Cautionary wisdom on Cdiff testing

Although most current rapid assays for Cdiff toxin A and B tout very high sensitivity and specificity, the cautionary wisdom elucidated in this study (abstract) is to consider the positive predictive value (PPV) of the test, which depends on the prevalence of the disease in the population you are testing. The authors note that only about 10% of hospitalized patients with diarrhea will have Cdiff. If there are 500 stool samples submitted, even if the sensitivity and specificity of your Cdiff test is 95% and 97%, the PPV is only 78% (which means that 22% of +tests do not represent true Cdiff).  The assays evaluated in this study had median sensitivites from 76-95% and specificities from 93-100%. You can determine the PPV of the cdiff test at your hospital by determining the sensitivity and specificity of the assay, and the # (and % of total) of positive tests .  This will greatly help you interpret your cdiff tests!

Danielle Scheurer

Dr. Scheurer is a clinical hospitalist and the Medical Director of Quality and Safety at the Medical University of South Carolina in Charleston, South Carolina, and is Assistant Professor of Medicine. She is a graduate of the University of Tennessee College of Medicine, completed her residency at Duke University, and completed her Masters in Clinical Research at the Medical University of South Carolina. She also serves as the Web Editor and Physician Advisor for the Society of Hospital Medicine.

1 Comment

  1. CArlos Gutierrez MD MSc on November 24, 2008 at 11:55 pm

    It is an amazing job summarizing a paper in just a few words. Congratulations.

    For a test with sensitivity of 95% and specificity of 97%, the PPV is 78% for a pretest probability (“prevalence”) of 10% as you correctly stated. However, 5% +test would “not represent true CDiff” (not 22%), if by this you meant the proportion of false positives.

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