A post-operative patient develops acute-onset watery diarrhoea on day 10 of broad-spectrum antibiotic therapy. Sigmoidoscopy reveals pseudomembranes. Stool enzyme immunoassay (EIA) tests for toxin A only returns negative. What is the most appropriate next step in diagnosis?

• A Repeat stool EIA testing for both toxin A and toxin B
• B Stool culture on CCFA selective agar with spore shock
• C PCR for C. difficile toxin genes (tcdA/tcdB) ✓
• D Cell cytotoxicity neutralisation assay (CCNA)

Explanation

Many clinical C. difficile isolates are toxin A-negative/toxin B-positive (binary combinations), so testing for toxin A alone misses a significant proportion; however, the most sensitive and specific single-step test is NAAT/PCR targeting tcdB (and tcdA). PCR has sensitivity >90% and specificity >97% and is recommended as part of two-step or standalone testing algorithms (IDSA 2021). CCNA is the historical gold standard but requires 24–48 hours and cell culture facilities. Repeat EIA for both toxins is inferior to NAAT in sensitivity.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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