Diagnosing FIP

With no specific test for FIP, vets evaluate the cat's symptoms, diagnostics, and rule out differential diagnosis.

How FIP Is Diagnosed

Until recently, FIP could only be confirmed via autopsy. In 2019, there are conclusive, definitive tests for FIP,  though not all vets are aware of the advancements in diagnostics just yet.   It is imperative that you arm yourself with as much information as possible, so you can best advocate for your cat and get a correct diagnosis.  

If there is a presumptive diagnosis of FIP, it is also possible to use GS diagnostically.  GS will only treat FIP; therefore, if the cat improves while on GS, the diagnosis is confirmed. 

Diagnosing Effusive ("wet") FIP 

Effusive FIP is characterized by a build up of fluid in the cat’s abdomen (ascites), lungs (pleural effusion) or heart (pericardial effusion).  Cats who have ascites will often appear pot-bellied and their belly may feel like a water balloon.  If the effusion is in the heart or lungs, the cat may not have any outward changes in appearance.  If the fluid build up is severe, breathing may sound congested or be visibly labored.  

Complete CBC & Chemistry Panel

Whenever wet FIP is suspected, a complete blood panel and chemistry panel should be the first step toward a diagnosis.  The typical FIP blood work will show low albumin, high globulin, high white blood cell count, low red blood cell count, high neutrophils, high protein, and high bili.  Most FIP cats will have non-regenerative anemia, as well as a persistent fever.  Not all blood results will look the same; however, if some markers are present and the cat’s symptoms suggest a presumptive diagnosis of FIP, further diagnostics are generally recommended.

FIV/FELV Test

In that both FIV and FELV are more common and can present similar to FIP, these should be tested for and ruled out as a primary or secondary diagnosis.   

RT-PCR of the effusion

Using a syringe, the veterinarian draws a sample of the fluid for testing.  This is a simple procedure that is done in the office, typically without the need for anesthesia or sedative. If FIP, the effusion is typically high in protein, straw or honey colored, and viscous.  Reverse transcriptase polymerase chain reaction (RT-PCR) detects the RNA of the FCoV – i.e. is a test which detects actual virus.   Quantitative RT-PCR (RT-qPCR) is an interesting recent development in which the amount of virus in the sample may be measured.  A positive RT-PCR on the fluid is a confirmed diagnosis of wet FIP.  A negative RT-PCR on fluid does not necessarily rule out FIP, as there is roughly a 30% chance of a false negative.  

DO NOT run a RT-PCR test on blood or feces RT-PCR on blood or feces can, and often does yield a false positive, or a false negative.

GS As A Diagnostic

In cases where FIP is the presumptive diagnosis, an alternative to extensive additional lab work or procedures is to use GS diagnostically.  GS441 does not treat or cure any condition other than FIP therefore, if the cat begins to respond after a few doses, FIP can be confirmed. If the cat does not have FIP, the GS will do no harm.  

NOTE: THERE IS NO SUCH THING AS AN “FIP TEST”.   A proper diagnosis requires a multitude of diagnostics both to confirm FIP, as well as to rule out out conditions that share similar symptoms.  What is sometimes referred to (incorrectly) as an “FIP test” measures the cat’s exposure to FCoV.  Ninety five percent of cats in the world have been exposed to FCoV. These titer tests are NOT conclusive. A positive titer for FCoV is NOT an indicator that the cat has FIP.    

We highly recommend downloading Dr Addie’s Diagnosis Algorithm below, and providing a copy to your veterinarian. (Diagnosis Algorithm is the work of Dr Diane Addie and is copyright of catvirus.com)

Diagnosing Non-Effusive ("Dry") FIP

Dry FIP may present with a variety of symptoms, many of which are consistent with other conditions as well, which makes for a more challenging diagnosis.  Cats with dry FIP often present with lethargy, stunted growth,  inappetence, fever, anemia, and/or protruding third eyelid(s). There is often a history of diarrhea. 

If there is ocular involvement, uveitis, keratic precipitates, aqueous flare, or retinal vessel cuffing may be present.  

If the virus has affected the central nervous system or brain, neurological symptoms may include ataxia, tremors, seizures, or nystagmus (darting eyes).

Complete CBC & Chemistry Panel

Whenever dry FIP is suspected, a complete CBC and chemistry panel should be the first step toward a diagnosis. The typical FIP blood work will show low albumin, high globulin, high white blood cell count, low red blood cell count, high neutrophils, high protein, and high bili.  Most FIP cats will have non-regenerative anemia, as well as a persistent fever.  Not all blood results will look the same; however, if some markers are present and the cat’s symptoms suggest a presumptive diagnosis of FIP, further diagnostics are generally recommended.

Ultrasound

Dry FIP causes lesions to whichever organs are affected by the virus. An ultrasound can reveal whether or not lesions are present, as well as check for enlarged mesenteric lymph node(s) and enlarged kidneys.  During the ultrasound, a fine needle aspiration of the lesions may be done in the office, and with typically no need for anesthesia.  Samples should be sent to a lab for immunostaining. 

Immunostaining

Direct staining of FCoVs within macrophages by immunofluorescence in cytocentrifuged effusions or immunohistochemistry in tissue is considered the most specific test to confirm FIP. Immunostaining cannot differentiate between the “harmless” FCoV and FIP-causing FCoV, but finding infected macrophages in characteristic pyogranulomatous lesions or in inflammatory effusions is highly associated with FIP. In a recent study in which a large number of cats with confirmed FIP and controls with other confirmed diseases were investigated, positive immunofluorescence staining of intracellular FCoV antigen in macrophages of the effusion was 100% predictive of FIP. Although immunostaining has a high positive predictive value, the negative predictive value is not high, which means that a positive result is likely to be a true positive, while a negative result may not be a true negative. Therefore, a negative result does not rule out a diagnosis of FIP. 

GS As A Diagnostic

In cases where FIP is the presumptive diagnosis, an alternative to extensive additional lab work or procedures is to use GS diagnostically.  GS441 does not treat or cure any condition other than FIP therefore, if the cat begins to respond after a few doses, FIP can be confirmed. If the cat does not have FIP, the GS will do no harm.  

NOTE: THERE IS NO SUCH THING AS AN “FIP TEST”.   A proper diagnosis requires a multitude of diagnostics both to confirm FIP, as well as to rule out out conditions that share similar symptoms.  What is sometimes referred to (incorrectly) as an “FIP test” measures the cat’s exposure to FCoV.  Ninety five percent of cats in the world have been exposed to FCoV. These titer tests are NOT conclusive.  A positive titer for FCoV does NOT indicate that the cat has FIP.    

We highly recommend downloading Dr Addie’s Diagnosis Algorithm below, and providing a copy to your veterinarian.  (Diagnosis Algorithm is the work of Dr Diane Addie and is copyright of catvirus.com)

Dedicated to all FIP angels.

All information contained on this website is compiled from real-life experiences of cat owners who are currently, or have previously treated their cats for FIP.  Most of us are not veterinarians and the  information provided within is not intended to substitute or replace medical care by a licensed veterinarian.