# Evidence-Based Test Selection for Infectious Diseases in Dogs and Cats ## Quick Answer - Select PCR when active infection must be confirmed and sample timing and tissue selection match the pathogen's biology. - Select serology when exposure history, vaccination status, or chronic infection status is the clinical question. - Interpret every result against test sensitivity, specificity, and population prevalence because predictive value changes with disease frequency. ## At a Glance | Test Category | What It Detects | Best Clinical Use | Key Limitation | | --- | --- | --- | --- | | PCR (conventional and real-time) | Pathogen nucleic acid | Confirming active infection, identifying carrier states, and detecting organisms that are difficult to culture | Requires appropriate sample type and timing, a negative result does not exclude infection if the organism is sequestered or intermittently shed | | Serology (ELISA, IFA, MAT, virus neutralization) | Host antibodies or antigen | Assessing exposure history, vaccine response, and chronic or past infection | Cannot distinguish active from resolved infection in many cases, sensitivity and specificity vary by assay and disease stage | | Antigen tests | Pathogen proteins | Point-of-care screening for chronic infections such as feline leukemia virus and heartworm | Window periods and low antigen levels can produce false negatives early in infection | | Culture and virus isolation | Viable organism | Confirming bacterial or viral infection when antimicrobial susceptibility is needed | Slow turnaround, requires specialized laboratories, and sensitivity depends on sample handling | ## Understanding Test Performance in Clinical Decision Making Veterinarians face a practical problem when selecting diagnostic tests for infectious diseases in dogs and cats. The same pathogen can be detected by multiple methods, and each method answers a different clinical question. Test selection should begin with a clear definition of what the clinician needs to know, not with a default preference for a particular laboratory platform. Sensitivity describes the proportion of infected animals that test positive. Specificity describes the proportion of uninfected animals that test negative. Both parameters are fixed properties of a test under defined conditions, but the predictive value of a positive or negative result changes with the prevalence of disease in the population being tested. A test with high sensitivity is valuable for ruling out disease when the result is negative. A test with high specificity is valuable for confirming disease when the result is positive. In populations with low disease prevalence, even highly specific tests generate false positives that require confirmation with a second method. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides background on infectious disease diagnosis and emphasizes that laboratory results must be interpreted in the context of clinical signs, signalment, and exposure history. No test result stands alone. The same principle applies across companion animal and zoonotic disease investigation, where the choice between molecular and serological methods can change the clinical interpretation substantially. ## Core Principles of Test Selection ### Matching Test Type to Clinical Question The first decision point is whether the clinician needs to confirm active infection, document past exposure, or establish immune status. Polymerase chain reaction detects nucleic acid from the pathogen itself. A positive PCR result indicates that pathogen genetic material was present in the sample at the time of collection. Serological tests detect antibodies produced by the host in response to infection or vaccination. A positive serology result indicates that the immune system has encountered the pathogen or vaccine antigen, but it does not prove that active infection is present. For example, in the diagnosis of leptospirosis, the microscopic agglutination test detects antibodies against Leptospira serovars, while PCR detects pathogenic Leptospira DNA in blood, urine, or tissue. A study of rats from wet market areas in Kuala Lumpur found that 34.7 percent of captured rats were positive for anti-leptospiral antibody by MAT, and 50 rats were positive for pathogenic Leptospira DNA by PCR amplification of the flaB gene. The study demonstrated persistent Leptospira infections among rats and identified these animals as important reservoir hosts. The parallel use of serology and PCR in that study illustrates a broader principle: serology identifies exposure and carriage patterns in populations, while PCR confirms the presence of the organism at the time of sampling. For an individual dog with suspected acute leptospirosis, PCR on blood or urine during the early phase of illness may be more informative than MAT, which requires paired acute and convalescent samples to demonstrate a rising titer. ### Timing of Sample Collection The timing of sample collection relative to the onset of clinical signs is one of the most important determinants of test accuracy. Pathogen nucleic acid may be present in blood or tissue only during a narrow window of infection. Antibodies may not reach detectable levels until days or weeks after exposure. A PCR test performed too early or too late in the course of infection can produce a false negative result even when the animal is infected. A review of traditional and contemporary assays for detection of equid herpesvirus 1 in clinical samples emphasized that successful identification depends on suitable case selection with emphasis on timing of sample collection, selection of appropriate samples based on clinical manifestations, and careful evaluation and interpretation of laboratory results. The review noted that PCR-based assays allow detection of nucleic acid in clinical specimens precisely and rapidly compared with traditional methods that detect the agent, antigen, or agent-specific antibodies in serum. The same timing principles apply to companion animal infectious disease testing. For respiratory pathogens, samples collected during the acute phase of viral shedding are more likely to yield positive PCR results than samples collected after clinical signs have resolved. ### Sample Type Selection Different pathogens localize to different tissues and body fluids. The choice of sample type should reflect the biology of the suspected pathogen. Blood, urine, respiratory secretions, cerebrospinal fluid, joint fluid, skin biopsies, and fecal samples each have specific diagnostic utility. Collecting the wrong sample type is a common cause of false negative results. For systemic bacterial infections such as brucellosis, blood is the primary sample for molecular testing. A study of acute human brucellosis in north-eastern Kenya used real-time PCR on blood samples and compared results with the febrile Brucella plate agglutination test. The study found that the plate agglutination test had poor diagnostic performance compared with PCR, with an estimated sensitivity of 36.6 percent and specificity of 69.3 percent. The authors concluded that understanding local epidemiology is important for directing veterinary and public health interventions and for informing clinical diagnostic decision making. For veterinary patients, the same principle applies: the sensitivity of serological tests for Brucella can be low in acute infection, and PCR on blood may provide a more accurate diagnosis when clinical signs are consistent with brucellosis. For leptospirosis, urine is often the preferred sample for PCR because Leptospira organisms are shed in urine during the chronic carrier state. Kidney tissue is another option for postmortem diagnosis. Blood may be positive during the acute leptospiremic phase, but organisms disappear from blood as antibodies develop. A negative blood PCR in a dog with suspected leptospirosis does not rule out the disease if the sample was collected after the leptospiremic phase. ## Serological Testing: Strengths and Limitations ### Antibody Detection Methods Serological tests detect the host antibody response to infection or vaccination. Common formats include enzyme-linked immunosorbent assay, immunofluorescence assay, microscopic agglutination test, and virus neutralization. Each format has different performance characteristics, and results from different formats are not directly interchangeable. ELISA-based tests are widely used for screening because they are inexpensive, rapid, and adaptable to high-throughput laboratory settings. A study of severe fever with thrombocytopenia syndrome virus infection in wild boars in Japan used ELISA to screen 1,506 serum samples and found significantly higher ELISA positivity in Miyazaki Prefecture (34.1 percent) than in Oita Prefecture (11.9 percent). A subset of 150 samples was also examined by an 80 percent plaque reduction neutralization test. The study found that regional differences in ELISA seropositivity broadly corresponded with the distribution of human SFTS cases, suggesting that ELISA-based surveillance in wildlife may be useful for assessing broad regional patterns of transmission. The study also illustrates an important limitation of ELISA: it measures antibody binding, which may not always correlate with neutralizing antibody activity. Confirmatory testing with virus neutralization can provide additional information about functional antibody responses. The microscopic agglutination test is the reference method for leptospirosis serology. MAT detects agglutinating antibodies against specific Leptospira serovars. The test requires a panel of live antigens representing the serovars prevalent in the region. A study of Leptospira infection in rats from wet markets in Kuala Lumpur used MAT and found that the most prominent serovar was Malaya, followed by a local strain designated IMR LEP 175. The study demonstrated that MAT can identify the infecting serovar, which is useful for epidemiological investigations. However, MAT has limitations in clinical practice. A single MAT titer does not confirm acute leptospirosis because antibodies may persist for months after infection. Paired samples collected two to four weeks apart are needed to demonstrate a fourfold rise in titer, which is the standard criterion for confirming acute infection. ### Interpretation Challenges in Serology Serological results must be interpreted with knowledge of vaccination history. Vaccinated animals may have antibody titers that are indistinguishable from titers produced by natural infection. This is particularly relevant for diseases such as leptospirosis, feline herpesvirus, and canine distemper, where vaccination is routine. A positive serology result in a vaccinated animal does not prove that the animal is infected. Cross-reactivity between related pathogens can also complicate serological interpretation. Antibodies against one Leptospira serovar may react with antigens from other serovars, leading to false positive results or incorrect identification of the infecting serovar. The same issue applies to other pathogen groups with shared antigens. Serological tests cannot distinguish between active and resolved infection in most cases. Immunoglobulin M antibodies appear early in infection and decline after resolution, while immunoglobulin G antibodies persist for longer periods. Some laboratories offer IgM and IgG separate testing to improve interpretation, but the clinical utility of this approach depends on the specific disease and the timing of sample collection. ## Molecular Testing: PCR and Its Applications ### Principles of PCR Testing Polymerase chain reaction amplifies specific DNA sequences, allowing detection of minute quantities of pathogen nucleic acid in clinical samples. Real-time PCR, also called quantitative PCR, measures the amount of amplified product during the reaction, providing information about the quantity of pathogen nucleic acid in the sample. Reverse transcription PCR is used for RNA viruses, converting RNA to complementary DNA before amplification. PCR offers several advantages over traditional diagnostic methods. It can detect organisms that are difficult or impossible to culture. It provides rapid results compared with culture or virus isolation. It can be performed on a wide range of sample types, including blood, urine, tissue, swabs, and body fluids. PCR can also detect organisms that are no longer viable, which is useful when samples have been improperly stored or transported. The review of equid herpesvirus 1 diagnostic assays noted that PCR-based assays have allowed detection of nucleic acid in clinical specimens precisely and rapidly compared with traditional methods. The review emphasized that veterinarians must be aware of the advantages and disadvantages of various real-time PCR assays, interpretation of viral genetic markers, and latency in order to provide the best standard of care. Latency is a particular concern for herpesviruses, which can persist in a dormant state in the host and reactivate under stress or immunosuppression. A positive PCR result in a latently infected animal may not indicate active disease. ### Limitations of PCR PCR is highly sensitive, but this sensitivity creates interpretation challenges. Contamination of samples with nucleic acid from the environment or from other samples can produce false positive results. Laboratories use rigorous contamination controls, including separate areas for sample processing and amplification, but the risk cannot be eliminated entirely. PCR detects nucleic acid, not viable organisms. A positive PCR result does not prove that the pathogen is alive or that it is causing disease. This distinction is important for pathogens that can persist in the host without causing clinical signs. For example, a dog that has recovered from leptospirosis may continue to shed Leptospira DNA in urine for weeks or months. A positive urine PCR in such a dog indicates shedding, but it does not necessarily indicate acute disease. PCR can produce false negative results when the pathogen is present in very low numbers, when the sample does not contain the target organism, or when inhibitors in the sample interfere with the reaction. Blood, urine, and feces contain substances that can inhibit PCR. Laboratories use internal controls to detect inhibition, but not all assays include these controls. ### Quantitative PCR and Cycle Threshold Values Real-time PCR provides cycle threshold values, which represent the number of amplification cycles required for the fluorescent signal to exceed the background threshold. Lower cycle threshold values indicate higher amounts of target nucleic acid in the sample. Higher cycle threshold values indicate lower amounts. Cycle threshold values can be useful for clinical interpretation, but they must be used with caution. The relationship between cycle threshold values and pathogen load is not linear across all assays. Different laboratories may use different thresholds for reporting positive results. A high cycle threshold value near the cutoff may represent a true low-level infection or a false positive result from contamination. Clinicians should interpret cycle threshold values in the context of the clinical presentation and other laboratory findings. ## Disease-Specific Test Selection ### Feline Leukemia Virus and Feline Immunodeficiency Virus Feline leukemia virus and feline immunodeficiency virus are retroviruses that cause chronic infections in cats. Testing strategies for these viruses differ because the biology of the two viruses is different. FeLV is diagnosed by detection of viral antigen, specifically the p27 core protein, in blood. Point-of-care ELISA tests detect free p27 antigen in serum, plasma, or whole blood. Immunochromatographic tests are also available for in-clinic use. A positive antigen test indicates that the cat is infected and shedding virus. However, some cats can clear the infection, and transient antigenemia can occur. The [American Animal Hospital Association](https://www.aaha.org/resources) provides practice guidance on preventive care and infectious disease testing in companion animals. Confirmatory testing with immunofluorescence or PCR may be recommended for cats that test positive on screening tests, particularly if the result is unexpected based on the cat's risk profile. FIV is diagnosed by detection of antibodies against the virus. Point-of-care ELISA tests detect FIV antibodies in serum, plasma, or whole blood. A positive antibody test indicates that the cat has been exposed to the virus and has mounted an immune response. Because FIV infection is lifelong, a positive antibody test is generally interpreted as evidence of infection. However, kittens born to infected queens may acquire maternal antibodies that persist for several months, producing false positive results. The [American Veterinary Medical Association](https://www.avma.org/resources-tools/pet-owners) provides pet-owner education materials on preventive care and infectious disease testing. Confirmatory testing with Western blot or PCR may be recommended for cats with positive screening tests, particularly in low-prevalence populations where false positives are more likely. ### Heartworm Disease Heartworm disease is caused by Dirofilaria immitis, a nematode parasite transmitted by mosquitoes. Testing for heartworm disease uses antigen tests that detect proteins produced by adult female worms. Point-of-care antigen tests are widely used in veterinary practice. Antigen tests are highly specific, but they have limitations. Antigen tests can be negative during the prepatent period, which lasts approximately six to seven months after infection. Antigen tests can also be negative in infections with only male worms or with low worm burdens. The [Merck Veterinary Manual](https://www.merckvetmanual.com/) provides background on heartworm disease diagnosis and emphasizes that antigen testing should be combined with microfilaria testing in some situations. Microfilaria testing detects circulating larvae in the blood. Not all infected dogs have detectable microfilariae, and the absence of microfilariae does not rule out infection. Antigen testing is the primary screening method, and microfilaria testing provides additional information about the stage of infection. ### Leptospirosis Leptospirosis is a zoonotic bacterial disease caused by pathogenic Leptospira species. Diagnosis is challenging because clinical signs are nonspecific and can include fever, lethargy, vomiting, diarrhea, and renal or hepatic dysfunction. The microscopic agglutination test is the traditional serological method for leptospirosis diagnosis. MAT detects antibodies against specific serovars, and a fourfold rise in titer between paired acute and convalescent samples confirms acute infection. A single high titer in a dog with compatible clinical signs and appropriate exposure history is suggestive of infection, but it is not definitive. PCR on blood, urine, or tissue can detect pathogenic Leptospira DNA. Blood PCR is most useful during the acute leptospiremic phase, which occurs in the first week of infection. Urine PCR is most useful during the chronic shedding phase, which can persist for weeks to months after clinical recovery. A study of Leptospira infection in rats from wet markets in Kuala Lumpur used PCR amplification of the flaB gene to detect pathogenic Leptospira in kidney samples. The study found that 50 rats were positive for pathogenic Leptospira DNA, demonstrating persistent infection in reservoir hosts. For veterinary patients, PCR can confirm infection when serology is inconclusive or when acute infection is suspected. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) provides official guidance on animal health surveillance and reporting. Leptospirosis is a reportable disease in many jurisdictions, and veterinarians should be aware of local reporting requirements when they diagnose the disease. ### Brucellosis Brucellosis is a zoonotic bacterial disease caused by Brucella species. In dogs, Brucella canis causes reproductive failure, epididymitis, and discospondylitis. In other species, different Brucella species cause disease. Serological tests for brucellosis include the rapid slide agglutination test, the tube agglutination test, agar gel immunodiffusion, and ELISA. These tests detect antibodies against Brucella antigens. False positive results can occur with the rapid slide agglutination test, and positive results should be confirmed with a more specific test. PCR can detect Brucella DNA in blood, tissue, or other samples. A study of acute human brucellosis in north-eastern Kenya found that the febrile Brucella plate agglutination test had poor diagnostic performance compared with real-time PCR, with an estimated sensitivity of 36.6 percent and specificity of 69.3 percent. The study concluded that understanding local epidemiology is important for directing veterinary and public health interventions and for informing clinical diagnostic decision making. For veterinary patients, PCR may be more accurate than serology in acute infection, but serology remains useful for population screening and for documenting exposure. A study of multiple bacterial zoonoses in febrile outpatients in Garissa County, Kenya found considerable PCR positivity for Brucella (19.4 percent) and Leptospira species (1.7 percent) in malaria-negative febrile patients. The study also found that patients aged 5 to 17 years had higher odds of infection with Brucella species and Coxiella burnetii than older patients. These findings illustrate the importance of molecular testing for diagnosing zoonotic bacterial infections in endemic areas. For veterinary patients, the same principle applies: PCR can detect active infection when serology is negative or inconclusive. ### Tick-Borne Diseases Tick-borne diseases are caused by a variety of bacterial, viral, and protozoal pathogens transmitted by ticks. Common tick-borne diseases in dogs include anaplasmosis, ehrlichiosis, Lyme disease, and babesiosis. Serological tests detect antibodies against tick-borne pathogens. Point-of-care ELISA tests are available for some pathogens, including Anaplasma, Ehrlichia, and Borrelia burgdorferi. These tests are useful for screening, but they cannot distinguish between active and past infection. A positive antibody test in a dog with compatible clinical signs and exposure history is suggestive of infection, but confirmation may require PCR or other testing. PCR can detect pathogen DNA in blood or tissue. PCR is most useful during the acute phase of infection, when organisms are present in the bloodstream. A study of severe fever with thrombocytopenia syndrome virus infection in wild boars in Japan used quantitative reverse transcription PCR to screen for viral RNA. The study found a low viral RNA detection rate (0.57 percent in Miyazaki and 0 percent in Oita), despite high ELISA seropositivity. This finding illustrates that serology and PCR measure different aspects of infection: serology documents exposure, while PCR documents current infection. For veterinary patients, a negative PCR result does not rule out infection if the sample was collected after the organism has been cleared from the bloodstream. ## Practical Workflow for Test Selection ### Step 1: Define the Clinical Question Before selecting a test, the veterinarian should define what information is needed to guide patient management. The clinical question may be one of the following: - Does this animal have an active infection that requires treatment? - Has this animal been exposed to a specific pathogen in the past? - Is this animal a chronic carrier that can transmit infection to other animals or humans? - Is this animal protected against a specific pathogen by vaccination or prior infection? The answer to this question determines whether PCR, serology, antigen testing, or a combination of methods is most appropriate. ### Step 2: Assess the Clinical Presentation Clinical signs, signalment, and exposure history provide important context for test selection. A dog with acute fever, lethargy, and thrombocytopenia may have a tick-borne disease, leptospirosis, or another systemic infection. A cat with chronic weight loss, gingivitis, and recurrent infections may have FIV or FeLV. The differential diagnosis list guides the initial testing strategy. ### Step 3: Consider Disease Prevalence and Risk Factors The prevalence of disease in the local population affects the predictive value of test results. In a region where leptospirosis is common, a positive MAT titer is more likely to represent true infection than in a region where the disease is rare. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) provides information on disease surveillance and reporting that can help veterinarians understand local disease patterns. Risk factors such as outdoor access, hunting behavior, tick exposure, and contact with wildlife or livestock should be considered when selecting tests. A dog with known tick exposure and clinical signs consistent with anaplasmosis is a better candidate for Anaplasma testing than a dog with no tick exposure. ### Step 4: Select the Test Platform Based on the clinical question, presentation, and risk factors, the veterinarian selects the test platform. The table below summarizes the key considerations for test selection. | Clinical Question | Recommended Test | Rationale | | --- | --- | --- | | Confirm active infection in an acutely ill animal | PCR on appropriate sample type | Detects pathogen nucleic acid during active infection | | Document past exposure or immune status | Serology (ELISA, IFA, MAT) | Detects antibodies that persist after infection or vaccination | | Screen for chronic infection in a healthy animal | Antigen test or serology | Identifies infected animals that may not show clinical signs | | Confirm a positive screening test | PCR or confirmatory serology | Reduces false positives from screening tests | | Monitor response to treatment | PCR or quantitative serology | Tracks pathogen clearance or antibody decline | ### Step 5: Interpret Results in Clinical Context Test results should never be interpreted in isolation. The veterinarian should integrate laboratory findings with clinical signs, physical examination findings, and other diagnostic information. A positive PCR result in a healthy animal may represent subclinical infection or contamination. A negative serology result in an acutely ill animal may represent early infection before antibodies have developed. ### Step 6: Document and Communicate Results Accurate record keeping is essential for patient management and for public health surveillance. The veterinarian should record the test type, laboratory, sample type, collection date, and result. Communication with the owner should include an explanation of what the test result means and what follow-up testing or treatment is recommended. ## Records and Measurements ### What to Record Veterinary practices should maintain accurate records of diagnostic testing for infectious diseases. The following information should be recorded for each test: - Patient identification and signalment - Clinical signs and duration of illness - Vaccination history - Exposure history and risk factors - Sample type and collection date - Test type and laboratory - Test result and interpretation - Follow-up testing or treatment recommendations ### Using Records for Quality Improvement Aggregate testing data can be used to monitor disease prevalence in the practice population and to evaluate the performance of diagnostic tests. For example, a practice that tests many cats for FeLV and FIV can track the proportion of positive results over time. An unexpected increase in positive results may indicate a change in the local disease prevalence or a problem with the testing protocol. Records can also be used to evaluate the clinical utility of different tests. If a practice frequently obtains positive serology results that are not confirmed by PCR, the serology test may have a low positive predictive value in the practice population. This information can guide future test selection. ## Common Failure Patterns in Test Selection ### Testing Too Early or Too Late A common error is collecting samples before the pathogen has reached detectable levels or after the pathogen has been cleared from the sample site. For PCR, this means collecting blood during the acute phase of infection and collecting urine during the shedding phase. For serology, this means collecting convalescent samples two to four weeks after the acute sample to demonstrate a rising titer. ### Using the Wrong Sample Type Each pathogen has a preferred sample type for molecular detection. Blood is appropriate for systemic infections, urine is appropriate for leptospirosis, respiratory secretions are appropriate for respiratory pathogens, and tissue biopsies are appropriate for localized infections. Collecting the wrong sample type produces false negative results. ### Misinterpreting Serology Results Serology results are frequently misinterpreted. A positive antibody test does not prove active infection. A negative antibody test does not rule out infection if the sample was collected before seroconversion. Vaccination history must be considered when interpreting antibody titers. ### Overreliance on a Single Test No single test provides a complete picture of infection status. Combining PCR and serology can provide complementary information. For example, a positive PCR result with a negative serology result suggests acute infection before seroconversion. A negative PCR result with a positive serology result suggests past infection or vaccination. ### Ignoring Prevalence and Predictive Value The predictive value of a test result depends on the prevalence of disease in the population being tested. In low-prevalence populations, positive results are more likely to be false positives. Confirmatory testing is particularly important in low-prevalence settings. ## Welfare and Safety Context ### Zoonotic Disease Considerations Many infectious diseases of dogs and cats are zoonotic, meaning they can be transmitted to humans. Leptospirosis, brucellosis, and Q fever are examples of zoonotic bacterial diseases that can be transmitted through contact with infected animals or their body fluids. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) provides official guidance on animal health surveillance and welfare. Veterinarians should consider the zoonotic potential of suspected pathogens when selecting tests and should advise owners about appropriate precautions. A study of multiple bacterial zoonoses in febrile outpatients in Garissa County, Kenya found considerable PCR positivity for Brucella and Leptospira species in patients with non-malarial fever. The study also identified risk factors for exposure, including water source and age. These findings illustrate the importance of considering zoonotic diseases in the differential diagnosis of febrile illness in endemic areas. For veterinary patients, the same principle applies: a diagnosis of leptospirosis or brucellosis has implications for the health of the owner and other household members. ### Sample Collection Safety Sample collection from animals with suspected zoonotic infections requires appropriate precautions. Gloves should be worn when handling blood, urine, and tissue samples. Needlestick injuries should be avoided. Urine from animals with suspected leptospirosis should be handled with care because Leptospira organisms can penetrate intact skin and mucous membranes. ### Animal Welfare During Testing Diagnostic testing should be performed with attention to animal welfare. Blood collection, urine collection, and tissue biopsy are invasive procedures that can cause pain or distress. The [World Small Animal Veterinary Association](https://wsava.org/global-guidelines) provides global guidelines on companion animal welfare and clinical practice. Veterinarians should use appropriate restraint, analgesia, and anesthesia when indicated. ## Professional Escalation Criteria ### When to Refer to a Specialist Veterinarians should consider referral to a specialist in the following situations: - Clinical signs persist or worsen despite appropriate treatment based on test results - Test results are discordant with clinical findings - The patient has a suspected zoonotic disease that requires specialized management - The patient has a complex or unusual presentation that requires advanced diagnostic testing - The practice lacks the equipment or expertise to perform or interpret specific tests ### When to Contact Public Health Authorities Certain infectious diseases are reportable to public health authorities. Leptospirosis, brucellosis, and other zoonotic diseases may require reporting depending on local regulations. The [World Organisation for Animal Health](https://www.woah.org/en/what-we-do/animal-health-and-welfare) provides information on disease reporting and surveillance. Veterinarians should be familiar with local reporting requirements and should contact public health authorities when they diagnose a reportable disease. ### When to Seek Laboratory Consultation Veterinary diagnostic laboratories can provide valuable consultation on test selection and result interpretation. Laboratory professionals can advise on appropriate sample types, sample handling, and test limitations. When test results are unexpected or difficult to interpret, consultation with the laboratory is appropriate. ## Limitations of Current Testing Approaches ### Sensitivity and Specificity Tradeoffs No test has perfect sensitivity and specificity. Tests with high sensitivity may have lower specificity, producing false positives. Tests with high specificity may have lower sensitivity, producing false negatives. The veterinarian must choose the test that best balances these tradeoffs for the clinical situation. ### Regional Variation in Test Performance Test performance can vary by region because of differences in pathogen strains, disease prevalence, and laboratory practices. A test that performs well in one region may perform poorly in another. The study of severe fever with thrombocytopenia syndrome virus in wild boars in Japan found significant regional differences in ELISA positivity, illustrating that test results can vary by geographic area. Veterinarians should be aware of regional disease patterns and should interpret test results accordingly. ### Emerging Pathogens and Test Availability New pathogens are continually emerging, and tests for these pathogens may not be widely available. The study of severe fever with thrombocytopenia syndrome virus in wild boars in Japan used ELISA and quantitative reverse transcription PCR to assess infection in wildlife. For emerging pathogens, veterinarians may need to rely on specialized laboratories or reference laboratories for testing. ### Cost and Accessibility Diagnostic testing can be expensive, and cost may limit the testing options available to owners. The veterinarian should discuss the costs and benefits of different testing strategies with the owner and should recommend the most appropriate test based on the clinical situation and the owner's resources. ## Frequently Asked Questions ### What is the difference between PCR and serology for diagnosing infectious diseases in dogs and cats? PCR detects the genetic material of the pathogen itself, confirming that the organism is present in the sample at the time of collection. Serology detects antibodies produced by the host in response to infection or vaccination, documenting exposure but not proving active infection. PCR is preferred when active infection must be confirmed, while serology is preferred for assessing exposure history or immune status. ### When should I use PCR instead of serology for leptospirosis testing? PCR on blood is most useful during the acute leptospiremic phase, which occurs in the first week of infection. PCR on urine is most useful during the chronic shedding phase. Serology with the microscopic agglutination test requires paired acute and convalescent samples to demonstrate a fourfold rise in titer. PCR can provide a more rapid diagnosis in acutely ill animals, while serology is useful for documenting exposure and for epidemiological investigations. ### Why did my patient test positive on a screening test but negative on a confirmatory test? Screening tests are designed to have high sensitivity, which means they produce few false negatives but may produce false positives. Confirmatory tests are designed to have high specificity, which means they produce few false positives but may produce false negatives. A positive screening test followed by a negative confirmatory test may represent a false positive screening result, particularly in low-prevalence populations. ### Can a negative PCR result rule out infection? A negative PCR result does not completely rule out infection. The pathogen may be present in a different tissue or body fluid than the one sampled, the pathogen may be present in very low numbers, or the sample may have been collected at a time when the pathogen was not being shed. PCR results should be interpreted in the context of clinical signs and other diagnostic information. ### How does vaccination status affect serology test interpretation? Vaccination can produce antibody titers that are indistinguishable from titers produced by natural infection. A positive serology result in a vaccinated animal does not prove that the animal is infected. The veterinarian should consider the vaccination history when interpreting serology results and should use PCR or other methods when active infection must be confirmed. ### What sample types are appropriate for PCR testing? The appropriate sample type depends on the pathogen and the stage of infection. Blood is appropriate for systemic infections, urine is appropriate for leptospirosis, respiratory secretions are appropriate for respiratory pathogens, and tissue biopsies are appropriate for localized infections. The veterinarian should consult the laboratory or the test manufacturer for specific sample collection instructions. ### How should I interpret a positive antibody test in a healthy animal? A positive antibody test in a healthy animal indicates that the animal has been exposed to the pathogen or has been vaccinated. It does not prove that the animal is currently infected. The clinical significance of a positive antibody test depends on the specific disease, the vaccination history, and the risk of transmission to other animals or humans. ### When should I refer a patient to a specialist for infectious disease testing? Referral to a specialist is appropriate when clinical signs persist or worsen despite treatment, when test results are discordant with clinical findings, when the patient has a suspected zoonotic disease that requires specialized management, or when the practice lacks the equipment or expertise to perform or interpret specific tests. ## Related Veterinary Guides - [Hematologic and Biochemical Changes in Infectious Diseases](/knowledge/veterinary-medicine/clinical-pathology/hematologic-biochemical-changes-infectious) - [Serology and Molecular Diagnostics in Infectious Disease](/knowledge/veterinary-medicine/veterinary-pathology-microbiology/serology-and-molecular-diagnostics-in-infectious-disease) - [Dental Disease In Dogs And Cats](/knowledge/veterinary-medicine/dental-care/dental-disease-in-dogs-and-cats) - [Evidence-Based Veterinary Medicine: Principles and Practice](/knowledge/veterinary-medicine/veterinary-research-methods/evidence-based-veterinary-medicine-principles-practice) - [Bovine Respiratory Disease Vaccine Selection: A Comparative Guide](/knowledge/veterinary-medicine/food-animal-medicine/bovine-respiratory-disease-vaccine-selection) ## 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. - [Risk factors for acute human brucellosis in Ijara, north-eastern Kenya.](https://pubmed.ncbi.nlm.nih.gov/32236091). PLoS neglected tropical diseases, 2020. - [A review of traditional and contemporary assays for direct and indirect detection of Equid herpesvirus 1 in clinical samples.](https://pubmed.ncbi.nlm.nih.gov/26472746). Journal of veterinary diagnostic investigation : official publication of the American Association of Veterinary Laboratory Diagnosticians, Inc, 2015. - [Serological and molecular assessment of severe fever with thrombocytopenia syndrome virus infection in wild boars from two adjacent endemic prefectures in Japan.](https://pubmed.ncbi.nlm.nih.gov/42178278). The Journal of veterinary medical science, 2026. - [Leptospira infection and carrier survey on rats from wet market areas in Kuala Lumpur, Malaysia.](https://pubmed.ncbi.nlm.nih.gov/38648405). Journal of vector borne diseases, 2024. - [Molecular and serological diagnosis of multiple bacterial zoonoses in febrile outpatients in Garissa County, north-eastern Kenya.](https://pubmed.ncbi.nlm.nih.gov/38806576). Scientific reports, 2024. > This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.