FIP diagnosis
Knowing which tests to ask for, how to read them and what to look for in the blood work can make the difference. Here we walk through the diagnostic process step by step.
Why is FIP diagnosis difficult?
FIP is caused by a mutated form of feline coronavirus (FCoV). The problem is that ordinary feline coronavirus is very common: many cats carry it without ever developing FIP. There is currently no commercially available test that can tell the benign coronavirus from the FIP-causing mutant directly from blood.
Diagnosis therefore relies on combining several pieces of evidence: clinical signs, blood results, fluid analysis where there is fluid, imaging, and increasingly PCR.
Diagnosing FIP is not always easy. The most common and recommended tests, blood work, ultrasound and the appearance of the fluid, point towards the diagnosis. Often the case is described as consistent with or suggestive of FIP, rather than a confirmed FIP. This should not delay treatment: an experienced vet may decide to start on a solid suspicion, and often that is the right call.
Tests to diagnose FIP
No single test confirms FIP on its own. The diagnosis is made by combining clinical signs, blood tests, imaging and, where possible, PCR on the right sample. These are the recommended tests, in the order in which they are usually done.
Complete blood work (always)
A first step. Findings consistent with FIP include: raised total protein (>10 g/dL), a low albumin/globulin ratio (<0.6), anemia, neutrophilia, lymphopenia and raised bilirubin and liver enzymes. None of these findings confirms the diagnosis on its own, but they make it more likely.
Abdominal ultrasound (always)
This shows whether there is an effusion (free fluid). FIP is an inflammatory disease, so even without fluid other signs may appear: thickened intestinal walls, enlarged organs and enlarged or reactive lymph nodes.
What to do depending on the result:
With fluid (effusive or wet FIP): analyze the fluid, as explained in point 4.
Without fluid (dry FIP): perform a fine-needle aspiration (FNA) of an affected lymph node and cytology of the sample. If there is enough cellularity, send it for PCR (point 5).
Chest X-ray
Recommended when breathing is rapid or labored. If there is fluid in the chest, drain it.
Fluid analysis (wet FIP)
If there is abdominal or thoracic fluid, it needs to be analyzed.
Handling the fluid: never drain an abdominal effusion (ascites). Take only the amount needed for analysis. Thoracic fluid, on the other hand, should always be drained, because it compromises breathing.
FIP fluid is typically yellowish or straw-colored and viscous, with protein above 35 g/L and low cellularity.
Recommendations: cytology and, if there is cellularity, PCR. The Rivalta test is usually positive and helps tell it from a transudate, but it does not confirm the diagnosis on its own, so cytology is preferred.
Specific PCR for the FIP mutation
This is the most specific test currently available. It detects the virus mutation associated with FIP and should be done on effusion fluid or lymph node tissue. Never PCR on blood, and it should not be confused with antibody tests (ELISA), which do not confirm FIP.
A positive result on fluid or lymph node tissue has very high specificity and takes a case from consistent with FIP to a confirmed diagnosis. A negative result, however, does not rule the disease out.
Biopsy (dry cases)
In dry cases, ultrasound shows granulomas, and some vets choose to biopsy them. Avoid it whenever possible: it is a very invasive procedure and is not usually necessary.
Forms of FIP and their specific diagnosis
Each form of FIP has diagnostic particularities that matter. Knowing them helps your vet make the right clinical decisions from the first moment.
Dry FIP (non-effusive)
The hardest form to diagnose and the slowest to progress. With no free fluid, the tests are more complex.
Diagnostic protocol
- Complete blood work with biochemistry
- Abdominal ultrasound: thickened walls, liver and kidneys affected, enlarged abdominal lymph nodes
- Cytology of lymph node tissue (macrophages, neutrophils and protein are typical findings)
- IDEXX PCR on a tissue sample: it detects 2 specific FIP mutations. A positive result for feline coronavirus without either of the two mutations does not rule FIP out, but neither does it confirm it: it has to be read alongside the rest of the picture
Wet abdominal FIP (ascites)
The fastest-moving form. Yellowish, straw-colored fluid in the abdomen is the most visible sign. Treatment should be started immediately.
Diagnostic protocol
- Blood work with biochemistry + ultrasound
- Abdominal fluid sample: cytology (macrophages, neutrophils, protein) and IDEXX PCR
- Careful: in wet FIP the proteins can be diluted in the fluid, making the albumin/globulin ratio look normal (>0.6 or even >0.8). Do not rule FIP out on that basis
Important clinical management
- Do not drain the abdominal effusion unless it presses on the chest and interferes with breathing: the fluid will build up again and it can cause electrolyte and protein imbalance
- Do not give diuretics unless there is concurrent heart disease
Wet pleural FIP
Pleural effusion tends to come with dyspnoea of varying degrees. The diagnostic tests are the same as in the abdominal form, with one critical difference in management.
Diagnostic protocol
- Blood work with biochemistry + ultrasound
- Pleural fluid sample: cytology and IDEXX PCR
Important clinical management
- Do drain the pleura: unlike the abdominal form, pleural drainage is necessary so the cat can breathe normally and to avoid cardiorespiratory arrest
Ocular and neurological FIP
Occurs when the virus crosses the blood-brain barrier. It can appear combined with any of the forms above. Important: blood work can look completely normal, which does not rule the diagnosis out.
Diagnostic protocol
- Blood work with biochemistry + ultrasound
- CT or MRI if it is financially viable
- Lumbar puncture (CSF) for cytology and PCR, if intracranial pressure allows and the vet considers it appropriate
- In the ocular form: cytology and PCR of the aqueous humor
Response to treatment: if the picture is consistent with FIP and the cat's condition does not allow waiting, an experienced vet may decide to start treatment before having the PCR. GS-441524 acts specifically on the mutated coronavirus, so a clear improvement in the first 3–7 days strongly reinforces the suspicion. Reinforces, not confirms: response to treatment is a clinical argument in favor, not a diagnostic test. FIP usually appears in cats under 24 months or over 15 years old.
FAQ
Questions about diagnosing FIP
Clear answers to the most common questions.
There is no single 100% definitive test. The most diagnostic combination includes: an albumin/globulin ratio below 0.8 (often below 0.5), raised total protein, lymphopenia and anemia. In wet FIP, a positive Rivalta test on the abdominal fluid adds specificity. PCR on fluid and immunohistochemistry on tissue are the only ones that allow the diagnosis to be confirmed. Without one of the two, the right wording is a case consistent with FIP, not a confirmed FIP.
The cost varies with the tests performed and the country. Full blood work can cost between 80 and 200 EUR. Specific PCR on fluid can be more expensive. Talk to your vet about the most cost-effective options for your situation.
If the cat is deteriorating quickly and the clinical picture is strongly suggestive, some experienced vets start treatment without waiting for definitive confirmation; a positive response in the first few days also reinforces the diagnosis. It is the vet's decision, and until the PCR arrives the case remains consistent with FIP rather than a confirmed FIP. A firmer diagnosis gives more confidence to decide, but time matters.
Does your cat have FIP?
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