A BLAICH¹, C CASTELLANO¹, C KIESSLING¹, A GESCHKE², K RENTSCH², A EGLI¹, R FREI<sup>1

1</sup>Division of Clinical Microbiology, University Hospital Basel, Basel, Switzerland, ²Division of Clinical Chemistry, University Hospital Basel, Basel, Switzerland

This study evaluated the automated LIAISON C. difficile glutamate dehydrogenase (GDH) assay (DiaSorin) in comparison to the C. DIFF CHEK-60 assay (TECHLAB/Alere) and in GDH-positive stool samples the automated LIAISON C. difficile Toxins A&B assay (DiaSorin) in comparison to PCR and toxigenic culture. DiaSorin offers a stool extract preparation in a sealed tube prior to testing on the LIAISON. Stool samples submitted for the detection of C. difficile were routinely screened for C. difficile specific antigen GDH with the C. DIFF CHEK-60 (GDH-Alere) and the LIAISON GDH assay. GDH-positive samples with one or both of these assays were further tested with the Xpert C. difficile PCR assay (Cepheid) and toxigenic culture. GDH-LIAISON-positive samples were analyzed for toxin in a reflex-testing with the LIAISON C. difficile Toxins A&B assay. PCR and toxigenic culture were considered as gold standard. Of the 545 samples tested with the GDH-Liaison 454 samples were true-negative, 13 false-positive, 77 true-positive, and 1 false-negative; with the GDH-Alere 460 samples were true-negative, 7 false-positive, 73 true-positive, and 5 false-negative. Thus, the GDH-LIAISON and the GDH-Alere possess a sensitivity of 98.7% and 93.6% and a specificity of 97.2% and 98.5%, respectively. If PCR for toxin detection was negative, but culture positive, a PCR from culture (toxigenic culture) was additionally performed. Overall, we found 65 toxin-positive and 26 toxin-negative samples by PCR and toxigenic culture. With the Liaison toxin A&B assay 24 samples were true-negative, 2 false-positive, 39 true-positive, and 26 false-negative. Thus, the Toxin-LIAISON assay possesses a sensitivity of 60% and a specificity of 92.3%. The LIAISON GDH assay compared to the GDH-Alere assay is more sensitive (98.7% vs 93.6%) but slightly less specific (97.2% vs 98.5%). For a screening test like the GDH-assay high sensitivity is more important than specificity. For reflex-testing the sensitivity of 60% of the LIAISON Toxins A&B assay is in accordance to the sensitivity of other immunoassays described in literature. The stool sample preparation and automated analysis on the LIAISON are convenient and neat.