Diagnosis via serology or reverse transcriptase PCR (RT-PCR) of infectious materials can be difficult, as subclinical cats may have high titers and severely ill cats may have no titers. Cats with FIP typically exhibit higher coronavirus titers than cats with FECV that are not persistently infected.18
Cats are often infected with multiple variants of FECV, most of which are benign and create false-positive results if primers do not adequately target likely mutation sites. PCR should therefore be specific enough to identify FIP variants without missing unique mutations. These mutations can also cause FIP but are outside the boundaries of PCR primers used on the S gene, which can generate false-negative results.3,19 RT-PCR results should be verified via biopsy with immunohistochemistry.20 Sequencing the S gene allows all mutations in the S gene to be identified but is not widely available; results should be interpreted in conjunction with other diagnostic results.
Surgical biopsy with histopathology or fine-needle aspiration with cytology of affected organs is often used with immunohistochemistry to support diagnosis and determine disease stage.21 Full fluid analysis of cavitary effusions associated with FIP commonly demonstrates yellow, viscous fluid with an elevated protein concentration (>3.5 g/dL) and disproportionately low cell count.22 Fluid analysis in addition to other diagnostics (eg, serology, RT-PCR, fine-needle aspiration) performed on a variety of tissues can increase the probability of confirming diagnosis of FIP based on cumulative test results, signalment, and patient history.3 Histopathology, immunohistochemistry, and RT-PCR have 40% to 85% sensitivity and 83% to 100% specificity (depending on tissue type), reinforcing the need for a multifaceted diagnostic approach.20,21