TABLE 20-5.

Assays Available for Diagnosis of Feline Infectious Peritonitis

Assay	Specimen Type	Target	Performance
Fluorescent or immunoperoxidase antibody staining	Wash or effusion specimens, tissue aspirates, tissues obtained at biopsy or necropsy	FCoV	Gold standard for diagnosis. False negatives can occur in specimens that contain low numbers of macrophages or virus particles, or when virus is complexed by antibody. Immunofluorescence is more sensitive than immunoperoxidase methods. Non-specific staining may be interpreted as positive results by untrained personnel.
Serology	Blood, CSF, aqueous humor, effusion fluid	FCoV antibody	Positive antibody titers reflect only antibodies to a coronavirus and are not specific for a diagnosis of FIP. Most cats in multicat households test positive. Negative titers can occur in cats with advanced FIP. High titers in cats that do not reside in multicat households and that have signs suggestive of FIP may support the diagnosis. Interlaboratory variation in methodology and titer reporting occurs.
RT-PCR	Blood, wash or effusion specimens, tissue aspirates, tissues obtained at biopsy or necropsy	FCoV RNA	Does not differentiate between virulent and avirulent FCoV strains, and avirulent strains may be found in tissues and blood. Sensitivity and specificity can vary depending on assay design. False negative results occur when virus levels are low, when variant virus strains are present, or as a result of degradation of viral nucleic acid during specimen transport.
Histopathology	Usually necropsy specimens, but also biopsies	Inflammatory lesions induced by FCoV (pyogranulomatous vasculitis)	Biopsy is often not feasible antemortem as a result of critical illness and coagulopathies.

FCoV, feline coronavirus; FIP, feline infectious peritonitis; RT-PCR, reverse transcriptase–polymerase chain reaction.