Feline Infectious Peritonitis (FIP): an in-depth look.
Please note: this section is intended for individuals who want to learn more and have a deeper understanding of feline coronavirus and FIP. Basic knowledge of veterinary/scientific terms is needed to fully comprehend this section which was created for information purposes ONLY.
If you are a veterinarian, veterinary student, vet tech or hold any other type of position in the veterinary field, please visit Dr. Diane D. Addie’s website catvirus.com.
If you are a pet parent with a sick cat, please consult a qualified veterinarian IMMEDIATELY.
The authors exclude all liability whatsoever for any loss or damage arising out of the use of this website or reliance upon its content and STRONGLY advises anybody with veterinary-related questions to consult a qualified veterinarian. No responsibility can be accepted.
FIP is one of the most difficult diseases to diagnose in the living animal because it can present with almost any clinical signs. Also, there is no single diagnostic test for FIP thus diagnoses can only be confirmed by histopathology. At the time of writing, the only way to be absolutely certain (100%) of an FIP diagnosis is to biopsy the affected tissues and have them examined by a veterinary pathologist.
Regardless of what some laboratories and manufacturers of test kits claim, there is NO single test for FIP. Most tests which claim to be FIP tests are really tests for feline coronavirus (FCoV) RNA or antibodies. While these tests have their uses, they are unable to differentiate the FCoV-infected (seropositive) cat suffering from FIP from the FCoV-infected (seropositive) cat with a lookalike condition. Diagnosis of FIP is a challenge to even the most competent veterinary clinician.
Dr. Diane Addie has developed a FIP diagnosis algorithm (click here) in which she recommends RT-PCR testing of the effusion or an organ sample drawn by a needle as a diagnostic technique preferable to biopsy. Only 18% of samples sent to her laboratory for FIP diagnosis actually turn out to be FIP. Since cats with FIP are usually euthanized, it is absolutely vital that FIP is accurately differentiated from other, treatable, conditions.
To learn more about diagnosing FIP, visit catvirus.com
Diagnosing effusive FIP
In recent years diagnosis of effusive FIP has become more straightforward. Detection of viral RNA in a sample of the effusion by reverse-transcriptase polymerase chain reaction (RT-PCR) is diagnostic of effusive FIP. 
To perform an RT-PCR test, the sample must be sent to an external and reliable veterinary laboratory. On the other hand, within the veterinary hospital there are a number of tests which can rule out a diagnosis of effusive FIP within minutes:
a. Measure the total protein in the effusion: if it is less that 35g/l – FIP is extremely unlikely.
b. Measure the albumin to globulin ratio in the effusion: if it is over 0.8 – FIP is ruled out and if it is less than 0.4 – FIP is a possible (but not certain) diagnosis. 
c. Examine the cells in the effusion: if they are predominantly lymphocytes then FIP is excluded as a diagnosis.
The effusion is a modified transudate, has the same consistency as plasma and it is straw colored and odorless, it clots on exposure to air and the amount varies from a few mLs to several hundred mLs in the worst cases. The effusion should be sterile. Two major differential diagnoses are bacterial peritonitis and pleurisy, where there are vast numbers of white blood cells in the effusion and bacteria is present. Cats with effusive FIP are often bright and eating though rapidly become dull and anorexic. Without effective treatment, death is usually within a few days.
Major differential diagnoses for effusive FIP:
Tumor (adenocarcinoma, lymphoma)
Liver disease (tumor, cirrhosis, lymphocytic cholangitis)
Bacterial peritonitis or pleurisy
Pregnancy or obesity
Send an effusion sample (not a blood sample) for RT-PCR testing.
A positive FCoV RT PCR on an effusion IS DIAGNOSTIC OF FIP.
However, a negative result does not rule out FIP: it depends on the sensitivity of the RT-PCR test.
Diagnosing non- effusive FIP
Non-effusive FIP is more difficult to diagnose than effusive FIP as fewer but larger pyogranulomata form over longer periods of time and the clinical signs are often directly attributable to the location of the lesions. Cats with non-effusive FIP are dull and typically present with weight loss, anorexia, moderate pyrexia (fever of 103 – 104F / 39 – 39.5C which is unresponsive to antibiotics or recurrent). Some have intra-ocular signs, such as uveitis and most cats with dry FIP have palpably enlarged mesenteric lymph nodes.
Because the clinical signs of non-effusive FIP tend to be vaguer and varied – the list of differential diagnoses is therefore much longer. Non-effusive FIP diagnoses should be considered when the following criteria are met: 
The cat is young (under two years old) and purebred: over 70% of FIP cases are in pedigree kittens.
The cat experienced stress such as recent rehoming, neutering or vaccination. 
The cat had an opportunity to become infected with FCoV – origins traced back to a breeding cattery, rescue organization, etc. or the recent introduction of a purebred kitten or cat into the household.
b. Clinical signs: 
The cat has become anorexic or is eating less than usual
The cat lost weight or failed to gain weight
The cat has pyrexia (fever) of unknown origin
The cat has intra ocular signs
The cat has icterus
Raised bilirubin without liver enzymes being raised
Either normal or slightly low albumin levels (giving a low albumin: globulin ratio)
AGP levels are greater than normal
Non-regenerative (usually mild) anemia
The cat has high FCoV antibody titers (use this parameter with extreme caution because of the high prevalence of FCoV in breeding and rescue catteries)
Non-effusive FIP can be ruled out if the cat is FCoV seronegative – provided the antibody test used has excellent sensitivity. 
The course of non-effusive FIP is chronic with the cats, frequently surviving weeks to months on treatments. Once neurological signs (nystagmus, ataxia, seizures, and paralysis) begin then death rapidly ensues.
A positive FCoV serology IS NOT DIAGNOSTIC OF FIP.
A positive FCoV antibody test does NOT mean the cat has FIP: only that the cat has been exposed to FCoV infection.
Send a blood sample (not effusion sample) for FCoV antibodies testing.
Blood samples should NEVER be sent for FCoV RT-PCR, even though some misinformed laboratories ask for blood samples for that test.
DISCLAIMER: The use of this website is at your own risk. This website is for information purposes ONLY, and it is NOT meant to replace a consultation with a fully qualified veterinary surgeon (veterinarian). It is NOT intended to be used to diagnose or treat any cat. The creators share their personal experiences, recommendations of treatments, foods, medications, supplements, and products for informative and educational purposes exclusively. The information in this site cannot and should not be used as a basis for diagnosis or choice of treatment. Creators and contributors exclude all liability whatsoever for any loss or damage arising out of use of this site or reliance upon its contents. Furthermore, creators and contributors strongly advise all users to always seek the advice of a qualified veterinarian and to obtain professional advice on the correct regimen for your cat and his or her particular situation. NO responsibility can be accepted.