High rate of false-negative results of the rectal swab culture method in detection of gastrointestinal colonization with vancomycin-resistant enterococci

The diagnostic accuracy of the rectal swab (RS) culture method in identifying gastrointestinal colonization with vancomycin-resistant enterococci (VRE) is not known. Serial quantitative stool cultures, skin cultures, and RS cultures were performed for patients with VRE infections to assess the false-negative rate of the RS and the prevalence of skin colonization, a prerequisite for cross-transmission, at varying VRE stool densities. A total of 35 stool samples were obtained from 13 patients. The sensitivity of the RS culture was 58%; it ranged from 100%, at VRE densities of greater than or equal to7.5 log(10) colony forming units (cfu) per gram of stool, to 0%, at densities of less than or equal to4.5 log(10) cfu per gram of stool. Skin colonization was detected at these low VRE stool densities, but it was more common at higher VRE densities (P<.001). Antibiotic exposure was significantly associated with higher VRE stool densities (P<.001). The high false-negative rate of the RS may be contributing to the continued increase in the prevalence of VRE.