Rivalta Test for Feline FIP: Accuracy, Interpretation, and Clinical Value

When a cat presents with abdominal or chest fluid, the first clinical question is not “is this FIP?” — it is “what kind of fluid is this?” The answer determines the urgency of the situation, the diagnostic path forward, and whether FIP treatment should begin immediately.

The Rivalta test is one of the fastest and most accessible tools for answering that question. It can be performed chairside in a veterinary clinic in under five minutes, requires only distilled water and acetic acid, and provides a result that is clinically meaningful — particularly when negative. Understanding what the Rivalta test actually measures, what its results mean, and where it fits in the full FIP diagnostic workup is essential for any owner navigating a suspected wet FIP diagnosis.

What the Rivalta Test Measures

The Rivalta test is a precipitation test that assesses the protein and fibronectin content of effusion fluid. It was first described by the Italian physician Fabio Rivalta in 1895 and has been validated for feline FIP diagnosis in modern veterinary literature, most notably by Hartmann et al. (2003, J Feline Med Surg).

FIP effusion fluid is a high-protein, high-fibronectin exudate — produced by inflamed, leaking blood vessels rather than by normal fluid dynamics. When a drop of high-protein exudate is added to dilute acetic acid solution, the proteins precipitate and form a visible cloud or strand that remains intact as it sinks. Low-protein transudates — the kind produced by heart disease, liver disease, or hypoalbuminaemia — dissolve immediately and leave the solution clear.

How the Test Is Performed

The Rivalta test requires a clear tube, distilled water, glacial acetic acid, and a sample of the cat’s effusion fluid collected by your veterinarian via abdominocentesis or thoracocentesis.

  1. Fill the tube with distilled water and add one to two drops of glacial acetic acid. Mix gently.
  2. Hold the tube up to a light source.
  3. Add one drop of the effusion fluid to the surface of the solution and observe.

Interpreting the result:

  • Positive: The drop remains intact, forms a visible white cloud or jellyfish-like strand, and sinks slowly while retaining its shape. This indicates a high-protein exudate consistent with FIP.
  • Negative: The drop dissolves immediately, leaving the liquid clear. This indicates a low-protein transudate, making FIP unlikely.
  • Equivocal: The drop partially dissolves but leaves some turbidity. Additional testing is required.

Accuracy: What the Numbers Actually Mean

The Hartmann et al. 2003 validation study reported:

  • Sensitivity: 91% — correctly identifies 91% of cats that actually have FIP as positive
  • Specificity: 66% — correctly identifies 66% of cats that do not have FIP as negative
  • Positive predictive value: ~86% — of cats with a positive result, approximately 86% actually have FIP
  • Negative predictive value: ~96% — of cats with a negative result, approximately 96% do not have FIP

A negative Rivalta test is highly informative. A 96% negative predictive value means FIP is very unlikely if the test is negative. A positive Rivalta test is less definitive — approximately 14% of positive results occur in cats that do not have FIP. Additional tests are needed to confirm before starting treatment.

What Causes a False Positive Rivalta Test

Conditions most likely to produce a false positive — a positive result in a cat that does not have FIP:

  • Bacterial peritonitis or pleuritis — septic effusion from a ruptured viscus or bite wound
  • Lymphoma — particularly mediastinal lymphoma causing pleural effusion
  • Chylothorax — chyle in the chest cavity, which is high in protein and fat
  • Haemorrhagic effusion — blood in the body cavity from trauma or a bleeding mass

Read our guide on conditions commonly confused with FIP for a full differential diagnosis discussion.

The Rivalta Test in the Full FIP Diagnostic Workup

The Rivalta test is a first-line screening tool, not a standalone diagnostic. The complete diagnostic workup for suspected wet FIP:

  1. Rivalta test on effusion fluid. First-line screening. A negative result makes FIP very unlikely. A positive result triggers the next steps.
  2. Albumin-to-globulin (A:G) ratio on blood work. Below 0.6 is highly suggestive; below 0.4 is strongly indicative. Ask specifically for this — it is not always included in a standard blood panel.
  3. Fluid protein analysis. FIP effusion typically has a total protein above 35 g/L with a high globulin fraction.
  4. Fluid cytology. FIP effusion is typically non-septic with a mixed cell population including macrophages and neutrophils.
  5. FCoV PCR on effusion fluid. A positive PCR on effusion fluid in a cat with a positive Rivalta test and low A:G ratio is strong confirmation of FIP.
  6. Ultrasound. Confirms fluid location and volume, assesses organ involvement, and can identify granulomatous lesions characteristic of dry FIP.

For a full explanation of all four diagnostic tests, read our guide to the 4 FIP diagnostic tests.

Starting Treatment After a Positive Rivalta Test

In a cat with a positive Rivalta test, a low A:G ratio, and clinical signs consistent with wet FIP, treatment with GS-441524 should begin as soon as possible — ideally within 24 to 48 hours of diagnosis. Waiting for PCR confirmation in a deteriorating cat costs days that cannot be recovered.

The correct dose for wet FIP is 6 mg/kg of injectable GS-441524 daily for 84 days. Use our dosage calculator to get the exact amount. Injectable administration is strongly preferred over oral capsules, particularly at the start of treatment, because cats with active wet FIP are frequently nauseous and have unpredictable oral absorption. Read our injectable vs oral guide.

Frequently Asked Questions

Can I perform the Rivalta test at home?
The Rivalta test requires effusion fluid collected by a veterinarian via abdominocentesis or thoracocentesis — a procedure that cannot be performed at home. If your cat has a visibly swollen abdomen or breathing difficulty, see a vet the same day.

My cat’s Rivalta test was positive. Does that confirm FIP?
A positive Rivalta test is strong evidence for FIP but not confirmation. Additional tests — A:G ratio, fluid protein analysis, cytology, and ideally FCoV PCR — are needed to confirm the diagnosis before starting treatment. Contact us for guidance on next steps.

My cat’s Rivalta test was negative. Can I rule out FIP?
A negative Rivalta test makes FIP very unlikely — the negative predictive value is approximately 96%. However, it does not completely rule out FIP, particularly in cats with dry FIP (which does not produce effusion). If clinical signs remain concerning, blood work including the A:G ratio should be performed.

How quickly does abdominal fluid resolve with GS-441524 treatment?
Most cats with wet FIP show visible reduction in abdominal fluid within 1 to 2 weeks of starting GS-441524 at the correct dose. Complete resolution typically occurs within 3 to 4 weeks. Fluid that is not reducing after 2 weeks warrants a dose review. Use our dosage calculator to verify.

The Rivalta test is one of the most useful tools in the FIP diagnostic workup — fast, inexpensive, and highly informative, particularly when negative. A positive result in a cat with a low A:G ratio and clinical signs consistent with wet FIP is sufficient to begin treatment while confirmatory tests are pending. Do not wait for every test result if the cat is deteriorating.

The FIP Cure Plus team is available in English and Arabic at info@fipcureplus.com.

View the FIP Cure Plus GS-441524 20 mg/ml injectable →

View the FIP Cure Plus GS-441524 30 mg/ml injectable →

FIP Cure Plus — transparent, evidence-based GS-441524 treatment for cats across the Gulf.

Back to blog