Stool culture for bacterial pathogens – If a stool culture is ordered, the laboratory will screen for Campylobacter spp., Salmonella serogroups, Shigella spp., Plesiomonas shigelloides, and Aeromonas spp. The most common pathogen causing bacterial gastroenteritis in Nebraska is Campylobacter jejuni. However, if Yersinia enterocolitica, Vibrio spp., or enterohemorrhagic E. coli (EHEC; E. coli O157:H7) are suspected, separate orders are required as different media are needed to isolate these pathogens. It is pertinent to note that 30-50% of all EHEC in Nebraska are not E. coli O157:H7 but other serotypes (e.g. O111:NM, O26:H11, O103:NM, etc.). The enterohemorrhagic E. coli culture will only isolate E. coli O157:H7 and not other serotypes. If EHEC is suspected, it is recommended that a shiga-toxin PCR test (offered by molecular diagnostics) be ordered as well as an enterohemorrhagic E. coli culture. The shiga-toxin PCR test will detect all EHEC serotypes. A culture for EHEC will automatically be performed on grossly bloody stool specimens.

Clostridium difficile toxin assay

a. Cultures for C. difficile are technically demanding, require two to three days for growth, and are not specific for distinguishing between toxin-negative or toxin-positive strains or asymptomatic carriage.

b. The C. difficile toxin assay is the test used for diagnosing C. difficile-associated colitis. It is typically recommended to order two assays because the sensitivity for the first sample is 72%, while the sensitivity for the second sample is 84%. However, if the first sample comes back positive, there is no need to order a second sample. Thus, the assays should be ordered one at a time and not “C. difficile x 3”. Moreover, the assays must be performed on separate stool samples, ideally 24 hours apart.

c. The C. difficile toxin assay should not be used to assess response to therapy because many patients will continue to carry the toxin without any clinical manifestations of colitis.