How to Diagnose FIP in Cats

FIP is notoriously difficult to diagnose definitively without a biopsy, but a combination of clinical signs, bloodwork, and fluid analysis allows for a high-confidence presumptive diagnosis — which is sufficient to begin treatment.

Clinical Signs to Watch For

Wet FIP Signs

  • Distended, fluid-filled abdomen
  • Difficulty breathing (fluid around lungs)
  • Lethargy and weakness
  • Persistent fever unresponsive to antibiotics
  • Weight loss and muscle wasting
  • Jaundice (yellowing of gums/eyes) in some cases

Dry FIP Signs

  • Chronic weight loss
  • Intermittent fever
  • Enlarged lymph nodes
  • Vomiting or diarrhea
  • Eye changes (cloudiness, uveitis)
  • Neurological symptoms in later stages

Ocular Signs

  • Cloudiness or color change in the iris
  • Visible blood or fibrin in the eye
  • Sensitivity to light

Neurological Signs

  • Loss of coordination (ataxia)
  • Nystagmus (rapid eye movement)
  • Seizures
  • Behavioral changes
  • Hind limb weakness or paralysis

Key Diagnostic Tests

1. Complete Blood Count (CBC) & Chemistry Panel

Typical FIP pattern: - Elevated globulins (hypergammaglobulinemia) - Low albumin (hypoalbuminemia) - Low A:G ratio (below 0.4 is highly suspicious) - Elevated bilirubin, liver enzymes - Lymphopenia (low lymphocytes) - Anemia

2. Albumin:Globulin (A:G) Ratio

A:G Ratio Interpretation
> 0.8 FIP unlikely
0.5 – 0.8 Indeterminate
< 0.5 FIP possible
< 0.4 FIP highly likely

3. The Rivalta Test (For Wet FIP)

A cheap, reliable in-clinic test for effusive FIP.

How it works: A drop of the abdominal or chest fluid is added to diluted acetic acid in water. In FIP, the high protein/fibrin content causes the drop to maintain its shape and float (positive result). In transudates (low protein), the drop disperses (negative).

Sensitivity: ~91% | Specificity: ~66%

A positive Rivalta, combined with clinical signs and bloodwork, provides strong diagnostic confidence.

4. FCoV Antibody Titer

Measures antibodies against Feline Coronavirus. High titers in a sick cat with other FIP signs support the diagnosis, but many healthy cats also have antibodies. Negative titer makes FIP less likely but doesn't rule it out.

5. PCR Testing

RT-PCR on effusion fluid for FCoV RNA can support diagnosis. Available through several veterinary diagnostic labs (IDEXX, Antech).

6. Protein Electrophoresis (SPEP)

Shows the characteristic globulin spike seen in FIP. Useful when bloodwork is borderline.

Diagnostic Confidence Scoring

Most FIP specialists use a combination of factors to reach >80% diagnostic confidence before starting treatment:

  • Signalment (young cat, breed, multi-cat household)
  • Clinical signs (fever, fluid, weight loss)
  • CBC/chemistry (A:G < 0.4)
  • Rivalta positive (if wet)
  • Response to treatment (often considered diagnostic if cat improves rapidly)

When to Start Treatment Without a Biopsy

Given the cost and invasiveness of definitive biopsy, and the rapid progression of FIP, most veterinarians and the FIP treatment community agree that starting treatment based on high clinical suspicion is appropriate — especially since response to GS-441524 is itself diagnostic.

A cat with FIP that begins antiviral treatment typically shows improvement within 48–72 hours. Failure to respond may indicate a different diagnosis.