FIP Diagnostics
and Testing

Treatment Trials with GS-441524

When FIP is the presumptive diagnosis or one of the differentials being considered, initiating a treatment trial with GS-441524 is highly recommended. GS has been proven to be a very safe antiviral, making it an effective and safe diagnostic tool. This approach can, in many cases, circumvent the need for more invasive and costly testing procedures (Taylor et al., 2025).

Cats with FIP typically exhibit a significant response to GS treatment within the first 2-5 days. In some cases, however, cats may exhibit a slower response to treatment of up to 10 days. Monitor for signs of progress, including improved overall behavior, normalized temperature and reduced severity or resolution of ocular and neurological symptoms. With effusive forms of FIP, pleural and abdominal effusions may take longer to resolve. If no improvement is seen with adequate GS dosing, further diagnostic evaluation is warranted to determine an alternative diagnosis (Taylor et al., 2025).

Taylor, S., Tasker, S., Barker, E. & Gunn-Moore, D., Sorrell, S., Cerna, P. & Coggins, S. (2025). icatcare_fipupdate_July25. https://icatcare.org/resources/icatcare_fipupdate_july25.pdf

FIP Presentation

FIP commonly affects young cats, particularly those under 2 years old. Cats in multi-cat facilities, such as shelters and catteries, purebred cats, and male cats may also be more susceptible. A genetic predisposition, co-infection such as FeLV/FIV, combined with environmental factors, such as rehoming, overcrowding, and other stressors can contribute to the mutation of the feline coronavirus into FIP (Thayer et al., 2022).

As Thayer et al. explains (2022), FIP can manifest in various ways, including:

– Inappetence

– Weight loss

– Lethargy

– Jaundice

– Enlarged lymph nodes

– Pyrexia

– Ascites in the abdomen and/or chest

– Pyogranulomatous lesions and masses

– Ocular signs such as uveitis, colour change to the iris, blindness, retinal detachment

Neurological signs including ataxia, anisocoria, seizures, abnormal behaviour/mentation, aggression, nystagmus, hyperesthesia (Thayer et al., 2022).

Effusive FIP is characterized by the accumulation of fluid in the abdominal and/or pleural cavities. A straw-coloured, sticky, protein-rich fluid is commonly seen, and, in some cases, it is possible for the fluid to be pink tinged or watery, and non sticky (Thayer et al., 2022).

Diagnostic Blood work:

*Important* FECV antibody titers are NOT recommended for diagnosis. Most cats have coronavirus, and it is not a useful diagnostic tool for FIP.

Thayer et al. (2022) recommends a CBC and Biochemistry panel to aid in diagnosis. Many cats with FIP will show markers in the bloodwork, including:

– Hyperglobulinemia,

– Hypoalbuminemia

– Low A/G ratio (below 0.6)

– Anemia

– Lymphopenia

– Neutrophilia

– Hyperbilirubinemia

– Abnormal hepatic values

Thayer, V., Gogolski, S., Felten, S., Hartmann, K., Kennedy, & M., Olah, G.A. (2022). AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. Journal of Feline Medicine and Surgery. 2022;24(9):905-933. https://doi.org/10.1177/1098612X221118761

Tests to aid a FIP Diagnosis

Effusive FIP testing:

– A Rivalta test can be performed in clinic and is an inexpensive and effective diagnostic tool. If the result is negative, FIP is highly unlikely.

(Instructions for the Rivalta test can be found here: https://pmc.ncbi.nlm.nih.gov/articles/PMC11891792/)

– Fluid cytology will typically show high protein levels (>35 g/l) and low white blood cell counts which can cause variation between modified transudate and exudate. However, in some cases (e.g., secondary bacterial peritonitis) the fluid may have high cell counts. Analysis often reveals a low albumin-globulin ratio (<0.4). It is common to see minimal lymphocytes, nondegenerate neutrophils and macrophages (Thayer et al., 2022).

– RT-PCR tests can be a useful diagnostic as they can identify specific mutations of feline coronavirus. IDEXX has a test that is able to distinguish between FIP and FECV biotypes. When testing effusion, these tests have good specificity and reasonable sensitivity but are not useful for testing blood. A positive result is considered reliable. However, the approximately 30% false negative rate means that a negative test result does not eliminate the possibility of it being FIP.

– The IDEXX PCR attempts to distinguish FIPV from FECV. However, as it cannot detect all FIPV mutations, in a cat that has supportive clinical signs and tests positive for general FCoV, but is still negative for FIPV bio typing, this case is likely still a true positive and a treatment trial is warranted.

Diagnostic imaging can be helpful in identifying characteristic features of FIP, such as:

– Enlarged lymph nodes, particularly mesenteric lymph nodes

– Granulomas and masses

– Ascites (abdominal, pleural or pericardial) (Thayer et al., 2022).

Diagnosis can also be confirmed through:

Immunocytochemistry on fluid samples

Immunohistochemistry with supportive histopathological findings on tissue biopsies (Thayer et al., 2022).

Thayer et al. (2022) discusses the importance of being aware that FIP can present in many different ways, including with clinical signs related to the affected organs, such the kidneys and liver. Some cats with FIP may not exhibit the typical abnormalities in bloodwork, such as elevated bilirubin, hyperglobulinemia, hypoalbuminemia and/or a low a/g ratio. Fever may or may not be present. While older cats from single-cat homes may not fit the classic picture, this does not exclude the possibility of FIP (Thayer et al., 2022).

FIP Diagnostics and Testing

Treatment Trials with GS-441524

FIP Presentation

Tests to aid a FIP Diagnosis

Questions? Contact Us

FIP Diagnostics
and Testing