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Diagnosing FIP

The diagnosis of  FIP has a reputation for being challenging, but can often be fairly straight forward.     With the advent of anti-viral treatment, there is now an additional diagnostic option, since a positive response to FIP anti-viral treatment is itself diagnostic.

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.  

How much diagnostic info is enough?

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.

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 (ultrasound, etc) 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.


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.  Important factors to consider are:

  • 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, 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

  • Common symptoms that should raise suspicion for FIP:

    • cyclical  antibiotic-unresponsive fever

    • jaundice

    • abdominal distention  (with suspicion of ascites)

    • dyspnea  (suspicion of pleural fluid)

    • ​uveitis or retinitis (unilateral or bilateral)

    • neurological symptoms (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 (evaluation of gait, wheelbarrow, positioning, placement, etc. tests)  Neurological and ocular symptoms can be present with any form of FIP, however they present more commonly with dry FIP than with wet FIP.  

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: 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 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 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 (abominal, pleural, or pericardial)

  • Abdominal lymphadenopathy 

    • particularly mesenteric lymph node​

  • organomegaly

    • many different organs, including the heart may be affected​

    • Cardio involvement can present similarly to HCM

  • 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.

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:

    • Yellow (due to bilirubin) ​

    • Mucinous (sticky or stringy)

  • 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 differentiate 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 can detect specific mutations of FECV -- in some tests (IDEXX) they are able to specifically identify the biotype as FIP vs FECV.

    • ​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.

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