An In-Clinic Test Option for FIP Diagnosis
Margie Scherk, DVM, DABVP (Feline Practice), catsINK
This study aimed to calculate sensitivity, specificity, and positive and negative predictive values (PPV, NPV, respectively) of the Rivalta test for feline infectious peritonitis (FIP). The Rivalta test relies on the formation of a precipitate when a fluid sample from an effusion is added to acetic acid.
Of 851 cats with effusions, 782 had conclusively positive or negative Rivalta test results; definitive diagnosis was reached in 497. Prevalence of FIP in cats with effusion and positive Rivalta test results was 34.6%—lower than the 41%–51% previously reported. Increased effusion-specific gravity and cholesterol concentration, decreased serum albumin:globulin, and hyperbilirubinemia positively correlated with positive Rivalta test results. The components of effusion that lead to a positive Rivalta test are unknown. The Rivalta test had a sensitivity of 91.3%, specificity of 65.5%, PPV of 58.4%, and NPV of 93.4% for FIP diagnosis. Sensitivity, specificity, and PPV were lower than values previously reported except when considering populations of cats ≤2 years of age. Values also increased when cats with lymphoma or bacterial infections were excluded. Although sensitivity was lower than previously reported, it is high enough to be a useful diagnostic aid. In particular, NPV is excellent, so FIP is unlikely as a cause for effusion in cats with negative Rivalta tests.
Commentary
For FIP diagnosis, many European countries rely on the Rivalta test. Although it is a simple in-clinic test, it does not add any significant information to that which one may receive from determining a high protein level using a refractometer ± cytology. Its use should be limited to clear fluids, and it is most accurate in cats <2 years of age. Its value may be in its high NPV (ie, if a negative Rivalta result is found on a clear cavity fluid, FIP is highly unlikely). Serum albumin:globulin ratios <0.45 with peripheral lymphopenia, lack of toxic change in neutrophils, and high fluid protein are highly suggestive in the diagnostic puzzle of FIP.
Editor's note: This article was originally published in December 2013 as "Rivalta Test for FIP Diagnosis"