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FIP in cats: the blood work clues and how it is diagnosed

Feline infectious peritonitis (FIP) develops when a common feline coronavirus mutates inside a cat. Once almost always fatal, it can now be treated with antiviral drugs, which makes an accurate diagnosis more important than ever. No single blood test proves FIP: vets combine the cat’s history, blood work and, when present, an analysis of the fluid in the belly or chest.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

FIP mostly affects young cats, often pedigree cats from multi-cat homes. Typical blood findings are high globulins with a low albumin to globulin (A:G) ratio, low lymphocytes, a mild anemia and sometimes a high bilirubin with normal liver enzymes. An A:G ratio below 0.4 makes FIP very likely; above 0.8 it is very unlikely. A coronavirus antibody test cannot diagnose FIP. Fluid analysis, PCR and immunostaining help confirm it. With modern antivirals, more than 80% of treated cats survived in a large study.

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Which cats get FIP

FIP disproportionately affects pedigree cats and cats under two years old; in Germany, 39% of 222 cats with FIP were under one year [1]. Most cats come from, or have lived in, multi-cat households, and a recent stress such as adoption or neutering is common [1]. Fewer than 10% of cats infected with the coronavirus develop FIP [1].

Most cats have fluid in the belly or chest, with fever, poor appetite and weight loss (the “wet” form); others have no fluid but lesions in organs, the eyes or the brain [1].

What the blood work shows

  • Globulins are often high, with a low or low-normal albumin; total protein is not always high [1]. See albumin and globulin.
  • Low lymphocytes, high neutrophils and a mild to moderate anemia are common [1]. See anemia in cats.
  • Bilirubin is raised in 22–84% of cats, often with normal liver enzymes; a high bilirubin without red cell destruction or liver enzyme rise raises the suspicion of FIP [1].
  • AGP, an inflammation protein, is often above 1.5 mg/mL [1].

How to read the A:G ratio

Here is an example line from a young cat with fluid in the belly.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
A:G Albumin to globulin ratio0.32–> 0.8L
  1. Abbreviation A:G, A/G or albumin/globulin ratio, calculated from the albumin and globulin results.
  2. Result 0.32 in this illustrative line: low.
  3. Unit A ratio, without a unit.
  4. Laboratory interval Above 0.8, FIP is very unlikely; the ratio is used as a guide, not as a laboratory interval [1].
  5. Flag L: low. Below 0.4, FIP is very likely in a cat with compatible signs; the same cut-offs apply to the fluid [1].

Confirming FIP

  • Coronavirus antibody test: there is no “FIP antibody test”. A positive coronavirus antibody result in a healthy cat never means FIP [1].
  • Fluid analysis: FIP fluid is usually yellow, sticky, protein-rich (often above 35 g/L) and low in cells [1, 2]. The Rivalta test helps mostly when it is negative [1].
  • RT-PCR for the virus is positive in most fluid samples from cats with FIP, but positives also occur in cats without FIP; on blood it is much less sensitive [1].
  • Immunostaining of virus in macrophages from the fluid or tissue is highly supportive; a definitive diagnosis relies on tissue changes plus immunostaining [1].

A transformed outlook

Before antivirals, cats with the wet form given steroids survived a median of 8 to 21 days [1]. In a UK study of 307 cats treated with legally compounded remdesivir and/or GS-441524, 84.4% were alive at the last follow-up, and 10.8% relapsed [3]. Blood values such as the globulins, A:G ratio and lymphocytes are rechecked to follow the response.

Related articles

En français : la PIF du chat.

Questions to ask your veterinarian

  1. Are the globulins high and the A:G ratio low?
  2. Is there fluid that can be sampled and analysed?
  3. Which tests can confirm FIP: PCR, immunostaining?
  4. Is antiviral treatment available, and how will we monitor it?
  5. Should the other cats in the home be tested or watched?
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Frequently asked questions

What are the symptoms of FIP in cats?

Most often fluid in the belly or chest with fever, poor appetite and weight loss; some cats have eye or nervous signs instead [1].

How did my indoor cat get FIP?

From its own feline coronavirus, which is common and usually harmless, especially in cats from multi-cat homes. FIP develops when the virus mutates inside the cat [1].

Can humans get sick from cats with FIP?

No. Feline coronavirus and FIP are not known to infect people.

Does a positive coronavirus test mean my cat has FIP?

No. Many healthy cats carry antibodies; there is no “FIP antibody test” [1].

How long can a cat live with FIP?

Untreated, often only days to weeks. With modern antiviral treatment, more than 80% of cats survived in a study of 307 cats [1, 3].

Sources

  1. Tasker S et al. Feline infectious peritonitis: European Advisory Board on Cat Diseases guidelines. Viruses. 2023;15(9):1847. doi.org/10.3390/v15091847
  2. eClinPath, Cornell University College of Veterinary Medicine. Effusions. eclinpath.com/cytology/effusions-2
  3. Taylor SS et al. Retrospective study and outcome of 307 cats with feline infectious peritonitis treated with legally sourced veterinary compounded preparations of remdesivir and GS-441524 (2020-2022). J Feline Med Surg. 2023;25(9):1098612X231194460. doi.org/10.1177/1098612X231194460

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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