Does empirical Clostridium difficile infection (CDI) therapy result in false-negative CDI diagnostic test results?

BACKGROUND Patients with suspected Clostridium difficile infection (CDI) often receive empirical therapy prior to collection of stool specimens for diagnostic testing. The likelihood that such empirical therapy might result in false-negative CDI test results is unknown. METHODS We conducted a prospective study of CDI patients to determine the time to conversion of CDI test results, including polymerase chain reaction (PCR) for toxin B genes, glutamate dehydrogenase, and toxigenic culture, from positive to negative during CDI therapy. We evaluated the frequency of and risk factors for persistence of positive CDI tests. For patients receiving empirical therapy, we assessed the frequency of conversion of positive CDI test results at the time of the test order to negative by the time clinical staff collected stool specimens for testing. RESULTS For 51 CDI patients, PCR, glutamate dehydrogenase, and toxigenic culture results converted to negative at similar rates. For PCR, 14%, 35%, and 45% of positive CDI tests converted to negative after 1, 2, and 3 days of treatment, respectively. Increased age and infection with North American pulsed-field gel electrophoresis strains were associated with persistent positive PCR results. For CDI patients diagnosed at the time of the test order, conversion to negative PCR results by the time clinical stool specimens were collected occurred in 4 of 9 (44%) patients who were prescribed empirical CDI therapy versus 0 of 23 (0%) who were not (P = .004). CONCLUSIONS Empirical treatment for suspected CDI cases may result in false-negative PCR results if there are delays in stool specimen collection.