Incidence of the Prozone Phenomenon in Syphilis Serology

ObjectiveTo determine the rate of the prozone phenomenon in our patient population.MethodsSera from 4328 patients—3504 females (2065 pregnant, 1439 nonpregnant) and 824 males—were tested for syphilis by the rapid plasma reagin (RPR) test, and then rechecked with serial twofold dilutions of up to 16-fold to detect the prozone phenomenon. Chi-square analysis with Yates correction was used, with P ≤ .05 considered significant.ResultsThe total positivity rate in females was 6% (213 patients); 13% (27) of the positive tests were false-positive reactions as confirmed by a negative anti-treponemal antibody test. Only one prozone reaction was detected, in serum from a male subject, but it was not missed on initial screening.ConclusionThe rate of prozone phenomenon is very low (95% confidence interval 0–0.4%), and routine serial dilutions are not cost effective.