### Pattern 4: Instrument Calibration Errors Instrument calibration errors can produce discordant results for all organisms and antimicrobials tested on the instrument. This pattern is characterized by a sudden change in the rate of discordance. **Resolution**: Review the instrument calibration and maintenance records and perform a calibration check. ### Pattern 5: Reagent Degradation Reagent degradation can produce discordant results for specific antimicrobials. This pattern is characterized by discordance that is limited to a specific antimicrobial or class of antimicrobials. **Resolution**: Verify the expiration dates of the reagents and replace any that are expired or degraded. ## Limitations of Rapid AST Methods ### Methodological Limitations Rapid AST methods have inherent limitations that can produce discordant results. These limitations include: - **Shortened incubation time**: Rapid methods may not allow sufficient time for the full expression of resistance mechanisms - **Detection sensitivity**: Rapid methods may not detect low-level resistance that is detected by conventional methods - **Organism-specific limitations**: Rapid methods may not be validated for all organisms ### Clinical Limitations The clinical significance of a discordant result depends on the context. A discordant result for a patient with a severe infection may have different implications than a discordant result for a patient with a mild infection. The laboratory should consider the clinical context when interpreting and reporting discordant results. ### Evidence Limitations The evidence base for rapid AST methods is still evolving. The performance of rapid methods may vary depending on the organism, the antimicrobial, and the clinical setting. The laboratory should be aware of the limitations of the evidence and should interpret the results with caution. ## Biosafety and Containment ### Biosafety Considerations The handling of bacterial cultures and antimicrobial agents requires appropriate biosafety practices. The CDC and NIH provide biosafety guidance in the BMBL, which emphasizes the importance of risk assessment, containment, and decontamination. The laboratory should follow the biosafety practices that are appropriate for the organisms being tested. ### Decontamination The laboratory should have procedures for the decontamination of work surfaces, equipment, and waste. This is important for preventing the spread of antimicrobial-resistant organisms and for ensuring the safety of laboratory personnel. ### Personal Protective Equipment Laboratory personnel should wear appropriate personal protective equipment when handling bacterial cultures and antimicrobial agents. This includes gloves, lab coats, and eye protection. ## Professional Escalation Criteria The laboratory should escalate the investigation of discordant results to a supervisor or a specialist when: - The discordance is persistent and cannot be resolved by the initial investigation - The discordance involves a critical result that could affect patient care - The discordance suggests a systematic problem with the AST system - The discordance involves a novel or emerging resistance mechanism The escalation should include a summary of the investigation, the findings, and the recommended next steps. ## Building a Discordance Investigation Record System and Decision Framework ### The Case for a Structured Record System Laboratories that investigate discordant rapid AST results on an ad hoc basis often repeat the same investigative steps for each event without capturing the institutional knowledge gained from prior investigations. A structured record system transforms discordance events from isolated incidents into a dataset that reveals patterns, quantifies risk, and supports continuous improvement. The Clinical and Laboratory Standards Institute emphasizes that quality control and method verification are ongoing processes, not one-time events. A discordance investigation logbook operationalizes this principle by creating a permanent, searchable record of every discrepancy, its investigation, and its resolution. The primary purpose of the logbook is not to assign blame but to identify whether a discordance event represents a random occurrence, a method-specific limitation, or a systemic failure that requires corrective action. Without a structured record, the laboratory cannot distinguish between a rare methodological variation and a recurring problem that demands intervention. The logbook also supports the professional escalation process by providing documented evidence of the investigation steps taken and the rationale for the final result. ### Core Components of the Discordance Logbook The logbook should capture standardized data fields for every discordance event. These fields fall into four categories: event identification, specimen and organism details, method and instrument details, and investigation outcomes. **Event identification fields** include the date and time the discordance was observed, the unique accession number, the technologist who performed the test, and the supervisor who reviewed the event. These fields establish the timeline and accountability for the investigation. **Specimen and organism fields** include the specimen source, the organism identification, the Gram stain morphology, the culture purity assessment, and the colony morphology description. These fields allow the laboratory to identify whether discordance clusters around specific specimen types or organisms. **Method and instrument fields** include the rapid AST platform, the instrument serial number, the software version, the reagent lot numbers, the antimicrobial agents tested, and the quality control results for the day of the test. These fields allow the laboratory to identify whether discordance clusters around specific instruments, reagent lots, or antimicrobial agents. **Investigation outcome fields** include the root cause category, the resolution action taken, the final reported result, the time to resolution, and whether the event was escalated to a supervisor or manufacturer. These fields allow the laboratory to track the effectiveness of its troubleshooting process and to identify recurring root causes. ### The Decision Framework for Discordance Resolution A structured decision framework guides the investigation from observation to resolution. The framework uses a series of decision points that direct the investigator through the pre-analytical, analytical, and post-analytical factors in a logical sequence. The framework is designed to be practical and to produce a defensible outcome for each discordance event. **Decision point one: Is the discordance clinically significant?** The first question is whether the discordance changes the categorical interpretation in a way that affects patient management. A minor MIC difference that does not change the susceptible, intermediate, or resistant category may not require a full investigation. A major discrepancy that changes the category from susceptible to resistant or from resistant to susceptible requires immediate investigation and escalation. The laboratory should define the threshold for clinical significance based on the antimicrobial agent, the organism, and the clinical context. **Decision point two: Is the culture pure and correctly identified?** The investigator should examine the primary isolation plate, review the Gram stain, and confirm the organism identification. If the culture is mixed or the identification is incorrect, the rapid AST result is invalid and the test should be repeated from isolated colonies. This decision point addresses the most common cause of discordance. **Decision point three: Were the test conditions within specification?** The investigator should verify the inoculum density, the time between inoculum preparation and testing, the incubation conditions, and the reagent expiration dates. If any test condition was out of specification, the result is invalid and the test should be repeated under correct conditions. **Decision point four: Did quality control pass?** The investigator should review the quality control results for the day of the test. If the quality control strains produced results outside the expected range, the test run is invalid and the result should not be reported. The investigator should also review the instrument calibration and maintenance records to determine whether the instrument was functioning correctly. **Decision point five: Does the discordance persist on repeat testing?** If the initial investigation does not identify a clear cause, the test should be repeated using the same method and conditions. If the repeat test produces the same result, the discordance is likely due to a methodological difference between the rapid and conventional methods. If the repeat test produces a different result, the initial result was likely due to a technical error. **Decision point six: Does the reference method confirm the rapid result?** If the discordance persists, the result should be confirmed with a reference method such as broth microdilution. The reference method should be performed in parallel with the rapid method to determine which result is correct. The reference method result should be reported to the clinician. ### Implementing the Decision Framework in Practice The decision framework is implemented through a structured workflow that guides the investigator through each decision point. The workflow begins with the observation of a discordant result and ends with the documentation of the investigation and the final reported result. **Step 1: Record the initial observation.** The investigator records the discordance event in the logbook, including the specimen details, the organism identification, the rapid AST result, the conventional method result, and the date and time of the observation. **Step 2: Assess clinical significance.** The investigator determines whether the discordance changes the categorical interpretation and whether the change is clinically significant. If the change is not clinically significant, the investigator documents the event and closes the investigation. If the change is clinically significant, the investigator proceeds to the next step. **Step 3: Verify the culture and identification.** The investigator examines the primary isolation plate, reviews the Gram stain, and confirms the organism identification. If the culture is mixed or the identification is incorrect, the investigator documents the finding, invalidates the rapid AST result, and requests a repeat test from a pure culture. **Step 4: Verify the test conditions.** The investigator reviews the inoculum preparation records, the incubation conditions, and the reagent expiration dates. If any condition is out of specification, the investigator documents the finding, invalidates the rapid AST result, and repeats the test under correct conditions. **Step 5: Review quality control.** The investigator reviews the quality control results for the day of the test and the instrument calibration and maintenance records. If the quality control results are out of range, the investigator documents the finding, invalidates the test run, and repeats the test after corrective action. **Step 6: Repeat the test.** If the initial investigation does not identify a clear cause, the investigator repeats the test using the same method and conditions. The repeat test result is recorded in the logbook. **Step 7: Confirm with a reference method.** If the discordance persists, the investigator performs a reference method such as broth microdilution in parallel with the rapid method. The reference method result is recorded in the logbook. **Step 8: Document and report.** The investigator documents the investigation steps, the findings, and the final result in the logbook. The final result is reported to the clinician with a note explaining the discordance and the resolution. ### Recording Measurements and Tracking Patterns The logbook provides the data needed to track discordance patterns over time. The laboratory should calculate the following measurements on a monthly or quarterly basis: **Discordance rate by platform.** The number of discordant results divided by the total number of rapid AST results for each platform. This measurement identifies whether one platform has a higher discordance rate than another. **Discordance rate by organism.** The number of discordant results divided by the total number of rapid AST results for each organism. This measurement identifies whether discordance clusters by specific organisms. **Discordance rate by antimicrobial agent.** The number of discordant results divided by the total number of rapid AST results for each antimicrobial agent. This measurement identifies whether discordance clusters by specific agents. **Discordance rate by error category.** The number of discordant results in each category, such as pre-analytical, analytical, or post-analytical. This measurement identifies the most common failure modes. **Time to resolution.** The time from the observation of the discordance to the final reported result. This measurement identifies whether the investigation process is efficient or whether delays are occurring. The laboratory should review these measurements on a regular basis and use them to identify trends. For example, if the discordance rate for a specific antimicrobial agent increases suddenly, the laboratory should investigate whether the reagent lot has changed or whether the breakpoints have been updated. If the discordance rate for a specific organism increases, the laboratory should investigate whether the identification algorithm has changed or whether the organism has developed a new resistance mechanism. ### Common Failure Patterns in the Decision Framework The decision framework is designed to identify and resolve the most common failure patterns. The following patterns are frequently observed in the laboratory: **Pattern 1: The culture is mixed.** The rapid AST result reflects the growth of a contaminant instead of the pathogen. The decision framework identifies this pattern at decision point two, and the resolution is to repeat the test from a pure culture. **Pattern 2: The inoculum is not standardized.** The rapid AST result is affected by the inoculum density. The decision framework identifies this pattern at decision point three, and the resolution is to repeat the test with a properly standardized inoculum. **Pattern 3: The quality control is out of range.** The rapid AST result is affected by a problem with the test run. The decision framework identifies this pattern at decision point four, and the resolution is to repeat the test after corrective action. **Pattern 4: The breakpoints differ between methods.** The rapid AST result and the conventional method result are discordant because the methods use different breakpoints. The decision framework identifies this pattern at decision point six, and the resolution is to report the result using the breakpoints that are appropriate for the clinical context. **Pattern 5: The discordance is due to a methodological difference.** The rapid AST result and the conventional method result are discordant because the methods measure growth differently. The decision framework identifies this pattern at decision point six, and the resolution is to report the result using the reference method. ### Professional Escalation Criteria The decision framework includes clear criteria for escalating a discordance investigation to a supervisor or a specialist. The laboratory should escalate the investigation when: **The discordance is persistent and cannot be resolved.** If the discordance persists after repeat testing and reference method confirmation, the laboratory should escalate the investigation to a supervisor or a specialist. The escalation should include a summary of the investigation steps, the findings, and the recommended next steps. **The discordance involves a critical result.** If the discordance involves a result that could affect patient care, the laboratory should escalate the investigation immediately. The escalation should include the clinical context and the potential impact of the result. **The discordance suggests a systemic problem.** If the discordance rate for a specific platform, organism, or antimicrobial agent increases suddenly, the laboratory should escalate the investigation to determine whether a systemic problem exists. The escalation should include the discordance rate data and the potential causes. **The discordance involves a novel or emerging resistance mechanism.** If the discordance suggests a resistance mechanism that has not been previously observed, the laboratory should escalate the investigation to a specialist laboratory. The escalation should include the organism identification, the antimicrobial agent, and the discordant result. ### Biosafety and Containment in the Investigation Process The investigation of discordant AST results involves the handling of bacterial cultures and antimicrobial agents. The laboratory should follow the biosafety practices that are appropriate for the organisms being tested. The CDC and NIH provide biosafety guidance in the BMBL, which emphasizes the importance of risk assessment, containment, and decontamination. The laboratory should ensure that personnel wear appropriate personal protective equipment, including gloves, lab coats, and eye protection, when handling cultures and antimicrobial agents. The laboratory should also have procedures for the decontamination of work surfaces, equipment, and waste. ### Training and Competency for the Decision Framework The decision framework is only effective if laboratory personnel are trained to use it consistently. The laboratory should provide training on the decision framework to all personnel who perform rapid AST testing. The training should include the logbook entry procedures, the decision point criteria, and the escalation criteria. The laboratory should also assess the competency of personnel on a regular basis to ensure that they can apply the framework correctly. The American Society for Microbiology provides professional education and laboratory practice resources that can support the training of laboratory personnel. The laboratory should use these resources to supplement its internal training program. ### Limitations of the Decision Framework The decision framework has limitations that the laboratory should acknowledge. The framework is designed to identify and resolve the most common causes of discordance, but it cannot identify every possible cause. The framework relies on the accuracy of the records and the quality of the investigation. If the records are incomplete or the investigation is not thorough, the framework may not identify the cause of the discordance. The framework also assumes that the reference method is the correct result. If the reference method is performed incorrectly, the framework may produce an incorrect conclusion. The laboratory should ensure that the reference method is performed according to the CLSI standards and that the quality control for the reference method is acceptable. The framework does not replace professional judgment. The laboratory should use the framework as a guide, but the final decision should be made by a qualified professional who considers the clinical context and the limitations of the methods. ### Integration with Antimicrobial Stewardship The discordance logbook and decision framework support antimicrobial stewardship programs by providing accurate susceptibility data for surveillance and reporting. The World Health Organization emphasizes the importance of accurate susceptibility testing for antimicrobial resistance surveillance and stewardship. The logbook provides the data needed to identify the trends in discordance and to implement corrective actions that improve the accuracy of susceptibility testing. The decision framework ensures that discordant results are investigated and resolved in a systematic manner, reducing the risk of reporting inaccurate results to clinicians. The laboratory should share the discordance data with the antimicrobial stewardship team on a regular basis. The data can be used to identify the areas where the rapid AST system may be unreliable and to guide the selection of antimicrobial agents for the treatment of infections. The data can also be used to identify the organisms and antimicrobial agents that require confirmation with a reference method before the result is reported. ### Practical Implementation Steps The laboratory should implement the discordance logbook and decision framework in a phased approach. The first phase is to design the logbook and the decision framework. The laboratory should identify the data fields, the decision points, and the workflow. The second phase is to train the personnel. The laboratory should provide training on the logbook entry procedures, the decision framework, and the workflow. The third phase is to pilot the logbook and the decision framework. The laboratory should use the logbook and the decision framework for a trial period to identify any issues and to refine the process. The fourth phase is to implement the logbook and the decision framework fully. The laboratory should use the logbook and the decision framework for all discordance events and should review the data on a regular basis. The laboratory should also consider the use of electronic tools to support the logbook and the decision framework. A spreadsheet or a database can be used to record the discordance events and to calculate the measurements. The electronic tool should be designed to support the data fields and the workflow. The laboratory should ensure that the electronic tool is secure and that the data is backed up regularly. ### Records and Measurements for the Logbook The logbook should be maintained as a permanent record. The laboratory should retain the logbook for a period that is consistent with the laboratory quality management system and the regulatory requirements. The logbook should be reviewed on a regular basis to identify the trends and to ensure that the investigation process is effective. The laboratory should track the following measurements to evaluate the effectiveness of the logbook and the decision framework: - The number of discordance events per month or quarter - The discordance rate by platform, organism, and antimicrobial agent - The proportion of discordance events resolved at each decision point - The time to resolution for each discordance event - The proportion of discordance events escalated to a supervisor or specialist These measurements provide the laboratory with the data needed to evaluate the effectiveness of the troubleshooting process and to identify the areas for improvement. ## Frequently Asked Questions ### What is the most common cause of discordance between rapid AST and conventional methods? The most common cause is a mixed culture or an inoculum that is not properly standardized. When the culture contains more than one organism, the rapid AST system may report the susceptibility profile of the organism that grows fastest under the test conditions. An inoculum that is too heavy or too light can also produce discordant results because rapid methods are sensitive to the initial cell density. ### How should I investigate a discordant rapid AST result? Start by verifying the specimen and culture, including the purity and identification of the organism. Then verify the inoculum and test conditions, review the quality control records, and repeat the test. If the discordance persists, confirm the result with a reference method such as broth microdilution. ### Can breakpoint differences explain discordance between rapid and conventional methods? Yes, breakpoint differences can explain discordance. Rapid AST systems may use breakpoints that differ from those used in conventional methods. If the breakpoints are different, the same MIC value can be interpreted as susceptible by one method and resistant by another. The laboratory should verify the breakpoints used by the rapid system and compare them with the current CLSI breakpoints. ### What should I do if the rapid AST result is resistant but the conventional method is susceptible? If the rapid AST result is resistant and the conventional method is susceptible, the laboratory should repeat the test and confirm the result with a reference method. The result should be reported to the clinician with a note explaining the discrepancy and the resolution. ### How can I prevent discordant results in the future? The laboratory should implement a quality control program that includes regular calibration and maintenance of the AST instrument, verification of the inoculum and test conditions, and monitoring of the rate of discordance. The laboratory should also provide training to personnel on the proper use of the rapid AST system. ### What is the role of quality control in troubleshooting discordant results? Quality control is essential for detecting the performance of the AST system. The laboratory should review the quality control results for the day of the test and verify that the quality control strains produced the expected results. If the quality control results are out of range, the test should be repeated. ### When should I escalate a discordant result to a specialist laboratory? The laboratory should escalate a discordant result when the discordance cannot be resolved by the initial investigation, when the discordance involves a clinical agent that could affect patient care, or when the discordance suggests a systematic problem with the AST system. ### What are the limitations of rapid AST methods? Rapid AST methods have limitations including shortened detection times that may not allow for the full expression of resistance mechanisms, detection systems that may not detect low-level resistance, and validation that may not cover all organisms. The laboratory should be aware of these limitations and interpret the results with caution. ## Using the Evidence | Source | Best use in this topic | Important limitation | |---|---|---| | [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html) | official guidance | Check the linked page for current local requirements | | [Microbiology Resources](https://asm.org/) | official guidance | Check the linked page for current local requirements | | [Antimicrobial Resistance](https://www.who.int/health-topics/antimicrobial-resistance) | official guidance | Check the linked page for current local requirements | ## Related Bacteriology Guides - [Tenacibaculosis in Marine Fish: Emerging Pathogen and Diagnostic Methods](/knowledge/bacteria/aquatic-bacteria/tenacibaculosis-marine-fish-emerging-pathogen-diagnostic-methods) - [Bacterial Contamination of Poultry Meat: Sources, Risks, and Mitigation](/knowledge/bacteria/avian-bacteria/bacterial-contamination-poultry-meat-sources-risks-mitigation) - [Chlamydiosis in Birds: Clinical Presentation and Advanced Diagnostic Methods](/knowledge/bacteria/avian-bacteria/chlamydiosis-birds-clinical-presentation-advanced-diagnostic-methods) - [Food Safety in Poultry: Effective Methods to Kill Bacteria in Chicken](/knowledge/bacteria/avian-bacteria/killing-bacteria-in-chicken-food-safety) - [Feline Respiratory Infections: Etiology, Zoonotic Risk, and Diagnostic Approach](/knowledge/bacteria/pet-bacteria/feline-respiratory-infections-etiology-zoonotic-risk-and-diagnostic-approach) ## References and Further Reading - [Biosafety in Microbiological and Biomedical Laboratories](https://www.cdc.gov/labs/bmbl/index.html). CDC and NIH. - [Microbiology Resources](https://asm.org/). American Society for Microbiology. - [Antimicrobial Resistance](https://www.who.int/health-topics/antimicrobial-resistance). World Health Organization. - [Bacteriological Analytical Manual](https://www.fda.gov/food/laboratory-methods-food/bacteriological-analytical-manual-bam). U.S. Food and Drug Administration. - [Antimicrobial Susceptibility Testing](https://clsi.org/standards/products/microbiology). Clinical and Laboratory Standards Institute. - [Culture Collections](https://www.atcc.org/). American Type Culture Collection. - [NCBI Pathogen Detection](https://www.ncbi.nlm.nih.gov/pathogens). National Center for Biotechnology Information. - [Point-of-Care Diagnostic Technologies for Antimicrobial Resistance: Principles, Platforms, Clinical Impact, and Future Directions.](https://doi.org/10.3390/diagnostics16081239). 2026. - [Clinical and Economic Value of Rapid Microbiological Diagnostics in Bloodstream Infections: A State-of-the-Art Evidence Review with Emphasis on PCR and MALDI-TOF.](https://doi.org/10.3390/microorganisms14050994). 2026. - [Practical tips and common mistakes in ocular microbiology sampling and processing.](https://doi.org/10.4103/ijo.ijo_2190_22). 2023. - [The Practical Problem With Carbapenem Testing and Reporting Accurate Bacterial Susceptibilities.](https://doi.org/10.3389/fphar.2022.841896). 2022. > This article is educational and does not replace validated laboratory procedures, clinical diagnosis, institutional biosafety review, or public-health guidance.