American Board of Internal Medicine (ABIM) Certification Practice Exam

The most appropriate initial test for suspected Clostridium difficile infection (CDI) is the enzyme-linked immunosorbent assay (ELISA) for Clostridium difficile toxins A and B. This test is specifically designed to detect the presence of the toxins produced by C. difficile, which is seminal in establishing the diagnosis of CDI.

The ELISA test is advantageous for its practicality, rapidity, and ability to provide results that correlate well with the clinical presentation of diarrhea and the presence of C. difficile infection. Detection of these toxins is crucial, as it indicates the pathogenic potential of the organism, distinguishing CDI from mere colonization or asymptomatic carriage in patients who may have C. difficile in their gut flora.

While polymerase chain reaction (PCR) testing is highly sensitive and can detect the presence of the toxin genes, it does not necessarily correlate with active disease since it can pick up non-toxigenic strains or residual genetic material after treatment. Stool culture is considered more sensitive but is not practical as an initial diagnostic step because it is labor-intensive and time-consuming. Urine analysis does not provide relevant information for diagnosing CDI, making it inappropriate for this context.

Thus, the ELISA test for detecting C. difficile toxins A and