High Agreement Between an Ultrasensitive Clostridioides difficile Toxin Assay and a C. difficile Laboratory Algorithm Utilizing GDH-and-Toxin Enzyme Immunoassays and Cytotoxin Testing

The Singulex Clarity C. diff toxins A/B (Clarity) assay is an automated, ultrasensitive immunoassay for the detection of Clostridioides difficile toxins in stool. In this study, the performance of the Clarity assay was compared to that of a multistep algorithm using an enzyme immunoassay (EIA) for detection of glutamate dehydrogenase (GDH) and toxins A and B arbitrated by a semiquantitative cell cytotoxicity neutralization assay (CCNA). The performance of the assay was evaluated using 211 residual deidentified stool samples tested with a GDH-and-toxin EIA (C. Diff Quik Chek Complete; Techlab), with GDH-and-toxin discordant samples tested with CCNA. The stool samples were stored at –80°C before being tested with the Clarity assay. For samples discordant between Clarity and the standard-of-care algorithm, the samples were tested with PCR (Xpert C. difficile; Cepheid), and chart review was performed. The testing algorithm resulted in 34 GDH(+)/toxin(+), 53 GDH(−)/toxin(−), and 124 GDH(+)/toxin(−) samples, of which 39 were CCNA(+) and 85 were CCNA(−). Clarity had 96.2% negative agreement with GDH(−)/toxin(−) samples, 100% positive agreement with GDH(+)/toxin(+) samples, and 95.3% agreement with GDH(+)/toxin(−)/CCNA(−) samples. The Clarity result was invalid for one sample. Clarity agreed with 61.5% of GDH(+)/toxin(−)/CCNA(+) samples, 90.0% of GDH(+)/toxin(−)/CCNA(+) (high-positive) samples, and 31.6% of GDH(+)/toxin(−)/CCNA(+) (low-positive) samples. The Singulex Clarity C. diff toxins A/B assay demonstrated high agreement with a testing algorithm utilizing a GDH-and-toxin EIA and CCNA. This novel automated assay may offer an accurate, stand-alone solution for C. difficile infection (CDI) diagnostics, and further prospective clinical studies are merited.