Feline Infectious Peritonitis (FIP) affects cats globally. There is no single, straightforward blood test that can definitively confirm FIP. Diagnosis requires a multi-step, investigative process combining a cat’s history, clinical signs, and results from multiple laboratory and imaging studies. This comprehensive approach is necessary because FIP is a complex disease with varied presentations that often mimic other common feline illnesses.
The Fundamental Challenge of FIP Diagnosis
The difficulty in diagnosis stems from the Feline Coronavirus (FCoV), the virus responsible for FIP. FCoV is extremely common, infecting up to 90% of cats in multi-cat environments, but usually causes only mild or no clinical signs, such as transient diarrhea. FIP develops only when the low-virulence FCoV mutates spontaneously inside an individual cat, transforming into the highly virulent FIP virus (FIPV). This internal, non-contagious mutation prevents direct testing for a contagious FIPV strain that spreads between cats.
The resulting systemic inflammatory disease produces varied clinical signs that overlap with many other conditions. Cats with FIP frequently show vague symptoms like a fever unresponsive to antibiotics, persistent lethargy, loss of appetite, and weight loss. The disease manifests in two primary forms: the “wet” or effusive form, characterized by fluid accumulation in the abdomen or chest, and the “dry” or non-effusive form, which involves inflammatory lesions in organs like the kidneys, liver, brain, or eyes. Diagnosis cannot rely on symptoms alone because these signs are common to many feline diseases.
Screening Tests for Feline Coronavirus Exposure
Veterinarians often begin the diagnostic workup by confirming exposure to the common FCoV, but these screening tests have significant limitations for confirming FIP. The FCoV Antibody Titer measures the level of antibodies produced against the virus. A positive titer indicates past exposure to FCoV, but it cannot differentiate between the common, benign FCoV and the mutated FIPV. Since FCoV is widespread, a high titer often reflects a healthy cat’s immune response to the common virus, not the presence of FIP.
The Polymerase Chain Reaction (PCR) detects the viral genetic material (RNA) in a sample. Interpretation depends heavily on the sample source. Fecal PCR detects the common enteric FCoV, which is prevalent in healthy cats, making a positive result meaningless for FIP diagnosis. A positive PCR result from blood is more concerning, suggesting the virus has become systemic, a prerequisite for FIP.
The most informative application of PCR is on effusions or tissue samples, where a positive result for FCoV RNA strongly supports FIP. Some specialized laboratories offer PCR tests attempting to detect a specific genetic sequence associated with the FIP-causing mutation, such as in the spike (S) gene. However, these tests remain controversial because a consistent, single FIP-specific mutation has not been confirmed. Viral screening tests are best utilized to establish FCoV infection, which must then be combined with other clinical evidence.
Clinical Indicators and Supportive Diagnostic Evidence
Since viral tests alone are ambiguous, suspicion of FIP relies on analyzing the cat’s general health markers through routine bloodwork. A common finding is a low Albumin-to-Globulin (A:G) Ratio, calculated by dividing albumin by globulin levels in the blood. Albumin is often low in FIP cases, while globulins (including antibodies) are typically elevated due to systemic inflammation. A ratio of less than 0.8 is highly suggestive of FIP, though it is not a perfect indicator.
For the “wet” form of the disease, fluid analysis from the chest or abdomen provides supportive evidence. The Rivalta Test is a simple, quick test performed on the collected effusion fluid. If a drop of fluid dissipates when added to distilled water and acetic acid, the test is negative, suggesting a non-FIP cause. If the drop holds its shape, it indicates a high concentration of protein and inflammatory substances typical of FIP effusions.
The Rivalta test has a very high negative predictive value, meaning a negative result reliably rules out FIP as the cause of the effusion. However, a positive result only indicates high-protein fluid and is not unique to FIP. Diagnostic Imaging, such as ultrasound or radiographs, also helps by revealing fluid presence or specific organ changes like enlarged lymph nodes, liver, or kidneys.
Achieving Definitive Diagnosis
The most confident diagnosis of FIP requires a “constellation of findings,” where multiple pieces of evidence consistently point toward the disease. This includes consistent clinical signs, suggestive bloodwork abnormalities like a low A:G ratio, and positive results from supportive tests like the Rivalta test or systemic FCoV RNA detection. When a definitive, ante-mortem (before death) diagnosis is required, the gold standard involves testing a tissue sample for the presence of the FCoV antigen within infected immune cells.
This procedure is called Immunohistochemistry (IHC) or Immunocytochemistry (ICC). It involves obtaining a biopsy of an affected organ or an aspirate of the effusion fluid, then using specialized antibodies to stain for the FCoV antigen. The defining feature of FIP is the virus’s ability to replicate within monocytes and macrophages. If the stain reveals FCoV antigen specifically within these immune cells in the tissue or fluid, the diagnosis of FIP is confirmed with very high specificity.
Newer molecular techniques are improving ante-mortem certainty by focusing on the unique aspects of the FIPV infection. These include advanced Reverse Transcriptase-PCR (RT-PCR) assays that target subgenomic messenger RNA (mRNA), which is only produced when the virus is actively replicating. Detecting this replicating viral RNA in peripheral blood mononuclear cells is a highly specific indicator of FIP. Techniques like RNA In Situ Hybridization (RNA ISH) allow for the visualization of viral RNA directly within tissue sections, offering a highly sensitive and specific method.