How many times must a....

....patient test negative for Clostridium difficile, before you can call him negative? The new PCR assays for toxin B detection are more sensitive, and therefore have higher negative predictive values, than the older enzyme immunoassays. And even for the EIAs, the data to support the common practice of sending “C. diff X 3” were pretty weak.

A simple retrospective study in the October Journal of Clinical Microbiology shows how low-yield a repeat C. difficile PCR is, when ordered within 7 days of a negative test. Among almost 300 patients with a negative test who were re-tested, only 10 positive results were obtained. One was a false positive (compared with gold standard cytotoxicity assay), and 7 of the remaining 9 were positive more than 7 days after the first test (and usually in the context of ongoing risk factors for C. difficile, or new onset of diarrhea).

If your lab uses PCR for C. difficile toxin detection, save money and time by abandoning the practice of sending repeated tests to "rule out C. diff".