This patient, whose diarrhea began more than 3 days after hospitalization, tested negative for Clostridium difficile infection (CDI), so she should be prescribed an antimotility agent for symptomatic relief. CDI is the most common infectious cause of diarrhea in hospitalized patients. Antibiotic administration is a major risk factor; however, infection can also be acquired through person-to-person spread or environmental contamination. Diagnostic evaluation for CDI is appropriate for all patients with hospital-acquired diarrhea, regardless of antibiotic history. Nucleic acid amplification tests, such as polymerase chain reaction (PCR) for C. difficile toxin genes, are highly sensitive and specific and offer the advantage of rapid results. Antiperistaltic agents should be avoided in CDI treatment because they may obscure symptoms and increase risk for toxic megacolon. In this patient, however, the negative PCR test result essentially excludes this diagnosis. For noninfectious diarrhea, antimotility agents may relieve symptoms.

Hospital-acquired bacterial gastroenteritis, other than CDI, is exceedingly rare, with one study identifying an alternative bacterial enteropathogen in less than 1% of stool cultures submitted at 3 days of hospitalization or longer. Because of the low yield, many laboratories reflexively reject stool cultures submitted on patients who develop diarrhea more than 72 hours after admission.

Patients with confirmed CDI require antimicrobial treatment. Oral metronidazole, as well as vancomycin, is effective in the treatment of mild to moderate CDI. However, it is not indicated in the setting of a negative PCR test result.

Repeat PCR testing after a negative result is rarely indicated and only increases the probability of a false-positive result. Stool enzyme immunoassay, which detects glutamate dehydrogenase, an enzyme ubiquitous in C. difficile, is an accepted alternative to PCR in diagnosing CDI. When the result of stool enzyme immunoassay is negative, no further testing is necessary; however, when it is positive, a second, more specific test, such as PCR, is required for diagnosis.