Objective. To describe a pseudo-outbreak of Clostridium difficile infection (CDI) caused by a faulty toxin assay lot and to determine the effect of sensitivity, specificity, and repeated testing for C. difficile on perceived CDI burden, positive predictive value, and false-positive results. design. Outbreak investigation and criterion standard. patients. Patients hospitalized at a tertiary care hospital who had at least 1 toxin assay for detection of C. difficile performed during the period from July 1, 2004, through June 30, 2006. methods. The run control chart method and the x 2 test were used to compare CDI rates and the proportion of positive test results before, during, and after the pseudo-outbreak. The effect of repeated testing was evaluated by using 3 hypothetical models with a sample of 10,000 patients and various assay sensitivity and specificity estimates. results. In November of 2005, the CDI rate at the hospital increased from 1.5 to 2.6 cases per 1,000 patient-days (P <.01), and the proportion of positive test results increased from 13.6% to 22.1% (P <.01). An investigation revealed a pseudo-outbreak caused by a faulty toxin assay lot. A decrease of only 1.2% in the specificity of the toxin assay would result in a 32% increase in perceived incidence of CDI at this institution. When calculated by use of the manufacturer's stated specificity and sensitivity and this institution's testing practices, the positive predictive value of the test decreased from 80.6% to 4.1% for patients who received 3 tests. conclusion. Specificity is as important as sensitivity when testing for CDI. False-positive CDI cases can drain hospital resources and adversely affect patients. Repeated testing for C. difficile should be performed with caution.