The Four FIP Diagnostic Tests Every Cat Owner Must Know

There is no single test that can conclusively confirm Feline Infectious Peritonitis (FIP) on its own. What experienced veterinarians do instead is combine multiple diagnostic tools — alongside clinical signs and their own trained assessment — to reach a confident diagnosis.

Understanding these four tests helps you ask the right questions at your vet appointment, interpret the results you receive, and make faster, more informed decisions about starting GS-441524 treatment. Time matters in FIP — read our guide on why delaying FIP treatment is fatal to understand why early action is the single biggest factor in survival.

Test 1: Blood Test (CBC and Biochemistry Profile)

The blood test is the most comprehensive and most commonly used tool for assessing the likelihood of FIP. It covers two panels:

Complete Blood Count (CBC)

  • White blood cells (WBC): May be low, normal, or elevated depending on how long FIP has been present and whether other conditions exist simultaneously.
  • Red blood cells (RBC): Often low in FIP cats, indicating anaemia — a common finding in active FIP infection.
  • Lymphocytes: Low lymphocyte count is a classic FIP pattern and one of the earliest blood markers to appear.

Biochemistry Profile

  • Total Protein (TP) and Globulins: Elevated total protein and globulins (hyperglobulinaemia) are among the most consistent findings in FIP-positive cats. This increase is caused by the inflammatory process as the cat’s immune system responds to the virus.
  • Albumin-to-Globulin (A:G) ratio: This is the single most important marker for FIP diagnosis. A ratio below 0.6 is highly suggestive of FIP; below 0.4 is strongly indicative. Vets typically look at the A:G ratio as the first sign of potential FIP infection.
  • ALT and bilirubin: Elevated levels indicate liver involvement, common in FIP.
  • BUN and creatinine: Elevated kidney values in a cat suspected of FIP may indicate FIP-related kidney disease, concurrent kidney disease, or dehydration caused by FIP — context and clinical signs are needed to interpret these correctly.

Blood test results must always be interpreted alongside your cat’s clinical symptoms and history. A single abnormal value is rarely the full picture. For a complete explanation of every blood marker and what it means during GS-441524 treatment, read our guide to reading your cat’s blood work during FIP treatment.

Test 2: Rivalta Test

The Rivalta test is a simple, fast, and highly useful test specifically for the wet (effusive) form of FIP. It is performed at the vet clinic using fluid collected from the cat’s abdominal or chest cavity.

A small sample of the fluid is added to a test tube of distilled water with one drop of acetic acid. A positive Rivalta result — where the fluid forms a visible precipitate or “jellyfish” shape — has approximately 91% sensitivity for FIP.

The Rivalta test is particularly valuable because it can be performed quickly at the point of care, without sending samples to an external laboratory. If your cat has a visibly swollen abdomen or difficulty breathing and your vet suspects wet FIP, ask specifically about the Rivalta test.

Note: the Rivalta test is only applicable when fluid is present. It cannot be used to diagnose dry, ocular, or neurological FIP.

Test 3: PCR Testing

PCR (polymerase chain reaction) testing detects the genetic material of the feline coronavirus (FCoV) in fluid, blood, or tissue samples. It is one of the most specific tests available for confirming FCoV presence.

Important context for PCR results:

  • A positive PCR confirms the presence of FCoV, but does not on its own confirm FIP — because many cats carry FCoV without developing FIP.
  • PCR performed on abdominal or chest fluid in a cat with clinical signs of FIP carries much higher diagnostic weight than PCR on blood alone.
  • A negative PCR does not rule out FIP — the virus may not be detectable in the specific sample tested.

PCR is most useful when combined with the A:G ratio, Rivalta test, and clinical signs. Experienced vets use it as one piece of the diagnostic picture, not as a standalone confirmation.

Test 4: Histopathology

Histopathology is the examination of tissue samples under a microscope by a veterinary pathologist. It is the closest thing to a definitive FIP diagnosis available — but it requires a surgical tissue biopsy, which is invasive and not always practical in a cat that is already unwell.

The process involves:

  1. Collecting a solid tissue sample surgically from an affected organ
  2. Preserving, thinly sectioning, and staining the tissue
  3. Examining the prepared sections under a microscope

A veterinary pathologist can identify the characteristic granulomatous inflammation of FIP in the tissue architecture. The accuracy of a histopathology diagnosis is high, and the pathologist can often provide a prognosis — an opinion on the likely course of the disease — which helps guide treatment decisions.

In practice, histopathology is most commonly used when the diagnosis remains uncertain after blood work, Rivalta, and PCR, or when a tissue sample is collected during another procedure.

How Vets Combine These Tests

No single test is definitive. The diagnostic confidence comes from combining results:

Test What It Detects Best Used For Sensitivity
Blood test (A:G ratio) Protein imbalance, inflammation markers All forms of FIP — first-line screening High when A:G < 0.4
Rivalta test Protein-rich fluid characteristic of FIP Wet FIP only ~91%
PCR FCoV genetic material Confirming FCoV presence in fluid or tissue Variable by sample type
Histopathology Tissue architecture and granulomas Definitive confirmation when biopsy is possible Highest — near definitive

A vet who combines blood work (including A:G ratio), Rivalta test where applicable, PCR, and clinical signs can reach diagnostic confidence above 90% in most cases — without requiring a tissue biopsy.

What to Do Once FIP Is Confirmed

Once your vet reaches a confident FIP diagnosis, the next step is immediate: start GS-441524 treatment at the correct dose for your cat’s form of FIP and weight. Do not delay.

FIP CURE Plus supplies GS-441524 in two concentrations:

All pricing is listed openly in AED. View the full FIP CURE Plus product range.

For a complete step-by-step guide on what to do from the moment FIP is suspected through to completing the 84-day protocol, read our FIP action guide.

Frequently Asked Questions

Is there a single definitive test for FIP?
No. FIP diagnosis requires combining multiple tests — blood work (especially the A:G ratio), Rivalta test where fluid is present, PCR, and clinical signs. Histopathology on a tissue biopsy is the closest to definitive but is not always practical.

What is the most important blood marker for FIP?
The albumin-to-globulin (A:G) ratio. A ratio below 0.6 is highly suggestive of FIP; below 0.4 is strongly indicative. Read our blood work guide for a full explanation.

What if two vets disagree on the diagnosis?
This is not uncommon, particularly with dry FIP. If FIP cannot be ruled out and your cat is deteriorating, the clinical case for starting GS-441524 is strong — the drug has no known complications, and a positive response within 7 to 14 days is itself strong evidence of FIP. Read our guide on why delaying FIP treatment is fatal.

Can the Rivalta test diagnose dry FIP?
No. The Rivalta test requires fluid from the abdomen or chest cavity. It is only applicable for wet (effusive) FIP. For dry, ocular, and neurological FIP, blood work, PCR, and histopathology are the primary tools.

What are the symptoms I should look for before requesting these tests?
Persistent fever above 39.5°C, weight loss, reduced appetite, lethargy, swollen belly, eye changes, or neurological signs. Read our complete FIP symptoms guide for all 12 early warning signs.

Conclusion

FIP diagnosis is a process, not a single test. The combination of blood work (especially the A:G ratio), Rivalta test, PCR, and clinical signs gives experienced vets the confidence to act — and acting early is what saves cats.

If your cat is showing signs of FIP, do not wait for a perfect result. Ask your vet about these four tests, and read our step-by-step FIP action guide to know exactly what to do next.

Questions about GS-441524 treatment for your cat? The FIP CURE Plus team is available in English and Arabic at info@fipcureplus.com.

View the FIP CURE Plus GS-441524 injectable range →

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

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