Comparative evaluation of the cobas Cdiff assay for detecting toxigenic Clostridioides difficile

Rapid and accurate diagnosis of Clostridioides difficile infection (CDI) is essential for patient management and infection control. The cobas Cdiff assay is a rapid nucleic acid amplification test (NAAT) that detects the toxin B gene within approximately 20 min.

We evaluated the clinical performance of the cobas Cdiff assay using 111 residual stool specimens from patients with suspected CDI. Specimens were initially tested using an immunochromatographic glutamate dehydrogenase (GDH)/toxin assay and subsequently analyzed using the cobas Cdiff, Xpert C. difficile, and BD MAX CDIFF assays. Agreement rates, including positive percent agreement (PPA), negative percent agreement (NPA), and overall percent agreement (OPA), were calculated. Cycle threshold (Ct) values were compared for specimens positive by all three NAAT platforms.

The cobas Cdiff assay demonstrated excellent agreement with comparator assays. Compared with Xpert C. difficile, the PPA, NPA, and OPA were 96.7%, 100%, and 99.1%, respectively; agreement with BD MAX CDIFF was 100% for all metrics. Among GDH-positive/toxin-negative specimens, a substantial proportion were NAAT-positive, highlighting the limited sensitivity of toxin-based assays. Cobas Cdiff Ct values were consistently lower than those obtained with the comparator assays, reflecting platform-specific analytical differences in Ct values. Discordant results were rare and were further evaluated by repeat testing and clinical review.

The cobas Cdiff assay demonstrated high concordance with established NAATs, consistently lower Ct values, and a markedly shorter turnaround time. These findings support its use as a rapid molecular testing option for the detection of toxigenic C. difficile, while highlighting platform-specific differences in Ct values.