Yes — this has changed completely. The antiviral GS-441524 (and oral remdesivir) now achieves survival rates near 80% in effusive FIP for cats that survive the first 48 hours of treatment. What was once a fatal disease is now, with prompt diagnosis, largely treatable.

What is the FCoV antigen test useful for?

Screening and outbreak management — identifying coronavirus shedders in multi-cat households, shelters and catteries to guide hygiene and isolation. It is a screening and epidemiological tool, not a standalone FIP diagnostic.

FIP in Cats: What a Coronavirus Test Can and Can't Say

Q: Does a positive coronavirus test mean my cat has FIP?

No. Feline coronavirus (FCoV) is a common, usually mild gut infection, and the vast majority of infected cats never develop FIP. A positive FCoV antigen test confirms coronavirus infection only. FIP requires a clinical workup — compatible signs, effusion analysis and blood markers — not a single test.

Q: Can an antibody titre diagnose FIP?

No, and it's a real trap. Many healthy cats carry high FCoV antibody titres, and about 10% of cats with confirmed FIP have no detectable antibodies at all. A single titre at one point in time is not diagnostic — high titres don't confirm FIP, and negative titres don't rule it out.

Q: Is a low A:G ratio enough to diagnose FIP?

Not on its own. A:G below 0.6 is a classic FIP finding, but common conditions — including dental disease — can also push A:G below 0.6. A:G below 0.4 is highly suggestive and above 0.6 lowers the likelihood, but it must be read with clinical signs and other tests.

Q: How is FIP actually diagnosed?

By combining clinical signs (persistent fever, weight loss, effusion), effusion analysis in the 'wet' form, blood markers (low A:G, high globulins), and ideally PCR on effusion or tissue. There is no single definitive blood test — it is a multimodal diagnosis.

TL;DR — Feline coronavirus (FCoV) and FIP are not the same thing. FCoV is common and usually harmless; FIP is a rare, mutated, historically fatal form. Two traps catch people: an antibody titre can’t diagnose FIP (healthy cats carry high titres, and ~10% of FIP cats have no antibodies), and a low A:G ratio isn’t proof either (dental disease can push it below 0.6 too). The real story is that treatment has changed — GS-441524 and remdesivir now save ~80% of effusive FIP cats who survive the first 48 hours, so getting the diagnosis right and fast matters more than ever.

Feline infectious peritonitis (FIP) is the diagnosis every cat owner fears and every clinician dreads getting wrong — because for years, getting it “wrong” in either direction was a disaster. This guide unpacks what the tests actually tell you, the two traps that cause false FIP scares, and why the whole picture changed once the disease became treatable.

Two Diseases People Keep Confusing

The confusion starts with the names. They share a root but are not the same disease:

Feline coronavirus (FCoV) is a common, usually mild enteric virus. In multi-cat households and shelters it’s almost everywhere — most cats carry it at some point, most show no signs or just a passing bout of diarrhoea, and it spreads through faeces and shared litter boxes. It is, in the vast majority of cases, a non-event.

FIP happens when FCoV mutates inside one individual cat into a form that infects macrophages and spreads systemically. That mutation is rare — only a minority of FCoV-infected cats ever develop FIP — but when it happens the disease is aggressive. It comes in two forms: the effusive (“wet”) form with fluid building up in the belly or chest, and the non-effusive (“dry”) form with inflammatory lesions in the organs.

The critical point, stated plainly: a positive coronavirus test means your cat has coronavirus. It does not mean your cat has FIP. Most positive cats will never develop FIP, and over-reading that positive has historically led to unnecessary anxiety — and, worse, unnecessary euthanasia.

The Antibody Trap

Here’s the first thing most people don’t know, and it’s the source of a lot of false FIP scares.

FCoV antibody titres are not diagnostic for FIP. Not high ones, and not low ones. Here’s why, with the numbers:

Healthy cats carry high titres. In multi-cat households especially, many perfectly healthy, asymptomatic cats have high FCoV antibody levels. High titre ≠ sick cat, and most of those high-titre cats will never develop FIP.
About 10% of FIP cats have no detectable antibodies at all. A negative antibody result does not rule FIP out — roughly one in ten confirmed FIP cases is antibody-negative, because titres can wane late in disease.
A single titre at one point in time is meaningless. Titres fluctuate with reinfection and time. One reading can’t tell you anything about whether a cat has FIP.

For context on how weak even the strongest titre signal is: in a study of 1,108 cats (488 with histopathologically confirmed FIP), the highest titre measured (1:1600) was the single best diagnostic marker available — and yet only 36 of the 488 confirmed FIP cases (7.4%) ever reached that titre. In other words, even the “best” antibody threshold would have caught fewer than 1 in 13 real FIP cases.

The practical rule: an antibody titre should not be used to confirm or rule out FIP. If someone tells you a titre “proves” FIP, they’re misreading the test.

The A:G Ratio Trap

The albumin:globulin (A:G) ratio is a better signal than antibodies — but it’s not a silver bullet either, and treating it as one creates false positives.

A low A:G is a classic FIP finding. A:G below 0.4 is highly suggestive; above 0.6 lowers the likelihood considerably. It’s one of the most consistent blood markers, present in roughly half of wet-form and ~70% of dry-form cases.
But other things lower A:G too. Common, unrelated conditions — including dental disease — can push A:G below 0.6. So a low ratio on a blood panel is a clue, not a diagnosis.
The false-positive risk depends on where you are. In a low-prevalence setting (a general practice seeing lots of vague illness, few real FIP cases), a low A:G has poor positive predictive value — you’ll flag far more non-FIP cats than real ones. The test’s usefulness rises when the clinical picture already points at FIP.

The takeaway: A:G is a strong supporting finding when it sits alongside compatible signs. On its own, it’s how healthy cats get a scary “possible FIP” note on their chart.

What Actually Diagnoses FIP

Because no single blood test is definitive, FIP is a multimodal diagnosis. It comes together from:

Clinical signs — persistent or waxing-waning fever, weight loss, abdominal or chest effusion, ocular or neurological signs.
Effusion analysis — in the wet form, the fluid has a characteristic profile (high protein, low cellularity) and its own low A:G ratio.
Blood markers — low A:G, high globulins, hyperbilirubinaemia without liver-enzyme elevation.
Molecular tests — PCR on effusion or tissue to detect the mutated virus; this is the closest thing to a confirmatory test.
Response to treatment — increasingly, a cat that’s suspected of FIP and responds to antiviral therapy confirms the picture in hindsight.

The Part That Changed Everything: FIP Is Now Treatable

This is the context that reframes the entire testing conversation. For decades FIP was a death sentence — which is why an uncertain diagnosis was so agonising. That’s no longer true.

GS-441524 and its prodrug remdesivir are antivirals that directly target FIP.
In effusive FIP, survival with treatment now runs near 80% for cats that survive the first 48 hours.
In an experimental model, GS-441524 achieved 100% (10/10) complete resolution.
The non-effusive (dry/neurological) form responds less well and may need higher or longer dosing — but it’s no longer automatically fatal either.

The consequence for testing is direct: because a correct, early diagnosis now leads to a real treatment, the cost of a false negative went up dramatically, and the value of a confident diagnosis went up with it. The old fatalistic “if it’s FIP there’s nothing to do” logic is gone — which means getting the answer right, fast, actually changes the outcome.

Where the FCoV Antigen Test Fits

So if the antibody titre is a trap and the A:G is only a clue, what is the coronavirus antigen test actually for?

The FCoV antigen test (detecting virus shed in faeces) is a screening and epidemiological tool, not a FIP diagnostic. Its real uses:

Multi-cat household and shelter management — identify shedders to guide hygiene, litter-box management and isolation, reducing viral load in the environment.
Investigating persistent diarrhoea — coronavirus as a possible cause.
Providing context — knowing coronavirus status when FIP is suspected, so the clinician interprets the rest of the workup correctly.

Related product: FCoV-Ag Feline Coronavirus Antigen Test — a quantitative antigen test (cat. MG-CA-R029, 10-test box, room-temperature storage) that reports viral load rather than a yes/no line.

What This Means for a Clinic

For a veterinary practice, FIP is one of the clearest cases where diagnostic confidence is the product:

The disease is now treatable, so a fast, well-founded FIP workup leads to a treatment plan — and a treated FIP cat is a high-value, long-term case (12+ weeks of antiviral therapy with monitoring).
The traps are known and avoidable. Knowing that antibody titres don’t diagnose FIP and that A:G needs context lets a clinic avoid both false alarms and missed cases — which is exactly the expertise owners are willing to pay for.
The screening test has its own market. FCoV antigen screening for catteries, shelters and multi-cat breeders is a recurring, high-volume need independent of any single suspected-FIP workup.

For the owner, the framing matters: FIP used to mean “nothing we can do.” Today the honest message is “we can test for this properly, and if it is FIP, there’s now real treatment” — which changes the entire conversation from despair to a plan.

FAQ

Does a positive coronavirus test mean my cat has FIP?

No — 1 positive coronavirus test only confirms FCoV, because most infected cats never develop FIP. FIP needs a clinical workup, not a single test.

Can an antibody titre diagnose FIP?

No — the titre is meaningless as a single number: healthy cats carry high titres, and about 10% of confirmed FIP cats have no detectable antibodies. High doesn’t confirm, negative doesn’t rule out.

Is a low A:G ratio enough to diagnose FIP?

Not alone — A:G below 0.6 is a classic clue, but dental disease and other common conditions can also push it below 0.6. It’s a supporting clue, not proof.

How is FIP actually diagnosed?

By combining 4 inputs — clinical signs, effusion analysis, blood markers (low A:G, high globulins) and PCR on effusion or tissue. There is no single definitive blood test.

Can FIP be treated?

Yes — GS-441524 and remdesivir now achieve ~80% survival in effusive FIP for cats surviving the first 48 hours. What was fatal is now largely treatable with prompt diagnosis.

What’s the FCoV antigen test for?

For 2 jobs — finding shedders in multi-cat households and shelters, and guiding hygiene. It’s a screening tool, not a FIP diagnostic.

Key Takeaways

FCoV ≠ FIP — 1 common benign virus can rarely mutate into a serious disease.
Antibody titres don’t diagnose FIP — healthy cats carry high titres, ~10% of FIP cats have none, and a single reading is meaningless.
A:G below 0.6 isn’t proof — dental disease and other conditions cause it too; read it with clinical signs.
FIP is a 4-part diagnosis — signs + effusion + blood markers + PCR, not one test.
Treatment changed everything — GS-441524/remdesivir save ~80% of effusive FIP cats, so an early, correct diagnosis now genuinely changes the outcome.
The antigen test is for 1 job — finding shedders for multi-cat hygiene and outbreak management, not diagnosing FIP.

References

Cosaro E, et al. “Efficacy of oral remdesivir compared to GS-441524 for effusive FIP.” Viruses 2023;15(8):1680. PMC10458979 / PMID 37632022.
EveryCat Health Foundation. “Value of tests to diagnose FIP.” — A:G positive-predictive-value in low-prevalence populations.
dvm360 / Hartmann K. “Diagnosis of FIP: Facing the challenge.” — 1,108-cat study (JVIM 2003;17:81–790), titre and A:G performance.
FIP Vet Guide. “Diagnosing FIP.” — A:G limitations and antibody-titre guidance.
VIN / WSAVA 2013. “Dealing with the Dilemmas of FIP.” — A:G reference range and diagnostic thresholds.

This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any cat with suspected FIP should be evaluated by a veterinarian. Test results must be interpreted alongside clinical findings. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.