What Blood Test Results May Suggest FIP in Cats

Feline Infectious Peritonitis (FIP) remains one of the most challenging diseases in feline medicine. Its clinical presentation is variable, and definitive diagnosis is difficult because there is no single blood test that confirms the disease. Nevertheless, certain blood test results, when interpreted alongside clinical signs and history, can raise suspicion and aid veterinarians in diagnosing FIP. This article reviews the blood chemistry, hematology, and special diagnostic tests that suggest the presence of FIP in cats, providing a comprehensive overview for cat owners and veterinary professionals.

Feline Infectious Peritonitis (FIP) is caused by a mutation of feline coronavirus, resulting in an aggressive disease that can be fatal. The varied symptoms, ranging from effusive (wet form) with fluid accumulation to non-effusive (dry form) with granulomatous lesions, make diagnosis challenging. For US veterinarians and cat owners, understanding which blood test results may suggest FIP is crucial for early detection and effective management.

Hematology Findings

Certain changes in the complete blood count (CBC) can prompt suspicion of FIP. These changes, although not pathognomonic, are commonly seen in affected cats:

Lymphopenia: A decreased lymphocyte count is found in many FIP cases. This depression of the immune system occurs due to the overwhelming infection and inflammation.

Neutrophilia: Elevated neutrophil counts, often with a left shift (band neutrophils), indicate inflammation or secondary infection.

Non-regenerative anemia: Mild to moderate anemia, characterized by a decrease in red blood cell count and hematocrit, is common. The anemia is often non-regenerative, meaning the bone marrow is not producing new red cells.

Monocytosis: Increased monocytes may be seen due to chronic inflammation.

Leukocytosis: Cats with FIP frequently exhibit an overall increase in white blood cells.

Although these findings are non-specific and seen in other diseases, the combination of lymphopenia and neutrophilia is particularly suggestive.

Serum Chemistry Abnormalities

Blood biochemistry panels provide further clues that may indicate FIP, especially when interpreted with clinical findings:

Hyperglobulinemia: One of the hallmark findings is a significantly increased globulin level, often above 4 g/dL. Hyperglobulinemia is primarily due to increased gamma globulins resulting from antigenic stimulation.

Low Albumin/Globulin Ratio: A low A/G ratio (<0.8) is a strong indicator. FIP tends to decrease albumin production while markedly increasing globulins.

Mild to Moderate Elevated Liver Enzymes: Increased ALT, AST, and ALP may be observed, especially if granulomatous lesions affect the liver or if there is immune-mediated damage.

Azotemia: Increased blood urea nitrogen (BUN) and creatinine can occur in late-stage disease, implicating renal involvement.

Protein Electrophoresis

One of the more useful diagnostic aids, protein electrophoresis, helps delineate globulin classes:

Polyclonal Gammopathy: The pattern most commonly seen in FIP, reflecting broad-spectrum immune stimulation, can help differentiate FIP from monoclonal gammopathies seen in lymphoproliferative neoplasia (e.g., lymphoma or myeloma).

Disproportionate increase in beta or gamma globulins: This supports chronic inflammatory or infectious processes, with FIP being a prime suspect.

Effusion Analysis (for Wet Form)

When a cat presents with abdominal or thoracic effusion, analysis of this fluid is paramount:

Straw-colored, viscous fluid: Typical of FIP, containing high protein levels (>3.5 g/dL), especially globulins.

High Rivalta test sensitivity: Rivalta test is a simple bedside test used to distinguish FIP-related effusions from others; a positive test result adds weight to a presumptive diagnosis.

Low cellularity, primarily mononuclear cells and neutrophils: This cytologic finding, combined with high-protein levels, supports FIP.

Acute Phase Proteins

Certain inflammatory markers are often elevated in FIP, reflecting systemic inflammatory response:

Alpha-1-acid glycoprotein (AGP): This acute phase protein is markedly increased in FIP, sometimes more reliable than CBC or chemistry panel changes.

Serology and PCR

While blood antibody titers for feline coronavirus are not definitive, some supportive findings occur:

High feline coronavirus antibody titers: High titers are consistent with exposure but not diagnostic, as many healthy cats carry antibodies.

PCR detection of FIP-specific mutations: Advanced PCR can sometimes detect FIP mutations in blood, though sensitivity is low—more reliable when performed on tissues or effusion fluid.

Interpreting Test Results in Clinical Context

No single laboratory finding is diagnostic for FIP. The disease is best diagnosed by combining history, clinical signs, physical examination, and an array of hematological and biochemical results:

Young or purebred cats, especially those from multi-cat environments, are at higher risk.

Classic FIP findings: Chronic fever unresponsive to antibiotics, weight loss, poor coat, and abdominal/thoracic effusion.

Supportive bloodwork: Combination of lymphopenia, neutrophilia, hyperglobulinemia, low A/G ratio, and positive Rivalta test in the context of compatible clinical signs is highly suggestive.

Differential Diagnosis

Similar laboratory findings may be seen in other chronic inflammatory or neoplastic diseases, including lymphoma, other infectious peritonitis, and immune-mediated disorders. Therefore, a presumptive diagnosis of FIP should be supported by a synthesis of all available data.

Case Example

Consider a 3-year-old Maine Coon presenting with lethargy, anorexia, intermittent fever, and abdominal distension. CBC reveals lymphopenia, neutrophilia, and mild anemia. Serum chemistry shows marked hyperglobulinemia (5.2 g/dL), hypoalbuminemia, resulting in an A/G ratio of 0.5. Effusion analysis detects straw-colored, viscous fluid with high protein and low cellularity. Rivalta test is positive. These findings, in the context of compatible clinical signs, are strongly suggestive of FIP.

Limitations of Blood Tests

Blood tests indicate a suspicion but not confirmation. Definitive diagnosis requires histopathological examination with immunohistochemical staining or advanced PCR targeting coronavirus mutations within affected tissues. Non-specific laboratory findings mean clinicians must be cautious.

Conclusion

Upon review of hematological and biochemical indicators, cats suspected of FIP typically show a constellation of blood test abnormalities: lymphopenia, neutrophilia, non-regenerative anemia, hyperglobulinemia, decreased albumin/globulin ratio, and positive effusion tests. These findings, woven into the broader clinical narrative, allow veterinarians to make informed presumptive diagnoses and advise cat owners on prognosis and care options.

References

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Hartmann, K. (2005). Feline infectious peritonitis: ABCD guidelines on prevention and management.

Stranieri, A., Lauzi, S., Paltrinieri, S., & Giordano, A. (2018). Laboratory diagnosis of feline infectious peritonitis.

Tasker, S. (2018). Feline infectious peritonitis: The role of laboratory diagnosis.

Shelly, S. M., & Smith, G. K. (2006). Diagnostic features of effusions associated with feline infectious peritonitis.

Gunn-Moore, D. A. (2020). Review of laboratory findings in feline infectious peritonitis.

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