Repeated enzyme immunoassays have limited utility in diagnosing Clostridium difficile

Many clinical laboratories use enzyme immunoassays (EIA) to diagnose Clostridium difficile-associated disease (CDAD). Clinicians frequently order three EIAs to "rule out" CDAD. We performed a retrospective cohort study to determine the clinical utility of repeating EIA testing to diagnose CDAD. We reviewed all EIAs performed by our laboratory during 2005, determined the total number of tests per patient and per testing episode, and calculated the relative negative predictive value (NPV) of one EIA compared to >= 2 EIAs. The laboratory performed 2,938 EIAs, of which 253 (8.6%) tests were positive. Most patients (85%) were diagnosed by the first EIA performed. Of >1,000 testing episodes that included >= 2 EIAs within 7 days, only 15 patients had a positive second or third test after negative initial testing. The relative NPV of the first EIA was 97.4%. These data suggest that using newer generation EIAs, repeated testing is of limited benefit in diagnosing CDAD.