Positive and Negative Controls in Diagnostic Assays: Selection and Interpretation

Diagnostic assays depend on control materials to verify that test systems perform as intended and that results can be trusted. Positive controls contain the target analyte or a substance that produces a known response, while negative controls lack the target and should produce no response. Selecting appropriate controls for molecular and serological assays requires understanding the assay format, the purpose of each control, and the limits of what control results can tell you. This article provides laboratory students, technicians, researchers, and diagnostic professionals with practical guidance on choosing controls, interpreting control outcomes, and troubleshooting unexpected results.

At a Glance: Control Types and Their Functions

The table above summarizes the main control categories used in diagnostic laboratories. Each control type answers a different question about assay performance, and a well-designed assay incorporates multiple controls to distinguish between test failure, sample problems, and true negative results.

Core Principles of Control Design

What Controls Actually Verify

Controls verify that the analytical system is functioning within established parameters. A positive control confirms that the assay can detect the target when it is present. A negative control confirms that the assay does not produce a signal when the target is absent. Neither control confirms that a patient result is correct, because patient samples have unique matrix characteristics that controls cannot fully replicate.

The World Health Organization Laboratory Quality Management System Handbook emphasizes that quality control materials must be handled and stored according to manufacturer instructions and that control results must be documented and reviewed before patient results are released. Control failures indicate that the test system may be compromised, and patient results generated during the failure period should be considered unreliable.

The Relationship Between Controls and Assay Validation

Controls are part of the broader validation framework for diagnostic assays. The U.S. Food and Drug Administration Bioanalytical Method Validation Guidance describes the need for validation studies that establish accuracy, precision, selectivity, sensitivity, and stability before a method is used for sample analysis. Controls used during routine testing are distinct from validation samples, but the principles overlap. Validation establishes the performance characteristics of the assay, while controls monitor whether the assay continues to perform within those established characteristics.

For immunohistochemistry assays, validation of antibody specificity requires careful design of positive and negative tissue controls. A weight of evidence approach combines multiple validation strategies, including tissue controls with known target expression, cell pellets with defined protein content, corroboration with western blots, and replacement of the primary antibody with an appropriate negative control reagent. Each approach has strengths and weaknesses, and cumulative evidence supports assay specificity.

Control Placement in the Testing Workflow

Controls must be positioned in the workflow to detect failures at the point where they are most likely to occur. In molecular assays, controls placed before nucleic acid extraction monitor the entire process from sample processing through detection. Controls added after extraction monitor amplification and detection only. The choice depends on what failure mode the laboratory needs to identify.

For assays that include a sample adequacy control, the control is processed with each individual specimen. A study evaluating the C-C Motif Chemokine Receptor 5 gene as a sample adequacy control in HPV molecular diagnostics found that cellularity values obtained from molecular quantification were comparable across different counting methods, supporting the use of this marker to reduce false-negative results from inadequate samples.

Selecting Controls for Molecular Assays

Amplification-Based Assays

Polymerase chain reaction and other amplification methods require controls that address the unique failure modes of these assays. Contamination is a primary concern because amplified products can be carried over into subsequent reactions. Negative controls that include water or extraction blanks processed alongside samples detect contamination events.

Positive controls for amplification assays should contain the target sequence at a concentration near the assay's detection limit to verify that the assay maintains adequate sensitivity. A positive control that produces a strong signal when the assay has lost sensitivity may not reveal the problem. Running positive controls at multiple concentrations, including a low concentration near the cutoff, provides more information about assay performance.

Internal controls are added to each reaction to detect inhibition. An internal control that fails to amplify in a sample that also tests negative for the target indicates possible inhibition, and the result should not be reported as a true negative without further investigation.

Quantitative Assays

Quantitative molecular assays, such as viral load tests, require calibrators to establish the relationship between signal and concentration. The National Center for Advancing Translational Sciences Assay Guidance Manual describes the use of standard curves and quality control samples at defined concentrations to monitor assay performance over time.

For quantitative assays, controls at multiple concentration levels are needed to verify performance across the reportable range. A control near the lower limit of quantification verifies sensitivity, while a control at a higher concentration verifies linearity and precision. Patient-based real-time quality control methods using moving rates of positive and negative patient results have been described for quantitative hepatitis B virus DNA testing, offering an additional monitoring approach that uses aggregate patient data.

Sample Adequacy Controls

Sample adequacy controls verify that the specimen contains sufficient biological material for testing. This is particularly important for assays where collection technique can vary, such as cervical screening specimens. The evaluation of CCR5 as a sample adequacy control in HPV diagnostics demonstrated that molecular cellularity quantification can confirm that a specimen contains enough cells for reliable testing, reducing the risk of false-negative results from inadequate samples.

Sample adequacy controls are distinct from internal amplification controls. The adequacy control measures the amount of human or host material present, while the internal control monitors the amplification reaction itself. Both may be needed in the same assay to distinguish between a sample that lacks target because it was not collected properly and a sample that lacks target because the assay failed.

Selecting Controls for Serological Assays

Immunoassay Controls

Serological assays detect antibodies or antigens in serum, plasma, or other biological fluids. Controls for these assays must account for the matrix effects of the specimen type and the variability inherent in antibody-antigen interactions.

Positive controls for antibody detection assays should contain antibodies to the target at concentrations near the assay cutoff. The National Center for Advancing Translational Sciences Assay Guidance Manual notes that control materials should be representative of the sample matrix and should be stable under storage conditions.

Negative controls for serological assays should be matrix-matched and should not contain antibodies or antigens that cross-react with the assay components. Nonspecific binding can produce false-positive results, and a negative control that produces a signal indicates that the assay has a specificity problem.

Bioassay Controls

Cell-based bioassays present additional control challenges because they depend on living cells that can vary in responsiveness. A study evaluating a novel thyroid-blocking immunoglobulin bioassay used cell-based reporter systems with luciferase readouts and compared results with binding immunoassays. The study found that the bioassay detected thyroid-blocking immunoglobulins in a higher percentage of samples than the binding assays, and results correlated with thyroid function.

For cell-based assays, controls must account for cell viability, responsiveness, and lot-to-lot variability. Positive controls should produce a defined response that falls within an established range, and negative controls should produce baseline activity. The assay guidance from the National Center for Advancing Translational Sciences emphasizes the importance of characterizing control performance during validation and monitoring control responses over time.

Immunohistochemistry Controls

Immunohistochemistry requires tissue-based controls that reflect the cellular context of the target protein. Positive tissue controls should contain the target protein in the appropriate cellular location, and negative tissue controls should lack the target. The Veterinary Pathology review on immunohistochemistry validation describes the use of positive and negative tissue controls as one approach within a broader weight of evidence strategy for establishing antibody specificity.

Negative control reagents for immunohistochemistry include replacing the primary antibody with an appropriate negative control reagent, such as normal serum from the same species or an isotype-matched immunoglobulin. This control detects nonspecific binding of the detection system to tissue components.

Practical Workflow for Control Implementation

Step 1: Define the Purpose of Each Control

Before selecting controls, document what each control is intended to verify. A positive control verifies detection capability. A negative control verifies specificity. An internal control verifies that the reaction conditions support amplification or detection. A sample adequacy control verifies specimen quality. Each control answers a different question, and no single control can substitute for another.

Step 2: Select Control Materials

Control materials should be appropriate for the assay format and matrix. Commercial controls are available for many assays and provide standardized performance characteristics. In-house controls may be prepared from characterized materials, but they require validation to establish expected ranges and stability.

The World Health Organization Laboratory Quality Management System Handbook advises that control materials should be stored under conditions that maintain stability and that inventory should be managed to prevent use of expired materials.

Step 3: Establish Control Ranges

Control ranges must be established during validation and documented in the laboratory's quality procedures. For qualitative assays, the range may be simply positive or negative. For quantitative assays, control values should fall within a defined range around the expected value.

A collaborative study on negative and positive control ranges in the bacterial reverse mutation test examined control data across multiple laboratories and established ranges for acceptable control performance. This approach demonstrates that control ranges should be based on actual performance data instead of arbitrary values.

Step 4: Document Control Results

Control results must be recorded for each run and reviewed before patient results are released. Documentation should include the control identity, lot number, expected range, observed result, and the identity of the person who reviewed the results. Trends in control values should be monitored over time to detect gradual changes in assay performance before they produce out-of-range results.

Step 5: Define Actions for Control Failures

Standard operating procedures should specify what actions to take when controls fail. These actions typically include repeating the run, investigating the cause of failure, and evaluating whether patient results from the failed run can be reported. The World Health Organization Laboratory Quality Management System Handbook emphasizes that control failures require investigation and corrective action before patient testing resumes.

Records and Measurements

What to Record

Laboratories should maintain records of control lot numbers, preparation dates, expiration dates, storage conditions, and results for each testing run. These records support troubleshooting when problems occur and provide data for monitoring long-term assay performance.

Control results should be recorded in a format that allows trend analysis. Levey-Jennings charts are commonly used for quantitative controls, with control limits set at established standard deviations from the mean. Westgard rules provide criteria for identifying systematic and random errors based on control value patterns.

Measurements That Matter

For quantitative assays, the key measurements are the control values themselves and their relationship to established ranges. For qualitative assays, the key measurement is whether the control produced the expected positive or negative result.

For sample adequacy controls, the measurement is the quantity of host or cellular material detected. The CCR5 study in HPV diagnostics demonstrated that molecular cellularity quantification can provide accurate measurements of sample adequacy across different concentration levels.

Monitoring Trends

Individual control results that fall within range do not guarantee that the assay is performing optimally. Trends in control values, such as a gradual increase or decrease over time, may indicate developing problems with reagents, equipment, or environmental conditions. Monitoring control trends allows laboratories to address issues before they produce out-of-range results.

Common Failure Patterns and Troubleshooting

Positive Control Fails to Produce Expected Signal

A positive control that produces no signal or a weak signal indicates that the assay failed to detect the target. Possible causes include degraded reagents, incorrect reagent preparation, equipment malfunction, or operator error. The first step is to repeat the assay with fresh reagents and verify that equipment is functioning correctly.

If the positive control continues to fail, the problem may be with the control material itself. Check the expiration date, storage conditions, and preparation records for the control. Consider whether the control was subjected to conditions that could degrade the target.

Negative Control Produces a Signal

A negative control that produces a signal indicates contamination or nonspecific binding. In molecular assays, contamination with amplified product is a common cause. Review laboratory practices for amplicon handling, reagent preparation, and workspace separation.

In serological assays, a reactive negative control may indicate nonspecific binding to assay components or cross-reacting substances in the control matrix. Evaluate whether the control matrix is appropriate for the assay and whether the detection system is producing background signal.

Internal Control Fails in Individual Samples

An internal control that fails in a specific sample while passing in other samples indicates that the sample contains inhibitors or that the sample was not processed correctly. The result for that sample should not be reported as negative without further investigation.

Options include diluting the sample to reduce inhibitor concentration, repeating the extraction, or requesting a new specimen. The specific approach depends on the assay and the suspected cause of inhibition.

Sample Adequacy Control Indicates Insufficient Material

A sample adequacy control that indicates insufficient cellular material suggests that the specimen was not collected properly or that the sample degraded during transport or storage. The test result may be unreliable, and a new specimen should be requested.

The CCR5 study in HPV diagnostics demonstrated that molecular cellularity quantification can identify samples with inadequate cellular content, reducing the risk of false-negative results. Laboratories should have procedures for communicating inadequate samples to the requesting clinician.

Control Results Are Within Range but Patient Results Are Questionable

Controls that pass do not guarantee that every patient result is correct. Patient samples can contain substances that interfere with the assay without affecting the controls. If a patient result is inconsistent with clinical findings, the laboratory should consider repeat testing, alternative methods, or consultation with the requesting clinician.

Quality Assurance and Regulatory Context

Laboratory Standards

The ISO 15189 guideline describes requirements for medical laboratories and provides a framework for quality assurance in molecular diagnostics. A review of quality assurance challenges in molecular diagnostics noted that ISO 15189 can serve as an orientation for integrating new test procedures into existing quality assurance measures.

Quality assurance programs should include internal quality control, external quality assessment, and ongoing staff training. External quality assessment programs, such as those provided by Quality Control for Molecular Diagnostics, allow laboratories to compare their performance with other laboratories using the same or similar assays.

External Quality Assessment

External quality assessment programs provide blinded samples that laboratories test and report results for comparison with expected values. A study of vancomycin resistance detection in enterococci over six consecutive years of external quality assessment demonstrated the value of these programs for monitoring laboratory performance over time.

Participation in external quality assessment is distinct from internal quality control. Internal controls monitor day-to-day assay performance, while external quality assessment provides periodic independent verification of testing accuracy.

Biosafety Considerations

Control materials may contain infectious agents or hazardous chemicals. The World Health Organization Laboratory Biosafety Manual provides guidance on safe handling of biological materials, including controls and patient samples. Laboratories should follow biosafety level requirements appropriate for the materials being handled and should ensure that staff are trained in safe practices.

Positive controls for infectious disease assays may contain inactivated or live organisms. The biosafety requirements for handling these materials depend on the organism and the degree of inactivation. Laboratories should verify the inactivation status of commercial controls and follow appropriate precautions.

Limitations of Controls

Controls Cannot Detect All Failures

Controls verify that the assay performs as expected under the conditions of the control material. They cannot detect failures that affect patient samples but not controls. For example, a control that is processed separately from patient samples will not detect errors in patient sample handling or identification.

Matrix Differences Between Controls and Patient Samples

Controls are typically prepared in a matrix that approximates the patient sample matrix, but the approximation is never perfect. Patient samples can contain substances that interfere with the assay without affecting controls. This limitation is inherent to the use of controls and cannot be completely eliminated.

Control Material Stability

Control materials degrade over time, and degraded controls can produce results that do not reflect assay performance. Laboratories must monitor control stability and replace controls according to manufacturer recommendations or established in-house stability data.

Historical Control Data

Historical control data provide context for interpreting current control results, but they have limitations. The Trp53 hemizygous mouse evaluation in pharmaceutical development noted that tumor incidence should be compared with historical control values, but historical data must be current and generated under comparable conditions. The same principle applies to diagnostic assay controls. Historical control ranges are useful only when the assay conditions have not changed.

Professional Escalation Criteria

Laboratory professionals should escalate control failures or questionable results to a supervisor or laboratory director when:

• Control failures cannot be resolved through routine troubleshooting
• Control failures suggest a systemic problem with reagents, equipment, or laboratory practices
• Patient results may have been affected by a control failure
• External quality assessment results indicate a performance problem
• Assay modifications are needed to address recurring control failures

The World Health Organization Laboratory Quality Management System Handbook emphasizes that quality control is a management responsibility and that laboratories must have procedures for investigating and documenting control failures.

Frequently Asked Questions

What is the difference between a positive control and a negative control?

A positive control contains the target analyte or a substance that produces a known positive response in the assay. It verifies that the assay can detect the target when it is present. A negative control lacks the target analyte and should produce no response. It verifies that the assay does not produce false-positive results from contamination, nonspecific binding, or reagent problems.

Why do I need both positive and negative controls in every run?

Positive and negative controls detect different types of assay failure. A positive control detects failures that prevent detection of the target, such as degraded reagents or equipment malfunction. A negative control detects contamination or nonspecific reactivity that could produce false-positive results. Running both controls in every run verifies that the assay can distinguish between the presence and absence of the target.

What is an internal control in molecular diagnostics?

An internal control is a known target sequence that is added to each sample or reaction to monitor the entire testing process. It detects inhibition of amplification, extraction failure, or other problems that could produce false-negative results. If the internal control fails to amplify in a sample that tests negative for the target, the result should not be reported as a true negative without further investigation.

How do I choose the right positive control concentration?

The positive control concentration should be appropriate for the purpose of the control. A positive control near the assay's detection limit verifies that the assay maintains adequate sensitivity. A positive control at a higher concentration verifies that the assay produces a signal within the expected range. For quantitative assays, controls at multiple concentrations are needed to verify performance across the reportable range.

What should I do when a negative control produces a positive result?

A reactive negative control indicates contamination or nonspecific binding. Stop testing and investigate the cause before continuing. Review reagent preparation, workspace cleanliness, and amplicon handling practices. Repeat the run with fresh reagents and new control materials. Document the investigation and corrective actions taken.

Can a passing control guarantee that patient results are correct?

No. Controls verify that the assay performs as expected under the conditions of the control material. Patient samples can contain substances that interfere with the assay without affecting controls. Sample collection, handling, and identification errors can also affect patient results without being detected by controls. Clinical correlation and repeat testing are important when patient results are questionable.

What is a sample adequacy control?

A sample adequacy control verifies that the specimen contains sufficient biological material for reliable testing. It is particularly important for assays where collection technique can vary, such as cervical screening specimens. The control measures the amount of host or cellular material present, and an inadequate result indicates that the specimen may produce a false-negative result.

How often should control ranges be reviewed?

Control ranges should be reviewed whenever there is a change in assay conditions, such as new reagent lots, new equipment, or modified procedures. They should also be reviewed periodically to ensure that they reflect current assay performance. Trends in control values should be monitored continuously, and control ranges should be updated when the data support a change.

Related Diagnostic Guides

• Understanding Positive Controls in PCR: Purpose, Selection, and Interpretation
• Negative Controls in ELISA: Setup and Interpretation for Reliable Results
• Positive Control Selection for Bacterial Transformation Efficiency Assays
• Positive Controls in Recombinant DNA Experiments: Selection, Preparation, and Interpretation
• Positive Controls in Western Blotting: Selection and Validation

References and Further Reading

• Laboratory Quality Management System Handbook. World Health Organization.
• Laboratory Biosafety Manual. World Health Organization.
• Assay Guidance Manual. National Center for Advancing Translational Sciences.
• Bioanalytical Method Validation Guidance. U.S. Food and Drug Administration.
• NCBI Literature Resources. National Center for Biotechnology Information.
• The vaccine hesitancy scale: Psychometric properties and validation.. Vaccine, 2018.
• The positive and negative suicide ideation inventory: development and validation.. Psychological reports, 1998.
• A novel bioassay for thyroid-blocking immunoglobulins.. Frontiers in endocrinology, 2024.
• Primary HPV screening for cervical cancer.. Best practice & research. Clinical obstetrics & gynaecology, 2020.
• Validating Immunohistochemistry Assay Specificity in Investigative Studies: Considerations for a Weight of Evidence Approach.. Veterinary pathology, 2021.
• Dietary patterns and cardiovascular disease.. The Proceedings of the Nutrition Society, 2013.
• The Trp53 hemizygous mouse in pharmaceutical development: points to consider for pathologists.. Toxicologic pathology, 2002.
• From correlation to causality: the case of Subdoligranulum.. Gut microbes, 2020.
• Diagnostic Discrimination of BOKE STARS, a Bimodal Continuous Performance Test, for Attention-Deficit/Hyperactivity Disorder Assessment in Chinese Children: Single-Center Case-Control Study.. 2026.
• Diagnostic accuracy of the Fujifilm SILVAMP TB LAM II (FujiLAM II) assay for the diagnosis of childhood tuberculosis.. 2026.
• A critical review of diagnostic methods for camel brucellosis. 2026.
• Urinary fatty acid binding protein 2 as a biomarker of intestinal injury in pediatric celiac disease.. 2026.
• 27Novel blood metabolomic tests for the diagnosis of Primary Sclerosing Cholangitis and associated Cholangiocarcinoma. 2026.
• Evaluation of Serum Vascular Endothelial Growth Factor (VEGF) and Placental Growth Factor (PlGF) Levels in Unexplained Early Pregnancy Loss: A Case-Control Study.. 2026.
• Quality Assurance, Management and Control in Molecular Diagnostics. 2019.
• Applications of Molecular Diagnostics for Quality Control in Rearing of Spodoptera frugiperda (Lepidoptera: Noctuidae) Larvae for Experimental Use. Florida Entomologist, 2023.
• Evaluation of C-C Motif Chemokine Receptor 5 (CCR5) as a Sample Adequacy Control in HPV Molecular Diagnostics. Diagnostics, 2024.
• Quality of molecular detection of vancomycin resistance in enterococci: results of 6 consecutive years of Quality Control for Molecular Diagnostics (QCMD) external quality assessment. European Journal of Clinical Microbiology and Infectious Diseases, 2019.
• Current and Future Challenges in Quality Assurance in Molecular Diagnostics.. Clinica chimica acta, international journal of clinical chemistry, 2021.
• Anti-drug Antibody Assay Validation: Improved Reporting of the Assay Selectivity via Simpler Positive Control Recovery Data Analysis. AAPS Journal, 2019.
• Patient-based real-time quality control for quantitative hepatitis B virus DNA test using moving rate of positive and negative patient results. Clinical Chemistry and Laboratory Medicine, 2022.
• Negative and positive control ranges in the bacterial reverse mutation test: JEMS/BMS collaborative study. Genes and Environment, 2018.

This article is educational and does not replace validated laboratory procedures, institutional biosafety review, manufacturer instructions, or professional interpretation.