We studied 557 nonduplicate fresh stool specimens from adult patients clinically suspected of having Clostridium difficile-associated diarrhea. All samples were tested in parallel with an in-house cytotoxin B tissue culture assay (CTA), the C DIFFICILE TOX A/B II test (TA/B; TechLab, Blacksburg, VA), and the Triage Micro C DIFFICILE Panel (Biosite Diagnostics, San Diego, CA). The Triage device detects toxin A (TA) and glutamate dehydrogenase (GDH) simultaneously. Of the specimens, 350 were negative and 95 were positive for all markers. Another 112 specimens yielded discrepant results. The CTA found 143 positive specimens. Results of the components of the Triage and TA/B were compared separately with those of CTA. GDH was the most sensitive but least specific marker, whereas TA and TA/B were less sensitive but highly specific. Because of these attributes and a quick turnaround time, GDH would be the best screening test for C difficile-associated diarrhea. CTA detected the highest number of cases of C difficile-associated diarrhea and would be most useful as a confirmatory test for GDH-positive and TA-negative specimens.