Effects of broadening the gold standard on the performance of a chemiluminometric immunoassay to detect Chlamydia trachomatis antigens in centrifuged first void urine and urethral swab samples from men

Abstract

Traditionally, evaluations of nonculture assays for Chlamydia trachomatis are based on a comparison with urethral culture in men and cervical culture in women as the standard for positivity of infection, but it is known that culture may be less than 100% sensitive. A chemiluminometric immunoassay, Magic Lite (Ciba Corning, Medfield, MA) that detects C. trachomatis antigens was performed on centrifuged first void urine samples and urethral swabs collected from men attending a sexually transmitted disease (STD) clinic. Immunoassay performance was compared to urethral culture and also to a broader gold standard: an infected patient with positive culture results or a confirmed positive Chlamydiazyme enzyme immunoassay (Abbott, Chicago) result. Two studies were performed on a retrospective group of stored first void urine samples from 200 men and a prospective group of urethral swabs and first void urine samples from 199 men. Expanding the gold standard showed that a urethral swab assayed by culture had a sensitivity between 70.3% and 87.5%, with the following effects on immunoassay performance in the prospective study: the sensitivity of urethral swabbing was reduced from 96.2% to 78.4% (specificity increased from 96.0% to 98.1%) and first void urine sensitivity increased from 92.3% to 94.6% (specificity went from 87.9% to 93.8%). In the retrospective study, sensitivity of first void urine testing went from 91.4% to 92.5%, with a corresponding increase in specificity from 93.9% to 96.9%. This maneuver had relatively little impact on the negative predictive values, but dramatically increased the positive predictive values, for both samples.(ABSTRACT TRUNCATED AT 250 WORDS)