The diagnosis is suggested by a history of diarrhea that develops during administration of antibiotics or within 2 weeks of their discontinuation. The diagnosis may be confirmed by endoscopy, which reveals characteristic yellowish plaques within the intestinal lumen. Lesions may be seen throughout the entire alimentary tract, although they are typically limited to the right colon. For this reason, colonoscopy may be required in some cases to establish the diagnosis, which may be missed by sigmoidoscopy alone. It should be noted, however, endoscopy is not routinely needed to establish a diagnosis of pseudomembranous colitis.

The diagnosis is confirmed by the demonstration of C. difficile in the stool and by the detection of toxin in stool filtrates. The organism is best identified by stool culture using a selective growth medium.17 This technique has a sensitivity approaching 100 percent but lacks specificity. In addition, culture results take between 28 and 72 h, thus limiting their utilization in establishing the diagnosis in patients with suspected pseudomembranous colitis. Instead, C. difficile toxin is detected directly using a number of techniques including tissue-culture assay, enzyme-linked immunosorbent assays (ELISA), latex agglutination, dot-immunobinding assays, and polymerase chain reaction (PCR). Tests vary in their sensitivity, specificity, and time to completion. While tissue-culture assays are considered the "gold standard," most laboratories utilize the ELISA technique to detect the clostridial toxins; it has a sensitivity of 63 to 94 percent and a specificity of 75 to 100 percent. 18

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