The Rivalta test (Figure 2) is an inexpensive and readily performed in-clinic test with an extremely high negative predictive value for FIP.5,13 This test is positive in effusions with high protein content (especially acute-phase proteins) and negative in pure transudates. A positive Rivalta test is consistent with FIP in kittens but is less specific in older cats, as septic peritonitis and neoplasia can result in a positive test; however, these conditions can be ruled out via cytology.
PCR is a valuable modern tool for FIP diagnosis because it allows direct detection of viral RNA; some experts question its use in all cases, but most consider it to be a useful tool. Detection of FECV by PCR in tissue or effusion is consistent with FIP but not definitive, as non-FIP FECV may also be present in the tissue of healthy animals; however, failure to detect FECV can indicate that FIP is unlikely.7 Some specific S protein gene mutations are associated with FIP, and PCR tests that target these mutations may have higher specificity17,18 (ie, FIP diagnosis is more likely if the detected virus has one of these mutations). Although potentially clinically useful, PCR testing is not comprehensive because novel mutations may be missed.17,19 PCR testing of feces is not useful because fecal coronavirus is not predictive of systemic spread.
PCR sample choice depends on disease presentation; in cases of wet FIP, effusion should be tested, and in cases of dry FIP, peripheral blood, CSF, aqueous humor, lymph nodes, and/or organ aspirates should be tested.16,20 Test results should be interpreted based on the sample submitted; a positive PCR that identifies specific S protein gene mutations in abdominal effusion likely indicates FIP, but a positive FECV result in peripheral blood is less specific.19,20 It should be noted that testing is done sequentially. Initially, reverse transcriptase PCR for FECV is performed. When the result is positive, a second PCR should be run to confirm the FECV is FIPV, not FECV, biotype.