Clostridioides difficile colitis in ICU — FFICM MCQ

Correct answer: C — Interpret results with clinical severity and consider repeat/toxin-based testing pathway

The correct answer is C, interpret results with clinical severity and consider repeat/toxin-based testing pathway. A GDH-positive, toxin EIA-negative stool result is a recognised discordant pattern in the two-stage UK testing algorithm: it confirms the organism is present but does not confirm active toxin-mediated disease, so the result cannot be read in isolation. Per UK Department of Health/PHE (now UKHSA) guidance, discordant GDH+/toxin- samples require a third confirmatory test (typically PCR or repeat toxin assay) and must be interpreted alongside the clinical picture. In this patient, ileus and a rising white cell count are features of severe, potentially fulminant C difficile infection, so clinical suspicion must drive further testing and consideration of empirical treatment rather than being dismissed by a negative toxin EIA alone. Why the other options are wrong: B. Diagnose active C difficile solely from GDH positivity: GDH detects the organism (toxigenic or non-toxigenic strains, or colonisation) but not active toxin production, so it cannot confirm disease on its own. D. Exclude C difficile because toxin is negative: toxin EIA has limited sensitivity and a single negative result does not reliably exclude infection, especially with a compatible severe clinical picture and ileus. E. Request colonoscopy as the routine next test: endoscopy is invasive, unnecessary for routine diagnosis, and reserved for diagnostic uncertainty (e.g. suspected pseudomembranous colitis when other tests are unhelpful), not a first-line step after discordant GDH/toxin results. A. Stop testing because diarrhoea is expected in ICU: this ignores a clinically significant discordant result with signs of severe disease (ileus, rising white cell count) and risks missing life-threatening fulminant CDI. Key point: a GDH-positive, toxin-negative result is discordant and non-diagnostic; it mandates a confirmatory third test (PCR or repeat toxin) plus clinical correlation, particularly when severity markers such as ileus are present.

Reference: UK Department of Health / Public Health England (now UKHSA), Updated Guidance on the Diagnosis and Reporting of Clostridium difficile Infection, 2012 (two-stage/three-stage testing algorithm for discordant GDH/toxin results), https://assets.publishing.service.gov.uk/media/6821b81bf16c0654b19060b2/withdrawn-dh_133016.pdf