# False Positives and False Negatives in Veterinary Serology ## Quick Answer - Serology false positives and false negatives arise from assay design, sample quality, timing of infection, vaccination history, and cross-reacting antibodies, so unexpected results require systematic troubleshooting before clinical action. - Confirmatory testing with a different method, such as immunochromatography, immunofluorescence, or virus isolation, is the standard next step when a result conflicts with clinical signs or patient history. - No single serological test has perfect sensitivity and specificity, and a negative result does not exclude early infection or immunosuppressed carriers. ## Understanding Serology Test Performance Serological assays detect antibodies or antigens in blood, serum, or other body fluids. The clinical value of any serology test depends on two performance characteristics: sensitivity and specificity. Sensitivity is the ability of a test to correctly identify animals that truly have the target condition. Specificity is the ability to correctly identify animals that do not have the condition. A test with high sensitivity produces few false negatives. A test with high specificity produces few false positives. In veterinary practice, no assay achieves 100 percent sensitivity and 100 percent specificity simultaneously. Trade-offs are inherent. A test designed to catch every infected animal will often flag some uninfected animals. A test designed to avoid false positives will miss some early or low-level infections. Understanding these trade-offs helps veterinarians interpret results in context instead of treating a single test result as absolute truth. The most common serological platforms in veterinary medicine include enzyme-linked immunosorbent assay, immunofluorescence assay, and agglutination tests. Each platform has distinct strengths and weaknesses. ELISA tests are widely used for screening because they are rapid, inexpensive, and adaptable to many pathogens. IFA tests use fluorescent-labeled antibodies to visualize antigen-antibody complexes under a microscope. Agglutination tests rely on visible clumping of particles when antibodies bind to antigens. Each method has its own sources of error. ## At a Glance | Assay Type | Common Use | Main False Positive Source | Main False Negative Source | Confirmatory Approach | | --- | --- | --- | --- | --- | | ELISA | FeLV antigen, heartworm antigen, antibody panels | Cross-reacting antibodies, nonspecific binding, contaminated sample | Antigen levels below detection threshold, early infection, immune complex interference | Repeat with a different ELISA kit, then IFA or PCR | | IFA | Antibody detection for toxoplasmosis, anaplasmosis, leptospirosis | Nonspecific fluorescence, prior vaccination, cross-reacting antibodies | Antibody level below detection threshold, sample collected too early | Paired titers, Western blot, or PCR | | Agglutination | Brucellosis, leptospirosis, blood typing | Nonspecific agglutination, contaminated sample, prior vaccination | Antibody level below detection threshold, prozone effect | Repeat with dilution, confirm with PCR or culture | ## Causes of False Positives A false positive occurs when a test indicates the presence of a target antigen or antibody when the animal is not actually infected. False positives create unnecessary concern, lead to additional testing, and can result in inappropriate treatment or euthanasia decisions. Several mechanisms produce false positives. ### Cross-Reactive Antibodies The immune system produces antibodies that bind to specific antigen structures. Some antibodies bind to similar structures on unrelated pathogens. This is called cross-reactivity. For example, antibodies against one bacterial species may bind to antigens from a different species in an ELISA. Cross-reactivity is a common cause of false positives in antibody tests for related pathogens. Vaccination is a major source of cross-reactive antibodies. Vaccines stimulate the immune system to produce antibodies against the vaccine antigen. These antibodies may be indistinguishable from antibodies produced during natural infection. A positive antibody test in a recently vaccinated animal does not confirm infection. The timing of vaccination relative to testing is essential for interpretation. ### Nonspecific Binding ELISA and IFA tests rely on antibodies binding to antigens. Nonspecific binding occurs when antibodies or other serum proteins stick to the test surface or to the detection reagent without a specific antigen-antibody interaction. This is more common in samples with high protein content, such as hemolyzed or lipemic samples. Nonspecific binding produces a weak positive signal that can be misinterpreted. ### Sample Contamination Contamination of the sample with bacteria, blood, or other substances can interfere with the assay. Bacterial contamination can produce enzymes that mimic the detection signal. Hemolysis can release hemoglobin that interferes with colorimetric detection. Lipemia can cause turbidity that affects optical density readings. Proper sample collection and handling reduce these risks. ### Recent Vaccination Vaccination history is a major cause of false positives in antibody tests. The immune response to a vaccine produces antibodies that can be detected by serology. The duration of vaccine-induced antibody varies by vaccine type and individual animal. A positive antibody test shortly after vaccination does not confirm infection. The veterinarian must know the vaccination history of the animal before interpreting the result. ### Immune Complex Interference In some infections, antibodies bind to antigens to form immune complexes. These complexes can interfere with the detection of free antigen or free antibody. In an ELISA that detects antigen, immune complexes can block the antigen from binding to the capture antibody, producing a false negative. In an antibody test, immune complexes can reduce the amount of free antibody available for detection. ## Key Causes of False Negative A false negative occurs when the test fails to detect the target when the animal is truly infected. False negatives are dangerous because they can delay treatment and allow the infection to spread. Several mechanisms produce false negatives. ### Timing of Sample Collection Antibody tests require time for the immune system to produce detectable levels of antibody. The window between infection and detectable antibody is called the seroconversion period. During this period, the animal is infected but the antibody level is below the detection threshold. Collecting a sample too early produces a false negative. Antigen tests also have timing constraints. Some antigens are present only during certain stages of infection. For example, FeLV antigen is present in the blood during the early stage of infection but may be cleared from the blood during the latent stage. A negative antigen test during the latent stage does not exclude infection. ### Low Antigen or Antibody Levels The concentration of antigen or antibody in the sample must be above the detection threshold of the assay. In early infection, the antigen level may be too low to detect. In chronic or latent infections, the antibody level may decline below the detection threshold. The sensitivity of the test determines the minimum detectable level. ### Prozone Effect The prozone effect occurs when the concentration of antibody is too high. In agglutination tests, high antibody levels can prevent the formation of visible clumps. The test appears negative even though the antibody is present. This is more common in agglutination tests than in ELISA or IFA. Diluting the sample can overcome the prozone effect. ### Immune Complex Interference Immune complexes can also cause false negatives in antigen tests. When the antigen is bound to antibody, the detection reagent cannot access the antigen. This is a common cause of false negatives in FeLV antigen tests. The antigen is present but hidden. ### Sample Degradation Antibodies and antigens are proteins that degrade over time. Improper storage, repeated freezing and thawing, or prolonged storage at room temperature can reduce the amount of detectable protein. A degraded sample can produce a false negative. Sample handling is a critical part of the diagnostic process. ## Confirmatory Testing When a serology result conflicts with clinical signs or patient history, the veterinarian should confirm the result with a different test method. Confirmatory testing is the standard approach to resolve false positives and false negatives. ### Repeat Testing Repeating the same test on a new sample is the first step. A repeat test can rule out sample contamination or a technical error. If the repeat test gives the same result, the result is more likely to be accurate. If the repeat test gives a different result, the original result was likely a false positive or false negative. ### Different Test Method Confirming with a different test method is more reliable than repeating the same test. Different methods have different sources of error. A positive result on two different methods is more likely to be a true positive. A negative result on two different methods is more likely to be a true negative. For example, a positive FeLV antigen ELISA can be confirmed with an IFA test. The IFA test detects the antigen in a different way. A positive result on both tests is strong evidence of infection. A positive ELISA with a negative IFA suggests a false positive. ### Paired Antibody Testing For antibody tests, a single positive result does not prove infection. A paired antibody test measures the antibody level at two time points. A four-fold or greater increase in antibody level between the two samples indicates an active infection. A stable antibody level suggests a past infection or vaccination. ### PCR Testing PCR testing detects the genetic material of the pathogen. PCR is not a serology test, but it is a powerful confirmatory tool. PCR can detect the pathogen directly, regardless of the antibody response. A positive PCR result confirms infection. A negative PCR result does not exclude infection, especially if the pathogen is present in low numbers. ## Practical Workflow for Unexpected Results When a serology result is unexpected, the veterinarian should follow a systematic approach to determine the cause and the next step. ### Step 1: Review the Clinical Picture The first step is to compare the serology result with the clinical signs and patient history. Does the result make sense? A positive FeLV test in a healthy, indoor-only cat is unexpected. A negative FeLV test in a cat with severe anemia is also unexpected. The clinical picture guides the interpretation. ### Step 2: Review the Test History The veterinarian should review the vaccination history, previous test results, and any recent treatments. Vaccination can cause false positives. Previous test results can show a trend. Recent treatment can affect the immune response. ### Step 3: Repeat the Test The veterinarian should repeat the test on a new sample. This confirms the result and rules out a technical error. The repeat test should be performed in the same laboratory or with the same test kit. ### Step 4: Confirm with a Different Method If the repeat test gives the same result, the veterinarian should confirm with a different method. The choice of confirmatory test depends on the pathogen and the available laboratory. ### Step 5: Interpret the Result The veterinarian should interpret the confirmed result in the context of the clinical picture. A confirmed positive result does not always mean the animal is sick. A confirmed negative result does not always mean the animal is healthy. ## Records and Measurements Accurate records are essential for interpreting serology results. The veterinarian should record the following information for each test: - The date of sample collection - The date of the test - The test method and kit - The result - The vaccination history - The clinical signs - The treatment history These records allow the veterinarian to track changes over time and to identify patterns. A single positive result is less informative than a series of results. ### Sample Collection Records The veterinarian should record the type of sample, the collection method, and the storage conditions. A sample that was stored improperly can produce a false result. The record should include the time between collection and testing. ### Test Kit Records The veterinarian should record the test kit lot number and the expiration date. An expired or defective kit can produce false results. The record should include the name of the person who performed the test. ## Common Failure Patterns ### Failure to Consider Vaccination History A common failure is to interpret a positive antibody test without knowing the vaccination history. A vaccinated animal can have a positive antibody test for months or years. The veterinarian must ask about vaccination before interpreting the result. ### Failure to Consider Timing A common failure is to collect a sample too early in the infection. The antibody level is below the detection threshold, and the test is negative. The veterinarian should repeat the test after the appropriate interval. ### Failure to Confirm A common failure is to accept a single positive result without confirmation. A false positive can lead to unnecessary treatment or euthanasia. The veterinarian should confirm a positive result with a different method. ### Failure to Consider the Prozone Effect A common failure is to interpret a negative agglutination test without considering the prozone effect. The sample should be diluted to rule out the prozone effect. ## Welfare and Safety Context Serology testing is a noninvasive procedure that requires a blood sample. The sample collection should be performed by a trained professional to minimize pain and distress. The veterinarian should follow the guidelines for animal welfare and handling. The World Organisation for Animal Health provides official guidance on animal health and welfare. The organization emphasizes the importance of accurate diagnosis for disease control and prevention. A false positive can lead to unnecessary treatment or euthanasia. A false negative can allow the disease to spread. The American Veterinary Medical Association provides resources for pet owners and veterinarians. The association emphasizes the importance of regular veterinary care and preventive care. The association also provides guidance on the interpretation of diagnostic tests. The American Animal Hospital Association provides practice guidelines for companion animal care. The guidelines emphasize the importance of evidence-based medicine and the use of diagnostic tests. The World Small Animal Veterinary Association provides global guidelines for companion animal care. The guidelines emphasize the importance of vaccination and preventive care. ## Limitations of Serology Serology is a valuable diagnostic tool, but it has limitations. The veterinarian should understand these limitations before interpreting a result. ### Serology Does Not Prove Infection A positive antibody test does not prove that the animal is currently infected. The antibody can be from a past infection or from vaccination. A positive antigen test is more specific, but it can also be a false positive. ### Serology Does Not Prove Immunity A positive antibody test does not prove that the animal is immune to the pathogen. The antibody level may be too low to provide protection. The immune response is complex and involves a broad antibodies. ### Serology is Time-Dependent The result of a serology test depends on the timing of the sample collection. A sample collected too early can be negative. A sample collected too late can be negative. The veterinarian must consider the timing of the infection. ### Serology is Pathogen-Specific A serology test is specific to a particular pathogen. A positive result for one pathogen does not rule out infection with another pathogen. The veterinarian must consider the differential diagnosis. ## Professional Escalation Criteria The veterinarian should escalate the case to a specialist or a reference laboratory in the following situations: - The result is unexpected and the clinical picture is unclear. - The result is positive and the animal is healthy. - The result is negative and the animal is severely ill. - The result is positive and the animal has been vaccinated. - The result is positive and the animal is a breeding animal. - The result is positive and the animal is a blood donor. The veterinarian should also escalate the case when the confirmatory test is not available in the practice. The reference laboratory can provide a more accurate test. ## Building a Serology Result Interpretation Record System A recurring source of diagnostic error in veterinary practice is the absence of a structured record that links a serology result to the specific conditions under which it was produced. Without a standardized record, a veterinarian cannot reliably distinguish a true positive from a false positive caused by a defective kit lot, a recent vaccination, or a mishandled sample. The practical decision framework presented here addresses this gap by defining a record system that captures the variables known to influence serology outcomes, a troubleshooting method for unexpected results, and a comparison of confirmatory strategies based on the clinical question at hand. ### The Core Problem: Results Without Context A serology result is a measurement of a biological interaction in a specific sample at a specific time. The same animal can produce a positive result on one day and a negative result on another day if the sample is collected at a different stage of infection or if the sample is handled differently. The veterinarian who interprets a result without a complete record of the sample, the test, and the patient is working with incomplete information. This is a common source of both false positives and false negatives in practice. For example, a positive FeLV antigen test in a healthy cat is only interpretable if the veterinarian knows the cat's vaccination status, the date of the last vaccine, the storage conditions of the sample, and the lot number of the test kit. A cat that received a vaccine within the previous few weeks may have vaccine-induced antigen that is detected by the test. A sample that was left at room temperature for several hours may have degraded antigen that produces a false negative. A kit that was stored improperly or used past its expiration date may produce a false positive. The record is the tool that allows the veterinarian to identify these causes. ## The Serology Result Record The record system proposed here is a single-page form or an electronic entry that is completed for every serology test performed in the practice. The record has four sections: animal identification, sample information, test information, and clinical context. Each section captures data that is known to influence the interpretation of the result. ### Animal Identification Section This section records the animal's signalment, including species, breed, age, sex, and reproductive status. It also records the animal's identification number and the owner's name. This information is essential for tracking the animal over time and for comparing results across multiple tests. The record should also include the animal's vaccination history. The veterinarian should record the type of vaccine, the date of the last vaccine, and the date of any previous vaccines. This information is critical for interpreting antibody tests because vaccine-induced antibodies can persist for months or years. A positive antibody test in a recently vaccinated animal does not confirm infection. ### Sample Information Section This section records the details of the sample that was tested. The veterinarian should record the date and time of sample collection, the type of sample (serum, plasma, whole blood), the collection method, and the person who collected the sample. The record should also include the storage conditions of the sample, including the temperature and the duration of storage before testing. The sample information section is essential for identifying false negatives caused by sample degradation. A sample that was stored at room temperature for more than a few hours may have degraded antigen or antibody. A sample that was frozen and thawed multiple times may have reduced protein levels. A sample that was hemolyzed or lipemic may produce nonspecific binding in an ELISA. The record allows the veterinarian to identify these issues. ### Test Information Section This section records the details of the test that was performed. The veterinarian should record the test method, the test kit name, the lot number, the expiration date, and the person who performed the test. The record should also include the date and time of the test. The test information is essential for identifying false results caused by defective kits or technical errors. A kit that is expired or stored improperly may produce false results. A technician who performs the test incorrectly may produce a false result. The record allows the veterinarian to trace the result to a specific kit and a specific person. ### Clinical Interpretation Section This section records the clinical signs, the patient history, and the reason for testing. The veterinarian should record the presenting complaint, the physical examination findings, and any relevant history, such as recent travel, exposure to other animals, or previous test results. This information is used to determine whether the result is consistent with the clinical picture. The clinical interpretation section also records the action taken. The veterinarian should record whether the result was accepted, repeated, or confirmed with a different method. This information is used to track the outcome of the case and to identify patterns of false results. ## The Troubleshooting Method for Unexpected Results When a serology result is unexpected, the veterinarian should use a structured troubleshooting method to identify the cause. The method is a series of questions that the veterinarian asks about the sample, the test, and the animal. The answers to these questions guide the next step. ### Question 1: Is the Result Consistent with the Clinical Picture? The first question is whether the result makes sense given the animal's clinical signs and history. A positive FeLV test in a healthy, indoor-only cat is unexpected. A negative FeLV test in a cat with severe anemia is also unexpected. If the result is consistent with the clinical picture, the veterinarian can proceed with the interpretation. If the result is inconsistent, the veterinarian should investigate the cause. ### Question 2: Was the Sample Collected and Stored Correctly? The second question is whether the sample was collected and stored correctly. The veterinarian should review the sample information section of the record. If the sample was stored at room temperature for several hours, the result may be a false negative. If the sample was hemolyzed or lipemic, the result may be a false positive. If the sample was contaminated, the result may be unreliable. ### Question 3: Was the Test Performed Correctly? The third question is whether the test was performed correctly. The veterinarian should review the test information section of the record. If the kit was expired, the result may be unreliable. If the kit was stored improperly, the result may be unreliable. If the technician performed the test incorrectly, the result may be unreliable. ### Question 4: Is the Vaccination History Known? The fourth question is whether the vaccination history is known. If the animal was vaccinated recently, the result may be a false positive. If the animal was vaccinated months ago, the result may still be a false positive. The veterinarian should ask the owner about the vaccination history if it is not recorded. ### Question 5: Is the Timing of the Sample Appropriate? The fifth question is whether the sample was collected at the appropriate time. If the sample was collected too early in the infection, the result may be a false negative. If the sample was collected too late, the result may be a false negative. The veterinarian should consider the timing of the infection and the seroconversion period. ### Question 6: Is the Result Reproducible? The sixth question is whether the result is reproducible. The veterinarian should repeat the test on a new sample. If the repeat test gives the same result, the result is more likely to be accurate. If the repeat test gives a different result, the original result was likely a false positive or false negative. ### Question 7: Does the Result Need Confirmation? The seventh question is whether the result needs confirmation. If the result is positive and the clinical picture is unclear, the veterinarian should confirm the result with a different test method. If the result is negative and the clinical picture is severe, the veterinarian should consider a different test method. ## Comparison of Confirmatory Strategies The existing article describes confirmatory testing with a different method, but it does not provide a structured comparison of the strategies available. This section provides that comparison. The veterinarian can choose among three confirmatory strategies: repeat testing, paired antibody testing, and PCR testing. Each strategy has distinct strengths and limitations. ### Repeat Testing Repeat testing is the simplest confirmatory strategy. The veterinarian repeats the same test on a new sample. This strategy is useful for ruling out a technical error or a contaminated sample. If the repeat test gives the same result, the result is more likely to be accurate. If the repeat test gives a different result, the original result was likely a false positive or false negative. The limitation of repeat testing is that it does not rule out a systematic error in the test kit. If the kit is defective, the repeat test will give the same false result. Repeat testing is most useful when the original result was unexpected and the veterinarian suspects a technical error. ### Paired Antibody Testing Paired antibody testing measures the antibody level at two time points. The veterinarian collects a sample at the time of the first test and a second sample two to four weeks later. A four-fold or greater increase in antibody level between the two samples indicates an active infection. A stable antibody level suggests a past infection or vaccination. The strength of paired antibody testing is that it distinguishes between an active infection and a past infection. This is important for interpreting antibody tests because a single positive result does not prove infection. The limitation of paired antibody testing is that it requires a second sample and a delay of two to four weeks. This is not useful when the veterinarian needs an immediate answer. ### PCR Testing PCR testing detects the genetic material of the pathogen. PCR is not a serology test, but it is a powerful confirmatory tool. PCR can detect the pathogen directly, regardless of the antibody response. A positive PCR result confirms infection. A negative PCR result does not exclude infection, especially if the pathogen is present in low numbers. The strength of PCR testing is that it is highly specific and can detect the pathogen directly. The limitation of PCR testing is that it is more expensive and may not be available in the practice. The veterinarian may need to send the sample to a reference laboratory. ### Choosing the Confirmatory Strategy The choice of confirmatory strategy depends on the clinical situation. If the veterinarian suspects a technical error, repeat testing is the first step. If the veterinarian needs to distinguish between an active infection and a past infection, paired antibody testing is the appropriate choice. If the veterinarian needs a detailed explanation, PCR testing is the appropriate choice. The veterinarian should also consider the cost and the availability of the confirmatory test. A reference laboratory can provide a more accurate test, but the test may take several days to complete. The veterinarian should discuss the options with the owner and make a decision based on the clinical picture. ## Practical Implementation Steps The record system and troubleshooting method described here are practical tools that can be implemented in any veterinary practice. The following steps describe how to implement the system. ### Step 1: Create the Record Form The first step is to create a record form that includes the four sections described above. The form can be a paper form or an electronic form. The form should be completed for every serology test performed in the practice. ### Step 2: Train the Staff The second step is to train the staff on the use of the form. The staff should understand the importance of recording the sample information, the test information, and the clinical interpretation. The staff should also understand the troubleshooting method and the confirmatory strategies. ### Step 3: Use the Record for Every Test The third step is to use the record for every test. The veterinarian should review the record before interpreting the result. The veterinarian should also use the record to track the results over time. ### Step 4: Review the Records for Patterns The fourth step is to review the records for patterns. The veterinarian should look for patterns of false positives or false negatives. For example, if a particular kit lot produces a high number of false positives, the veterinarian should contact the manufacturer. If a particular technician produces a high number of false negatives, the veterinarian should review the technician's technique. ### Step 5: Escalate to a Reference Laboratory The fifth step is to escalate to a reference laboratory when the confirmatory test is not available in the practice. The reference laboratory can provide a more accurate test. The veterinarian should send the sample to the reference laboratory with a complete record of the sample and the test. ## Common Failure Patterns in the Record System The record system is only useful if it is used consistently. The following are common failure patterns that undermine the record system. ### Failure to Complete the Record A common failure is to complete the record incompletely. The staff may forget to record the sample storage conditions or the kit lot number. This makes it impossible to interpret the result. The veterinarian should check the record for completeness before interpreting the result. ### Failure to Review the Record A common failure is to interpret the result without reviewing the record. The veterinarian may be in a hurry and may not review the record. This can lead to a misinterpretation of the result. The veterinarian should review the record before interpreting the result. ### Failure to Use the Record for Troubleshooting A common failure is to use the record for troubleshooting. The veterinarian may repeat the test without reviewing the record. This can lead to a repeated false result. The veterinarian should use the record to identify the cause of the unexpected result. ## Welfare and Safety Context The record system and troubleshooting method are consistent with the principles of animal welfare and safety. The World Organisation for Animal Health provides official guidance on animal health and welfare. The organization emphasizes the importance of accurate diagnosis for disease control and prevention. A false positive can lead to unnecessary treatment or euthanasia. A false negative can allow the disease to spread. The record system reduces the risk of both. The American Veterinary Medical Association provides resources for pet owners and veterinarians. The association emphasizes the importance of regular veterinary care and preventive care. The record system supports this by ensuring that serology results are interpreted accurately. The American Animal Hospital Association provides practice guidelines for companion animal care. The guidelines emphasize the importance of evidence-based medicine and the use of diagnostic tests. The record system is an evidence-based tool that supports the interpretation of diagnostic tests. The World Small Animal Veterinary Association provides global guidelines for companion animal care. The guidelines emphasize the importance of vaccination and preventive care. The record system supports the interpretation of vaccination-related serology results. ## Professional Escalation Criteria The veterinarian should escalate the case to a specialist or a reference laboratory when the record system does not resolve the unexpected result. The following situations require escalation: - The result is unexpected and the record does not identify a cause. - The result is positive and the animal is healthy, and the confirmatory test is not available in the practice. - The result is negative and the animal is severely ill, and the confirmatory test is not available in the practice. - The result is positive and the animal is a breeding animal, and the confirmatory test is not available in the practice. - The result is positive and the animal is a blood donor, and the confirmatory test is not available in the practice. The veterinarian should also escalate the case when the record system reveals a pattern of false results that cannot be explained. For example, if a particular kit lot produces a high number of false positives, the veterinarian should contact the manufacturer and the reference laboratory. ## Frequently Asked Questions ### What is the most common cause of a false positive in a veterinary serology test? The most common cause is cross-reacting antibodies from a previous infection or vaccination. The immune system produces antibodies that can recognize similar antigens from different pathogens. A positive result should be interpreted in the context of the vaccination history. ### How long after infection does a serology test become positive? The time depends on the pathogen and the test. For most pathogens, the antibody level becomes detectable within one to three weeks after infection. The veterinarian should repeat the test after a later interval if the first test is negative. ### Can a vaccinated animal have a positive serology test? Yes. Vaccination stimulates the immune system to produce antibodies that can be detected by serology. The duration of the antibody varies with the vaccine and the pathogen. The veterinarian should know the vaccination history before interpreting the result. ### What is the prozone effect? The prozone effect occurs when the antibody concentration is too high. In agglutination tests, the high antibody level prevents the formation of visible clumps. The test appears negative even though the antibody is present. Diluting the sample can overcome the prozone effect. ### How can I confirm a positive serology result? The veterinarian should confirm a positive result with a different test method. For example, a positive ELISA can be confirmed with an IFA or PCR. A positive result on two different methods is more likely to be a true positive. ### What should I do if the serology result conflicts with the clinical picture? The veterinarian should review the sample, the test history, and the vaccination history. The veterinarian should repeat the test on a new sample. If the result is still unexpected, the veterinarian should confirm with a different method. ### Can a false negative serology result delay treatment? A false negative can delay treatment because the veterinarian may not suspect the infection. The veterinarian should consider the clinical picture and the timing of the infection. A negative result does not exclude the infection. ### What is the difference between a screening test and a confirmatory test? A screening test is designed to be highly sensitive and to catch most infections. A confirmatory test is designed to be highly specific and to rule out false positives. The veterinarian should use a confirmatory test when the screening test is positive. ## Using the Evidence | Source | Best use in this topic | Important limitation | |---|---|---| | [Pet Care](https://www.avma.org/resources-tools/pet-owners) | official guidance | Check the linked page for current local requirements | | [AAHA Guidelines](https://www.aaha.org/resources) | official guidance | Check the linked page for current local requirements | | [Global Guidelines](https://wsava.org/global-guidelines) | official guidance | Check the linked page for current local requirements | ## Related Veterinary Guides - [Cat Sneezing: Common Causes and When to Worry](/knowledge/veterinary-medicine/online-vet-and-care-costs/cat-sneezing-causes) - [Preanalytical Errors in Veterinary Laboratory Testing: Prevention and Troubleshooting](/knowledge/veterinary-medicine/clinical-pathology/preanalytical-errors-veterinary-laboratory) - [Cat Limping on a Front Leg: Common Causes and Urgency Guide](/knowledge/veterinary-medicine/vet-visit-decisions/cat-limping-front-leg) - [Cat Skin Rash on Face: Common Causes and Home Care](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-skin-rash-on-face-causes) ## References and Further Reading - [Pet Care](https://www.avma.org/resources-tools/pet-owners). American Veterinary Medical Association. - [AAHA Guidelines](https://www.aaha.org/resources). American Animal Hospital Association. - [Global Guidelines](https://wsava.org/global-guidelines). World Small Animal Veterinary Association. - [Merck Veterinary Manual](https://www.merckvetmanual.com/). Merck Veterinary Manual. - [Cornell University College of Veterinary Medicine](https://www.vet.cornell.edu/). Cornell University. - [Animal Health and Welfare](https://www.woah.org/en/what-we-do/animal-health-and-welfare). World Organisation for Animal Health. - [Canine brucellosis.](https://pubmed.ncbi.nlm.nih.gov/15271453). Animal reproduction science, 2004. - [False Positives in Brucellosis Serology: Wrong Bait and Wrong Pond?](https://pubmed.ncbi.nlm.nih.gov/37235322). Tropical medicine and infectious disease, 2023. - [Ghostbuster-A Western Blot-Based Panel Method to Resolve False-Positive Brucellosis Serology Test Results.](https://pubmed.ncbi.nlm.nih.gov/40142466). Microorganisms, 2025. - [Modelling the probability and impact of false-positive serology for Borrelia burgdorferi sensu lato: A case study.](https://pubmed.ncbi.nlm.nih.gov/32385952). Equine veterinary journal, 2021. - [Introduction to Antigen and Antibody Assays.](https://pubmed.ncbi.nlm.nih.gov/27154595). Topics in companion animal medicine, 2015. > This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.