Feline coronavirus tests (antibody titre, RT-PCR, FIP mutation PCR) in cats

What it shows

A form can show three different coronavirus lines:

• FCoV antibody titre — shows that the cat’s immune system has met the virus. There is no blood test that tells antibodies to a harmless strain from antibodies to an FIP-causing one. In a meta-analysis, signs of infection (antibodies or virus in faeces) were found in about 32% of cats.
• FCoV RT-PCR — looks for the virus’s genetic material (RNA) in a sample. In faeces it shows shedding; in fluid or tissue, that the virus is there. This test does not tell the mild gut form of the virus from the FIP form.
• FIP virus (“mutation”) RT-PCR — looks for changes in the spike gene that have been linked with FIP. IDEXX runs it only after a positive FCoV RT-PCR and only on fluid, tissue or aspirates: blood carries too little virus, and faeces may contain the gut form even in a cat with FIP. The decisive mutations are still not known.

Higher than reference — what it may be a sign of

• Positive antibody titre — contact with FCoV, now or in the past. Most cats with antibodies never develop FIP, and a healthy cat with a high titre is not more likely to develop FIP than one with a low titre.
• FCoV detected by RT-PCR — in faeces, the cat is shedding virus. In blood, low levels of virus can occur in cats that never develop FIP. In fluid from the abdomen or chest, a positive result supports FIP but does not confirm it.
• “Mutation” PCR positive — it supports FIP in a cat with clinical signs. Compared with testing for FCoV alone, it may only slightly improve diagnosis, and false positives have been documented.

A definitive diagnosis usually needs tissue examined under the microscope with detection of coronavirus antigen. The vet weighs all findings together and decides on the next steps.

Lower than reference

A negative antibody result makes recent contact with FCoV less likely, but it does not exclude FIP: titres can fall to zero over months to years after infection, some cats with FIP have no antibodies, and a negative test with advanced signs does not rule FIP out. A negative RT-PCR does not exclude infection either, as false negatives occur. On the mutation test IDEXX reads “not mutated” (FECV) as FIP being unlikely, while “indeterminate” or “below the limit of detection” does not rule FIP out.

Look at together with

• Fluid analysis — when there is fluid in the abdomen or chest, sampling it for cytology, biochemistry and coronavirus detection is a priority.
• Globulin, albumin and bilirubin — high globulin, low albumin and high bilirubin are among typical findings, though high globulin also occurs in other conditions.
• Complete blood count — low lymphocytes, high neutrophils, anaemia and low platelets are typical.

Units and conversion

None of the three results has a unit. The antibody test is reported either as positive or negative or as a titre — the highest dilution of serum, written 1:N, at which antibodies are still found; the larger N, the more antibody. RT-PCR is reported as detected or not detected. The mutation test may name the result, for example the FIP or the gut (FECV) type, “indeterminate” or “below the limit of detection”. In the app these results are stored as text.

Reference range

The reference range is on your lab’s form; for PCR it is “not detected”, and for antibodies the lab states which titre it calls negative. There is no single, minimally invasive test for FIP during life; diagnosis usually rests on a combination of history, examination findings and laboratory tests.

Cats and dogs

Sources

1. ABCD (European Advisory Board on Cat Diseases) — Guideline for feline infectious peritonitis (updated 2026)
2. Merck Veterinary Manual — Feline infectious peritonitis
3. Cornell Feline Health Center — Feline infectious peritonitis
4. VCA Animal Hospitals — Feline infectious peritonitis testing
5. IDEXX — Feline infectious peritonitis virus RealPCR: diagnostic update (2015)