FIP Diagnosis
Diagnosing FIP can be complex. No single test is 100% definitive. Here we explain what tests exist, what they mean, and how vets reach a diagnosis to begin treatment.
Why is FIP diagnosis difficult?
FIP is caused by a mutated form of feline coronavirus (FCoV). The problem is that ordinary feline coronavirus is very common — many cats carry it without developing FIP. There is currently no commercially available test that can distinguish the benign coronavirus from the FIP-causing mutant directly from blood.
Diagnosis therefore relies on combining multiple pieces of evidence: clinical signs, blood test results, fluid analysis (if present), imaging, and increasingly PCR tests. A skilled vet with FIP experience can reach a reliable clinical diagnosis in most cases.
Diagnostic tests explained
Complete blood count (CBC)
SupportiveOften shows non-regenerative anemia, lymphopenia, and neutrophilia. Non-specific but part of the diagnostic picture.
Biochemistry panel
SupportiveElevated total protein (>7.8 g/dL), elevated globulins, low albumin, elevated bilirubin, and elevated liver enzymes are common but non-specific.
Albumin:Globulin (A:G) ratio
UsefulA ratio below 0.4 is highly suspicious for FIP. Above 0.8 makes FIP unlikely. Between 0.4–0.8 requires additional tests.
Coronavirus antibody titer
LimitedPositive result only means exposure to FCoV — does NOT confirm FIP. A negative result makes FIP less likely but does not exclude it. This test is often misinterpreted.
Rivalta test (on effusion)
Useful for wet FIPSimple, inexpensive test on effusion fluid. Positive (drop stays suspended) indicates high protein consistent with FIP. ~86% specificity when combined with cytology.
Fluid protein analysis + cytology
High for wet FIPStraw-colored, viscous fluid with high protein (>35 g/L), high globulin fraction, and low cell count (predominantly neutrophils and macrophages) is highly characteristic of FIP.
PCR on fluids or tissue
High specificityPCR detecting FCoV RNA in effusion, CSF, or tissue. High specificity but sensitivity varies. Positive PCR in effusion or aqueous humor strongly supports diagnosis.
Histopathology + IHC (biopsy)
Gold standardOnly definitive test. Tissue biopsy showing pyogranulomatous inflammation with positive FCoV immunohistochemistry confirms FIP 100%. Invasive and not always feasible in advanced cases.
FAQ
Questions about FIP diagnosis
Clear answers to the most common questions.
A 100% definitive diagnosis can only be made by histopathology (biopsy with immunohistochemistry). However, the combination of clinical signs, fluid analysis (Rivalta test + protein ratio + cytology) and PCR allows a high-probability clinical diagnosis that supports starting treatment.
A positive FCoV antibody test only indicates the cat has been exposed to feline coronavirus. It does NOT confirm FIP. Most cats with positive coronavirus titers never develop FIP. This test alone is insufficient for diagnosis.
Yes. Dry FIP is harder to diagnose because there is no fluid to analyze. Diagnosis relies on combination of clinical signs, elevated total protein, high globulin fraction, low A:G ratio, imaging findings, and PCR from biopsy or aqueous humor if ocular form is present.
The Rivalta test is a simple test performed on effusion fluid. A drop of fluid is placed in water with acetic acid. If the drop stays suspended (positive), it indicates high protein content consistent with FIP effusion. It is inexpensive and rapid, with approximately 86% specificity.
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