The high sensitivity of PCR assays for diagnosing Clostridium difficile infection (CDI) has greatly reduced the need for repeat testing after a negative result. Nevertheless, a small subset of patients do test positive within 7 days of a negative test. The aim of this study was to evaluate the clinical characteristics of these patients to determine when repeat testing may be appropriate. The results of all Xpert C. difficile PCR (Cepheid, Sunnyvale CA) tests performed in the clinical microbiology laboratory at New York-Presbyterian Hospital, Columbia University Medical Center (NYPH/CUMC) from 1 May 2011 through 6 September 2013, were reviewed. A retrospective case-control study was performed, comparing patients who tested positive within 7 days of a negative test result to a random selection of 50 controls who tested negative within 7 days of a negative test result. During the study period, a total of 14,875 tests were performed, of which 1,066 were repeat tests (7.2%). Eleven of these repeat tests results were positive (1.0%). The only risk factor independently associated with repeat testing positive was history of a prior CDI (odds ratio [OR], 19.6 [95% confidence interval {CI}, 4.0 to 19.5], P < 0.001). We found that patients who test positive for C. difficile by PCR within 7 days of a negative test are more likely to have a history of CDI than are patients who test negative with repeat PCR. This finding may be due to the high rate of disease relapse or the increased likelihood of empirical therapy leading to false-negative results in these patients. The incidence of Clostridium difficile infection (CDI) has been rising over the last 2 decades, leading to significant morbidity and mortality, particularly in elderly and hospitalized patients (1). PCR assays for the diagnosis of CDI have gained in popularity in clinical microbiology laboratories in recent years. These PCR tests are rapid, simple to perform, and offer a high degree of sensitivity and specificity for detecting toxigenic C. difficile strains, usually by detection of the tcdB gene that encodes toxin B production. The high sensitivities of these assays have greatly reduced the need for repeat testing after a negative result, especially within the first 7 days of a negative result. Multiple studies have demonstrated that repeat testing within 7 days yields positive results in only about 1 to 3% of cases (2-5). However, clinical microbiology laboratories continue to receive test requests for patients with a recent negative test result, presumably in part due to the perceived poor sensitivity of previously used enzyme immunoassays (EIAs). These unnecessary PCR tests lead to increased costs and inefficient utilization of resources.Our laboratory began using PCR testing for C. difficile on 1 May 2011, and due to the low yield of repeat testing, instituted a policy on 30 October 2013 to reject stool specimens received within 7 days of a negative PCR result unless permission was granted by the laboratory director. Despite the low probability of a positive ...
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