Value of tests to diagnose FIP

Jeffery U, Deitz K and Hostetter S. Positive predictive value of albumin: globulin ratio for feline infectious peritonitis in a mid-western referral hospital population. J Feline Med Surg. 2012; 14: 903-5. 
 
The definitive method to diagnose feline infectious peritonitis (FIP) is biopsy and histopathology. A previous report has indicated that a low serum albumin to globulin (A:G) ratio has a high positive predicative value for FIP in cats with clinical signs highly suggestive of the disease. Yet, FIP can present with more vague clinical signs. Predictive values (positive and negative) depend significantly on the prevalence of the disease in the tested population. The positive predictive value is the probability that an animal with a positive test truly has the target disease. When prevalence of the disease is high, the positive predictive value of a test will be greater than when prevalence is low. The authors of this retrospective study investigated the performance of low A:G ratio as a diagnostic test for cats with vague signs of illness consistent with a possible diagnosis of FIP. They wanted to establish the positive predictive value of A:G less than 0.8 and less than 0.6 for FIP in a referral hospital in a mid-west population of cats with one or more clinical signs compatible with FIP (lethargy and inappetance, along with weight loss, pyrexia, effusion, neurological signs, diarrhea, mild upper respiratory tract signs, ocular lesions, and jaundice).

The findings of this study show the importance of determining diagnostic test performance in relation to the population in which the test will be performed. In this study’s results, a cat with an A:G of 0.6 or more was highly unlikely to have FIP. Also, in this population of cats (those visiting a referral hospital in mid-west United States) where the prevalence of FIP is low, a low A:G is of little value in confirming FIP. Therefore, using an A:G of less than 0.8 or less than 0.6 to support a diagnosis of FIP in a patient from a population where FIP would be one of several differential diagnoses carries a high risk of a false-positive diagnosis. [VT]

See also: Fischer Y, Sauter-Louis C and Hartmann K. Diagnostic accuracy of the Rivalta test for feline infectious peritonitis. Vet Clin Pathol. 2012; 41: 558-67. 

Related blog posts:
Diagnosis of FIP (May 2011)
Development of new therapies for FIP (March 2012)