Clostridioides difficile laboratory testing algorithms and performance: a review of the Royal College of Pathologists...

Laboratory diagnosis of Clostridioides difficile infection (CDI) is challenging, and several recommended guidelines exist to guide testing. We sought to describe testing approaches used by Australian and New Zealand laboratories enrolled in the Royal College of Pathologists of Australasia Quality Assurance Programs (RCPAQAP) external quality assessment (EQA) program for the detection of C. difficile to compare laboratory practices with published recommendations and to report on participant performance. We analysed laboratory methods in use for the period 2012 to 2023 and collated participant performance for the period 2015 to 2023. Between 62 and 88 laboratories were enrolled in the RCPAQAP C. difficile EQA each year. Most participants used a multistep testing algorithm, but a significant minority (24/84, 29%), in 2023, used nucleic acid amplification tests (NAATs) in a single-test approach. Use of NAATs has increased over time, whilst use of a toxin A/B enzyme immunoassay as a single test method has ceased. Most participants performed well in identifying C. difficile -toxin-positive and C. difficile --negative samples (usually >95-98% concordance), whereas concordance was lower (<85%) for non-toxigenic C. difficile (p<0.001). Testing methods have changed over time. Many laboratories use NAAT as a single test, but it is unknown if this is combined with the recommended pre-test sample selection criteria. Changes to the test items are recommended to assess the ability of participants to detect strains with unexpressed toxin genes. Laboratories are recommended to regularly review their testing algorithms to enhance reporting accuracy.