TABLE 21-4.

Diagnostic Assays Available for Feline Immunodeficiency Virus Infection

Assay	Specimen Type	Target	Performance
Serology (ELISA, Western blotting, IFA)	Blood, serum	Antibody to FIV	Positive test results in cats >6 months of age that have not been vaccinated for FIV equal infection, but confirmation is recommended in healthy cats using a second test from a different manufacturer or Western blot. False positives occur in cats <6 months of age (maternal antibody) and cats vaccinated with the FIV vaccine. False negatives can occur in the first 2 months of infection or with advanced disease and severe immunosuppression.
PCR	Blood	FIV proviral DNA or viral RNA	Sensitivity and specificity vary between laboratories. Even well-designed assays may be insensitive because of viral variability and low-level viremia. PCR assays are the only assay that can definitively diagnose infection in cats vaccinated for FIV. Never use without performing serology.
Virus isolation	Blood	FIV	Difficult, not widely available. Used primarily as a research tool.

IFA, immunofluorescent antibody.