Diagnosing FIP

Overview

FIP diagnosis, treatment response, and practical decision-making

The option of treatment also presents the challenge in shifting diagnostic patterns and goals to balance both time and financial resources between diagnostics and treatment. Initiating treatment quickly can be key to success, especially for wet FIP or FIP cats in critical condition.

FIP can be difficult to definitively diagnose, but it's not necessary, and rarely advantageous, to exhaust all diagnostic options, since response to treatment can itself be the confirming diagnostic for a presumptive diagnosis.

Key idea

A positive response to FIP antiviral treatment can support the diagnosis.

Time matters

Rapid treatment may be important, especially for wet FIP or critical cats.

Practical goal

Balance diagnostic confidence, time, cost, and patient stability.

How Much Diagnostic Info Is Enough?

It depends on the case, patient stability, and available evidence

So how much diagnostic info is enough? It depends on the situation -- what pieces of information you have, how strongly the history, signalment, and diagnostics that you do have point to FIP, as well as the stability of the patient, and the financial constraints of the owner.

History and signalment are important guides here -- for example: A young male cat who was recently neutered and now presents as febrile and lethargic, with bloodwork showing A/G = 0.4 and lymphopenia.

In a case such as this, further diagnostics other than exams to check for presence of ocular or neurological involvement, as that would affect the treatment plan, should not be necessary to arrive at a presumptive diagnosis of FIP. If a mature adult cat were to present in this way, further workup such as ultrasound would not be unreasonable -- however a treatment trial would still be a valid option.

When effusion is present in a young cat, confirming that the distention is due to ascites and sampling it to confirm that it is consistent with effusion due to FIP is probably enough to begin discussion about a treatment trial due to the rapid progression of effusive FIP. Further diagnostics can be continued in parallel if necessary.

In cases where the diagnostic picture is more cloudy, a treatment trial may be preferable to invasive procedures like biopsy or exploratory surgery, both from the standpoint of impact on the patient, and financially for the owner.

Building a Diagnostic Wall

Diagnosis is usually built from multiple pieces of evidence

While it can be difficult or impossible to get definitive identification of FIP, in most cases a confident diagnosis can be made from a variety of evidence and diagnostics. Making a diagnosis relies on building a "diagnostic wall" of evidence as appropriate to the case.

The first place to start is to obtain the history and signalment of the cat, and consider the symptoms as presented upon exam and from description of the owner.

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History & Signalment

Age, breed, origin, stress events, and clinical history.

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Clinical Signs

Fever, lethargy, weight loss, jaundice, effusion, ocular or neurological signs.

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Lab Patterns

CBC, chemistry, A/G ratio, globulins, bilirubin, and inflammatory patterns.

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Imaging & Samples

Ultrasound, effusion analysis, cytology, PCR, aspirates, or CSF when appropriate.

History and Signalment

Important factors to consider

Age

70% of FIP cases occur in cats 1.5 years of age or less.

Breed

Pure-bred cats have three times greater incidence of FIP than random-bred cats due to genetic factors passed in bloodlines.

Origin

Cats from crowded or stressful multi-cat environments, for example hoarding situations or crowded shelters, are more likely to develop FIP.

Profiles of cats who developed FIP commonly show the following in their history:

Failure to thrive Recent stressful event Surgery Re-homing Vaccination Other illness Weight loss

Symptoms That Should Raise Suspicion for FIP

Common warning signs seen in FIP discussions

🌡️ Cyclical antibiotic-unresponsive fever

🟡 Jaundice

🫧 Abdominal distention with suspicion of ascites

🫁 Dyspnea with suspicion of pleural fluid

👁️ Uveitis or retinitis, unilateral or bilateral

🧠 Neurological symptoms such as ataxia, seizures, rear leg weakness, changes in gait, tremors, etc.

When doing the exam, for treatment purposes, it is NOT of importance to treatment to differentiate between wet and dry FIP, however it is crucial to ascertain if neurological or ocular symptoms are present, as this will affect the dosing and treatment plan necessary.

A basic eye and neurological exam, including evaluation of gait, wheelbarrow, positioning, placement, and related tests, may be important. Neurological and ocular symptoms can be present with any form of FIP, however they present more commonly with dry FIP than with wet FIP.

Bloodwork and Basic Diagnostic Tests

CBC and chemistry panel are useful for diagnosis and baseline monitoring

The next step is basic diagnostic tests. Obtaining a CBC and chemistry panel is useful for both diagnostic purposes and to serve as a baseline for treatment progress and is therefore recommended. Note though that not all cats will have strong indications of FIP in bloodwork, and that ocular and neurological FIP often compartmentalizes in the brain and eyes and can often present with unremarkable bloodwork.

Important: FECV titer is NOT diagnostic for FIP

A FECV titer is NOT diagnostic for FIP, and neither a positive nor negative result should be used to confirm or rule out FIP. FECV antibody titers can vary greatly over time, therefore measurement of a single antibody titer at a single random time point is not diagnostically significant. High and rising titer values can also be found in healthy FECV-infected cats, particularly in conjunction with FECV reinfection and therefore cannot confirm a suspicion of FIP. Similarly, a negative FECV antibody test result does not exclude FIP. Low or negative titers are seen in cats with both wet and dry FIP, in fact approximately 10% of cats with FIP do not have serum antibodies. FECV titer testing is not recommended and discouraged except in very rare cases.

The following bloodwork findings are of diagnostic significance, but are not definitive:

Anemia

Leukocytosis

Usually associated with neutrophilia

Lymphopenia

Hyperproteinemia

Hyperglobulinemia

Hypoalbuminemia

Albumin:globulin ratio <0.6

Hyperbilirubinemia and associated hyperbilirubinuria

Due to destruction of red blood cells

May or may not have abnormal hepatic values

When analyzing bloodwork, it is important to remember that none of these findings are present in all cats with FIP and many other conditions can cause any of these findings. FIP cannot be definitely diagnosed or ruled out based on bloodwork alone.

For example, while an A/G ratio of less than 0.6 is a classic diagnostic finding for FIP, FIP cats of all forms have been found to have higher ratios -- and conversely many conditions, including common ones like dental disease can cause an A/G ratio below 0.6.

Diagnostic Imaging

Imaging can help detect effusion, organ involvement, or lymph node changes

Diagnostic imaging can often be indicated to detect the presence of effusion, and/or organ or central lymph node involvement. Findings of diagnostic significance would include:

Ascites

Abdominal, pleural, or pericardial effusion.

Abdominal lymphadenopathy

Particularly mesenteric lymph node involvement.

Organomegaly

Many different organs, including the heart, may be affected.

Cardiac involvement

Cardio involvement can present similarly to HCM.

Granulomatous changes

Evidence of granulomatous changes or masses.

Similar to the hematological findings, it is important to remember that none of these findings are present in all cats with FIP and many other conditions can cause any of these findings.

In cases where neurological FIP is suspected, MRI may be indicated to look for changes consistent with FIP.

Ascites Fluid and Effusion Testing

Fluid appearance, Rivalta test, protein, cytology, and PCR

When ascites is present, there are several tests that are diagnostically significant. Importantly a FIP PCR test can be run on ascites fluid and some of these can provide definitive confirmation of FIP, although it cannot rule it out.

Ascites fluid from FIP typically appears:

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Yellow

Due to bilirubin.

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Mucinous

Sticky or stringy fluid appearance.

The Rivalta test is a quick, inexpensive test which can can be run in-house with only distilled water and vinegar or acetic acid. Instructions can be found here. The Rivalta test does not detect the FIP virus, instead, it differentiates between a transudate from an exudate. This test has a 86% PPV and 97% NPV for FIP.

Fluid analysis/cytology findings include:

Protein > 3.5 g/dl

Low cellularity

Non-degenerative neutrophils

Monocyte/macrophages

Large foamy macrophages

Lymphocytes

RT-PCR Tests and Advanced Sampling

Useful when positive, but negative results cannot rule out FIP

RT-PCR Tests can detect specific mutations of FECV -- in some tests such as IDEXX they are able to specifically identify the biotype as FIP vs FECV.

RT-PCR has very good specificity and fair sensitivity for effusion but it is not useful for blood. Positive results are considered reliable, but there is approximately a 30% false negative rate meaning that a negative result cannot rule out FIP.

One note of significance with these tests is that the turnaround time to get a result may affect the choice as to which tests are appropriate or whether or not it is prudent to start treatment in parallel with diagnosis: a Rivalta test can be done immediately, protein values on fluid can be done in house or usually with 24 hour turnaround from a diagnostic lab, but RT-PCR tests frequently require as much as a week before getting results.

When there are inflamed lymph nodes or other lesions, a fine needle aspirate can be obtained and can also be tested via RT-PCR. Invasive biopsies and exploratory surgeries should generally be avoided unless absolutely necessary -- a treatment trial often is more appropriate in those cases.

RT-PCR tests can also be run on aqueous humor when there is ocular involvement and CSF samples.

Medical Disclaimer

Educational content only

Important Notice

This content is provided for educational purposes only and does not constitute veterinary diagnosis, treatment, prescription, or medical advice. Always consult a licensed veterinarian for diagnosis and treatment decisions.