What Tests Are Commonly Used to Diagnose FIP

Feline infectious peritonitis (FIP) remains one of the most challenging and perplexing diseases for cat owners and veterinarians alike. This disease, which arises from a mutation of feline coronavirus, is almost always fatal if left untreated, often confounding even the most experienced clinicians. Accurate diagnosis is essential for managing FIP and making informed decisions regarding care. Over the last few decades, extensive research has been dedicated to refining and validating diagnostic tools. Below, you’ll find a comprehensive overview of the commonly used tests to diagnose FIP in cats, covering clinical signs observation, blood work, serology, PCR, imaging, cytology, and advanced laboratory testing—organized to inform both pet owners and veterinary professionals.

Clinical Evaluation and History

Diagnosis typically begins with a detailed clinical history and a physical examination. FIP primarily affects young cats or those living in multi-cat environments. Symptoms vary depending on whether the cat has the “wet” (effusive) or “dry” (non-effusive) form. Signs can include persistent fever, lethargy, weight loss, abdominal swelling, eye inflammation (uveitis), neurological symptoms, and difficulty breathing. A veterinarian’s initial suspicion is crucial, but alone it is not enough; laboratory and imaging tests are essential for confirmation.

Complete Blood Count (CBC) and Biochemistry Profile

Routine blood work forms the foundation of FIP diagnostics. Cats with FIP frequently exhibit a combination of nonspecific laboratory abnormalities:

Lymphopenia: A reduction in lymphocytes is common.

Neutrophilia: Increased neutrophils may suggest inflammation.

Nonregenerative anemia: A mild to moderate decrease in red blood cells.

Hyperglobulinemia: Elevated globulin levels, especially gamma globulins, are present in most FIP cases.

Low albumin-to-globulin (A/G) ratio: An A/G ratio below 0.8 is highly suggestive, although not exclusive, to FIP.

Elevated liver enzymes: Mild increases may reflect disease extension.

Although none of these findings are specific to FIP, their combination with suggestive clinical signs supports the diagnosis.

Effusion Analysis

In the “wet” form, FIP causes fluid accumulation in body cavities, namely the abdomen (ascites) or chest (pleural effusion). Sampling and analyzing this fluid provides important clues:

Physical characteristics: Effusions associated with FIP are frequently straw to yellow in color, viscous, and clear to slightly cloudy.

Protein concentration: FIP effusion typically reveals high total protein (>3.5 g/dL).

Rivalta test: A simple bedside technique, the Rivalta test, can rapidly suggest FIP. A drop of effusion fluid is added to acetic acid solution; cloud formation indicates a positive result.

Cytology of the effusion typically shows few to moderate numbers of cells (mainly neutrophils, macrophages, and lymphocytes) in a protein-rich background.

Serology for Feline Coronavirus Antibodies

Blood tests are available to measure antibody levels against feline coronavirus (FCoV). However, a positive result indicates exposure, not necessarily FIP:

High titers: Increased antibody levels may support the diagnosis, but many cats carry FCoV with no symptoms.

Interpretation limitations: Serology cannot distinguish between benign enteric coronavirus infection and FIP.

Veterinarians often use serology in conjunction with other findings, rather than as a definitive test.

Polymerase Chain Reaction (PCR) Testing

PCR testing identifies feline coronavirus genetic material in body fluids or tissues—one of the more advanced and sensitive diagnostic tools available.

Sample types: PCR can be performed on blood, effusion fluid, tissue biopsies, or cerebrospinal fluid.

Detection of FIP-associated mutations: Modern PCR assays can identify specific genetic mutations (such as those affecting the spike protein) strongly associated with FIP.

Limitations: FCoV presence does not guarantee disease; some healthy cats may test positive. PCR sensitivity and specificity vary depending on sample type, laboratory, and viral load.

When combined with clinical and laboratory findings, PCR increases diagnostic certainty, especially in atypical or dry forms of FIP.

Immunohistochemistry (IHC) and Tissue Biopsy

Obtaining tissue for histology and immunohistochemistry is considered the gold standard for FIP diagnosis. This involves microscopic examination of affected tissues, with staining that highlights coronavirus antigens inside macrophages.

Biopsy sites: Common tissues sampled include affected organs such as omentum, lymph nodes, liver, or brain in neurological cases.

IHC staining: Demonstrates viral antigens in characteristic lesions of FIP.

Diagnostic specificity: A positive result virtually confirms FIP; however, obtaining tissue samples may be invasive and sometimes impractical in very ill patients.

Imaging: Ultrasound and Radiography

Imaging plays a supportive role, mainly in identifying effusions and organ involvement.

Ultrasound: Useful for detecting fluid accumulation, organ enlargement, and granulomatous lesions in abdominal organs.

Radiographs: May reveal chest or abdominal effusions, but are less sensitive than ultrasound for fine detail.

Imaging is not diagnostic on its own, but helps guide further testing and identify areas for tissue sampling.

Advanced Laboratory Tests: Alpha-1-Acid Glycoprotein

Some advanced tests, such as measurement of alpha-1-acid glycoprotein (AGP), can help differentiate FIP from other causes of illness. AGP is an acute-phase protein, meaning it increases in response to inflammation, infection, or malignancy.

Serum AGP levels: Elevated levels are frequently seen in FIP, above those of most other diseases.

Diagnostic utility: While not specific, high AGP may add supporting evidence in challenging cases.

Differential Diagnosis: Rule-Out Strategies

Because FIP has signs that overlap with other conditions, such as lymphoma, bacterial infections, heart disease, and other types of viral infections, diagnostic protocols commonly rule out these alternatives.

Testing for blood parasites: Especially if anemia or fever is prominent.

FeLV/FIV screening: Cats may have coexisting immunosuppressive viral infections.

Bacterial cultures: Especially for pyothorax or other infectious effusions.

A holistic approach, combining multiple modalities, is crucial for accurate diagnosis and management.

Diagnosing Dry (Non-Effusive) FIP

Diagnosing the non-effusive “dry” form is particularly difficult, as these cats usually lack the classic abdominal or chest fluid. Here, imaging, PCR, AGP, and biopsy play elevated roles.

Organ-specific presentations: Ocular FIP may be diagnosed based on uveitis and confirmatory PCR of aqueous humor. Neurological FIP often requires cerebrospinal fluid analysis with PCR and imaging.

Biopsy: Surgical tissue sampling for histology and IHC may be required.

Emerging Diagnostic Advances

The landscape of FIP diagnosis is rapidly evolving. Molecular techniques are becoming more sensitive and specific, with ongoing research into new biomarkers and point-of-care tests.

Genotyping of coronavirus spikes: A promising method to distinguish FIP-causing strains.

Automated PCR platforms: Enable quicker results from effusion, blood, or tissue samples.

Veterinary laboratories continue to develop newer protocols to improve accuracy and reduce turnaround times.

Prognostic Value of Diagnostic Testing

Not all tests are equally predictive of disease outcome. Combining multiple findings—history, effusion analysis, serology, PCR results, and biopsy—connects clinical suspicion to a reasonably certain diagnosis.

A/G ratio and globulin levels: Often associated with more advanced disease.

PCR and AGP: Higher levels tend to correlate with active, severe FIP.

Veterinarians rely on stepwise testing, adjusting the protocol depending on initial results and the cat’s stability.

Sampling and Laboratory Best Practices

Reliable diagnosis depends on proper sample collection, storage, and handling.

Fresh samples: Effusion and blood samples should be sent promptly to avoid degradation.

Tissue biopsies: Require precise identification of affected areas, ideally guided by imaging.

Laboratory communication: Detailed histories and clinical observations should accompany all samples.

Pet owners are encouraged to work with veterinary teams experienced in infectious disease management.

Understanding False Positives and Negatives

It is important to be aware that none of the tests are perfect. False positives and negatives occur due to sample quality, timing, or technical limitations.

PCR limitations: May not detect low-viral-load samples, especially in dry FIP.

Serology: May yield positives in healthy carriers.

Biopsy: Sampling error may miss focal lesions.

A comprehensive approach that uses all available modalities maximizes diagnostic accuracy.

Future Directions in FIP Diagnosis and Care

With new therapies for FIP emerging, quick and accurate diagnosis is more important than ever. Research is focused on developing better point-of-care test kits, next-generation sequencers for coronavirus mutations, and novel biomarkers.

Digital imaging and artificial intelligence: May assist veterinarians in identifying characteristic changes in tissue and effusion fluid.

Multiplex PCR: To distinguish FIP-causing strains from benign coronaviruses.

Continued investment in research and clinical trials promises improved diagnostic options.

Optimal Testing Strategy in Suspected Cases

Veterinarians typically develop a tailored testing protocol. For effusive cases, fluid analysis combined with PCR is often recommended. In dry cases, advanced imaging, AGP, and biopsies may be prioritized. Decisions depend on the cat’s symptoms, stability, laboratory accessibility, and owner resources.

Conclusion

Accurate diagnosis of feline infectious peritonitis relies on a holistic, stepwise approach. Veterinarians integrate physical examination, laboratory tests, imaging, and molecular diagnostics to reach a high level of diagnostic certainty. As technology and research progress, further advances will provide hope for earlier detection and improved outcomes for affected cats.

References

1. Dr. Niels C. Pedersen. “Feline Infectious Peritonitis: An Update.” Veterinary Clinics of North America: Small Animal Practice (2020).

2. Hartmann, K., Binder, C., et al. “Diagnosis of feline infectious peritonitis: a review.” The Journal of Feline Medicine and Surgery (2003).

3. Tasker, S. “Diagnosis of feline infectious peritonitis: Update on evidence supporting laboratory testing.” The Journal of Feline Medicine and Surgery (2018).

4. Kipar, A., Meli, M.L. “Feline Infectious Peritonitis: A Review of Pathogenesis and Immunity.” The Journal of Feline Medicine and Surgery (2014).

5. Addie, D.D., et al. “Coronavirus infections in cats: Requirements for increased awareness and contemporary diagnostic tools.” The Veterinary Journal (2012).

6. Hirschberger, J., Hartmann, K. “Feline infectious peritonitis: An update.” The Veterinary Clinics of North America: Small Animal Practice (2020).

7. Dewerchin, H., et al. “Feline infectious peritonitis: diagnostic challenges and recent advances.” Compendium: Continuing Education for Veterinarians (2014).

8. Felten, S., Hartmann, K. “Diagnosis of feline infectious peritonitis: Pathogenesis, clinical signs, and recent diagnostic advances.” Journal of Small Animal Practice (2019).