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. A skilled vet with FIP experience can reach a reliable clinical diagnosis in most cases and start treatment on it, while the case remains formally consistent with FIP until PCR or histopathology confirms it.
FIP diagnostic tests
No single result confirms FIP. Blood work, imaging and the appearance of the fluid all point towards the diagnosis, but none of them closes it. Until there is a positive PCR for the mutation in the fluid —abdominal or pleural— or histopathology with immunohistochemistry in the dry forms, the right wording is a case consistent with or suggestive of FIP, and not a confirmed FIP. This does not delay treatment: an experienced vet may decide to start on a solid suspicion, and often that is the right call. What changes is how it is named.
Complete blood work
The essential first step. In FIP you usually find: raised total protein (>10 g/dL), hypergammaglobulinaemia, a low albumin/globulin ratio (<0.4), non-regenerative anaemia, lymphopenia and sometimes raised bilirubin and liver enzymes.
Abdominal ultrasound
Lets you see fluid accumulation in the abdomen, organ lesions, lymphadenopathy and thickened intestinal loops. It is essential in dry FIP to assess how far the lesions extend.
Fluid analysis (wet FIP)
If there is abdominal or thoracic fluid, analysing it is essential. Features typical of FIP: yellowish, straw-coloured, viscous fluid, protein >35 g/L, low cellularity. The Rivalta test usually comes back positive, which helps tell it from a transudate, though on its own it does not confirm the diagnosis.
Specific PCR for the FIP mutation
The most specific test currently available. It detects the specific mutation of the FIP virus in fluid, tissue or, in some laboratories, blood. A positive PCR in abdominal fluid has very high specificity, and it is the test that takes a case from consistent with FIP to a confirmed diagnosis.
Biopsy (dry cases)
In dry FIP, when there is no fluid to analyse, a biopsy of an affected organ (kidney, lymph node, intestine) with histopathology and immunohistochemistry is the route to confirmation, though it is more invasive.
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-coloured 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 humour
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 favour, 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 anaemia. 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?
Tell us your situation and we'll tell you where to start.