An unusual cause of empyema thoracis.

The second observation was the error made in calculating the positive predictive value. By deÞ nition, positive predictive value is “the probability that a person actually has the disease given that he or she tests positive”.[2] In the paper under discussion, the disease was represented by VZV ELISA positives (as the gold standard) and the test was represented by “recalled history of chicken pox”. According to this deÞ nition, the positive predictive value (PPV) of the recalled history of chicken pox should be calculated by using following formula.