Diagnosing feline infectious peritonitis (FIP) is rarely settled by one test. Veterinarians build suspicion from the whole cat, sample the most affected site (fluid first, then an abnormal organ), interpret the results against the clinical picture, and, when suspicion is high enough, start antiviral treatment before a definitive result is available. Current guidance from the European Advisory Board on Cat Diseases (ABCD) favors oral GS-441524 where it is available, with remdesivir used when it is the only antiviral option or when a cat cannot tolerate oral medicine. This guide walks through that pathway step by step for clinicians and informed caregivers who want to understand what the veterinarian is doing and why.
Why FIP is a probability rather than a single test result
FIP is associated with feline coronavirus (FCoV), but FCoV infection or detection is not by itself the same thing as FIP, and most cats that carry FCoV never develop the disease. No single routine sign or laboratory result is pathognomonic. Fever, appetite loss, weight loss, effusion, eye changes, neurological signs and organ abnormalities all occur in FIP, and each also occurs in many other conditions. In living cats, diagnosis is therefore usually a judgment about probability, built from the whole case.
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For scale, an AAFP/EveryCat estimate from 2022 attributes around 0.3% to 1.4% of feline deaths at veterinary institutions to FIP. That figure describes deaths in institutional settings. It is not a measure of how common FIP is across the cat population, and it is not a risk estimate for an individual cat.
Step 1: Build suspicion from the whole case
Before ordering tests, the veterinarian works through the history, signalment, physical examination and the way this particular cat presents. The goal is to choose the tests most likely to change the assessment, not to run every available panel. Features that commonly prompt FIP to be considered include:
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- Fever, appetite loss or weight loss that is persistent or unexplained
- Abdominal or chest fluid (effusion)
- Ocular (eye) signs
- Neurological signs
- Abnormal organs on examination or imaging
- Routine hematology or biochemistry changes, which point toward illness but do not identify FIP on their own
Step 2: If fluid is present, sample it first
When effusion is present, ABCD guidance prioritizes sampling the fluid. It is tested in three ways:
- Cytology, to look for a cell pattern compatible with FIP.
- Biochemistry, including protein content. Protein-rich fluid is a key feature.
- FCoV detection, by RT-PCR or antigen immunostaining.
Protein-rich fluid with compatible cytology and FCoV detected in the fluid makes FIP very likely. A single finding in isolation carries less weight, as the interpretation table below shows.
Step 3: If there is no fluid, sample the affected tissue
Without effusion, the target is the abnormal organ or tissue itself. Two routes are used.
Fine-needle aspirate of an abnormal organ
Aspirates of abnormal organs or tissues, examined by cytology and tested for FCoV by RT-PCR or immunostaining, are often helpful in non-effusive disease. The sample is less invasive than surgical sampling, but the result is still read against the sampled site and the cat’s clinical findings.
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Histopathology with FCoV antigen detection
This is generally the definitive route. Invasive biopsy, however, may not be practical in a sick cat, so the choice is made with the veterinarian based on the cat’s condition and what the procedure would add to the decision.
Reading the results together
The same finding can mean different things depending on what else is present. The table sets out how common result patterns are interpreted.
Rank #3
| Result pattern | What it supports | What it does not establish |
|---|---|---|
| Protein-rich effusion, compatible cytology, FCoV detected in the fluid | FIP very likely (ABCD guidance) | Final interpretation still rests on the clinical picture |
| Effusion present, but cytology or FCoV detection does not match | Not the combination the guidance describes for a “very likely” call | Either FIP or its exclusion; the other findings decide |
| FCoV detected by RT-PCR in fluid or tissue, without compatible cytology or clinical picture | FCoV present in that sample | FIP. A positive molecular test alone is not universal confirmation |
| Abnormal organ aspirate with compatible cytology and FCoV detection (non-effusive disease) | FIP, and often helpful for that reason | A definitive diagnosis on its own; read against the sampled site and clinical findings |
| Histopathology with FCoV antigen detection | Generally the definitive route | Practicality: invasive, and may not be practical in a sick cat |
| Routine hematology or biochemistry abnormality | Illness and a changing picture that can be tracked | FIP; these findings are nonspecific |
| Fecal FCoV detection or an FCoV antibody result alone | Exposure to FCoV | FIP; not diagnostic on its own |
When to start trial antiviral treatment
ABCD’s guideline states: “When a diagnosis of FIP is very likely, trial treatment with antivirals is often warranted as improvements typically occur in cats with FIP within a few days of starting treatment.” (European Advisory Board on Cat Diseases, current guideline, updated 24 March 2026.)
A trial of treatment is a clinical decision about how likely FIP is, weighed against the cat’s condition. It is typically made once sampling has been done as far as the cat’s condition allows, rather than replacing that sampling.
Antiviral options: GS-441524 and remdesivir
Treatment has changed the outlook for cats with FIP, but access and rules differ by location. The table compares the factors that matter when choosing between the two antivirals named in current guidance.
Rank #4
| Factor | GS-441524 (oral, where available) | Remdesivir |
|---|---|---|
| Current ABCD position | Favored where available (2026 treatment update) | Used when it is the available antiviral, or when the cat cannot tolerate oral medicine |
| Route | Oral | Injectable |
| Patient suitability | Cats able to take oral medicine | Cats that cannot tolerate oral medicine, or where it is the only option |
| Reported success | 80% to 100% across published studies (ABCD, 2026). This is a range across heterogeneous studies, not a guarantee for an individual cat | No success range is established in the guidance cited here |
| Access | Depends on country; not stated for any specific jurisdiction | Depends on country; not stated for any specific jurisdiction |
| Monitoring | Follows the schedule below | Follows the schedule below |
Treatment choice, dose, duration and monitoring are decisions for the veterinarian.
Monitoring the response
Monitoring is built into the plan from the start. ABCD and a recent veterinary diagnostic algorithm both recommend the following schedule:
- Review any effusion after two weeks.
- Repeat hematology and serum biochemistry at two weeks, then monthly where feasible.
- Follow any values that were abnormal at the outset, and adjust testing to the cat’s clinical improvement and treatment course.
When improvement is not what you expect
Lack of the expected improvement is a prompt to review the case, not simply to continue the same plan. The veterinarian reviews:
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- The differential diagnoses still in play
- The formulation being used
- The overall treatment plan
Questions to ask your veterinarian
- Is there an effusion, and if so, has it been tested for cytology, biochemistry and FCoV detection?
- If there is no effusion, which organ or tissue is being sampled, and by which FCoV test?
- How likely is FIP on the current findings, and what would change that assessment?
- Is an antiviral available here, and which one, in what formulation?
- When are the fluid review and blood tests scheduled?
Where you live changes the options
Antiviral access and rules vary by country, and prescribing and compounding requirements differ by jurisdiction. The guidance summarized here does not establish which products are legal in any specific country, so the answer for your location has to come from a local veterinarian. Do not source antivirals through unverified sellers or self-prescribe. A veterinary assessment and prescription are the route to legitimate treatment, and the veterinarian is the person who can say what is actually available to you.
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