FIP Titres in Kittens: What 1:400 Actually Means

A coronavirus antibody titre of 1:400 is one of the most misread numbers in cat medicine. Up to 40% of pet cats carry antibodies to feline coronavirus (FCoV), and in catteries the figure climbs to 90%, yet only 5% to 12% of infected cats ever develop feline infectious peritonitis (FIP). A positive titre is evidence of contact with a common gut virus — not a diagnosis, not a prognosis. In kittens under a year old, where most FIP cases occur, the result is harder still to read: maternal antibodies and recent exposure both push the number up.

What an antibody titre actually measures

A cat makes antibodies against FCoV within 7 to 10 days of infection, and the titre is the highest dilution at which those antibodies are still detectable. A result of 1:400 means the laboratory could dilute the serum 400-fold and still see a signal. It says nothing about viral load, strain, or whether that strain has mutated to infect white blood cells — the change that produces FIP. ELISA and immunofluorescence assays also use different scales, so 1:400 from one laboratory is not 1:400 from another; repeat tests belong in the same laboratory. And the figure moves: up after exposure, down over months, up again on re-infection, because immunity to FCoV is short-lived.

Why 1:400 does not mean FIP

Titre distributions overlap heavily between healthy cats and cats with FIP. The European Advisory Board on Cat Diseases (ABCD) notes that median titres in the two groups do not differ, which limits the value of a single result. The trend is still directional: few healthy cats reach 1:1600, and 1:3200 or above is highly suggestive of FIP. Suggestive is not diagnostic. Dry-form FIP can present at 1:200, and a cat in a breeding household can carry 1:3200 and stay well for years. Age matters too: about 70% of FIP cases are diagnosed under 18 months, so a young cat with fever and a high titre needs imaging, not reassurance.

A negative result is not an all-clear

About one cat in ten with FIP has a low titre or tests negative: circulating virus can bind the antibodies the assay is looking for, and the damage in FIP is driven by immune complexes rather than free virus. A negative titre does make FIP less likely, but it cannot exclude it in a cat with effusion, weight loss and persistent fever. This is where owners and clinics go wrong — reading a negative coronavirus test as proof the cat is fine, then waiting two weeks before examining the abdomen.

The results that carry real weight

Diagnosis is built from a pattern, not a single number. With ascites or pleural fluid, sampling the effusion is the highest-yield step. These cut-offs come from the ABCD diagnostic tool and published reviews of test performance:

Test	What it shows	Cut-off that matters	Main limitation
FCoV antibody titre	Exposure to coronavirus	1:3200 or above is suggestive	Overlaps in healthy cats; not diagnostic
Albumin:globulin ratio	Systemic inflammation	Below 0.4 supports, above 0.8 argues against	15% of FIP cats sit at 0.8; 80% sensitivity, 82% specificity
Globulin	Immune stimulation	Above 46 g/L (4.6 g/dL)	89% of FIP cats are hyperglobulinaemic
Alpha-1-acid glycoprotein	Acute-phase protein	Above 1.5 g/L supports, above 3.0 g/L strong	Not run by every laboratory
Effusion protein and Rivalta	Exudate versus transudate	Above 3.5 g/dL, Rivalta positive	Sensitivity 87% and 65–98%
Effusion RT-PCR	Viral RNA in the fluid	Positive supports FIP	Negative does not exclude it
Histopathology plus immunostaining	Antigen in the lesions	Gold standard	Needs biopsy tissue

Timing: why an 8-week kitten gives a misleading result

Kittens absorb antibodies from colostrum and are protected until about 6 weeks; those antibodies then fade. Test at 8 weeks and you may be measuring the mother's antibodies rather than the kitten's own response. Merck's guidance is to delay serology until 10 to 11 weeks, and catteries working to eliminate FCoV usually sample between 10 and 16 weeks. Even then, a positive result means exposure — the norm at that age in any multi-cat environment. The test works as a colony screen, poorly as an identity check on one pet kitten.

When paying for a titre is reasonable

Three uses hold up, and all are about context rather than diagnosis:

Monitoring a breeding colony, where the group trend matters more than any single value.
Confirming that a cat responded to natural exposure before joining a closed, tested household.
Adding background to an ambiguous case, read alongside albumin, globulin and AGP.

It adds little to the two requests that come up most: whether a healthy shelter kitten is safe to adopt (most will be positive), and whether to start antivirals. Treatment rests on compatible signs, imaging, effusion analysis and the first week's response.

What a diagnosis actually requires

ABCD's position is that only histopathology showing FIP-typical lesions, with FCoV antigen in those lesions, confirms FIP. Few cats are biopsied before treatment, so clinicians assemble a likely-FIP case: a young cat, fever unresponsive to antibiotics, high-protein effusion with a positive Rivalta test, albumin:globulin below 0.6, AGP above 1.5 g/L, and FCoV RNA in the fluid. Antivirals such as GS-441524 run for 84 days, with published survival of roughly 78% to 96% in cats that complete a course. A cheap titre cannot replace that work-up, and should not delay it.

If the result comes back positive

Do not isolate or rehome the cat on the strength of the number. FIP arises from mutation inside one cat, and feline coronavirus is not zoonotic.
Record weight weekly and temperature daily for two weeks: progressive weight loss with fever says more than any single titre.
In a multi-cat home, submit three faecal samples or rectal swabs for RT-PCR across one week to find heavy shedders; one negative sample proves little.
Reduce crowding and stress: one litter tray per cat plus one, daily scooping, no rehoming during the observation window.

Group housing and stress are the factors most consistently linked to progression from infection to disease.

This article does not replace veterinary diagnosis. A titre can neither confirm nor exclude FIP, and any cat with fever, weight loss, abdominal swelling or neurological signs needs examination, imaging and laboratory work by a veterinarian.

Terms mentioned in this article

FIP

Frequently asked

Does a 1:400 coronavirus titre mean my kitten has FIP?

No. A titre of 1:400 shows that the kitten has met feline coronavirus, which is common: up to 40% of pet cats and up to 90% of cats in multi-cat homes are seropositive. Only about 5% to 12% of infected cats ever develop FIP. A titre becomes suggestive at 1:3200 and above, and even then it is one line of evidence, read next to albumin:globulin ratio, globulin, alpha-1-acid glycoprotein and effusion analysis.

Can I test a kitten at 8 weeks of age?

You can, but the result is hard to interpret. Kittens carry maternal antibodies until about 6 weeks and lose them gradually, so an 8-week sample may measure the mother's antibodies rather than the kitten's own response. Merck's guidance is to wait until 10 to 11 weeks, and catteries usually sample between 10 and 16 weeks. Before that age, a positive result is neither proof of infection nor a reason to rehome the kitten.

If the titre is high, should I isolate the cat from my others?

Not on the titre alone. FIP arises when coronavirus mutates inside an individual cat, so a high titre in one cat is not evidence that it will infect the others, and feline coronavirus is not zoonotic. What does help is reducing load and stress: one litter tray per cat plus one, daily scooping, three faecal RT-PCR samples to identify heavy shedders if you need to, and no rehoming during observation.