Repeated immunoassays are not helpful in diagnosing an infectious form of colitis and diarrhea
Research Activities, January 2009, No. 341
Enzyme immunoassay (EIA) is the widely used technique to diagnose Clostridium difficile-associated disease (CDAD), an infectious form of colitis and diarrhea. Clinicians often repeat EIAs three times to definitely rule out CDAD, but a new study finds that repeating the newest version of the C. difficile EIA rarely aids in diagnosing the disease.
The researchers, all affiliated with Tufts-New England Medical Center, reviewed all C. difficile EIAs performed by their institution's microbiology laboratory during 2005. The EIA used at the medical center has been shown to be nearly as good in ruling out CDAD as the "gold standard" cellular cytotoxin assay (negative predictive values [NPVs] of 98.8-99.8 percent). Because the researchers were primarily interested in comparing a single EIA with repeated EIAs, they calculated a NPV relative to a single immunoassay. They found that of the 2,938 C. difficile EIAs performed in 2005, 91 percent were negative for the bacterium (which is ubiquitous in air, soil, water, feces, and on most surfaces) and 9 percent were positive. Patients were diagnosed with CDAD on the first test in 85 percent of the confirmed cases. Only 15 patients had a positive second or third test within 7 days of a negative previous EIA, giving a relative NPV of 97 percent for just 1 test.
Based on their findings, the researchers conclude that, when using the most modern version of the C. difficile EIA, repeated diagnostic testing is only warranted when there is high suspicion of CDAD based on clinical observations. In addition, they recommend that no more than two EIAs should be performed for this bacterium within a 7-day period.
The study was funded in part by a training grant from the Agency for Healthcare Research and Quality (T32 HS00060). More details are in "Repeated enzyme immunoassays have limited utility in diagnosing Clostridium difficile," by Marci Drees, M.D., David R. Snydman, M.D., and C.E. O'Sullivan in European Journal of Clinical Microbiology and Infectious Diseases 27, pp. 397-399, 2008.