A 40-year-old post-appendectomy patient develops abdominal pain, diarrhea (up to 15 watery stools/day), and fever on day 8 of ceftriaxone therapy. Sigmoidoscopy shows yellowish plaques on colonic mucosa. Stool testing for Clostridioides difficile toxin (A/B) by EIA is negative. The NEXT BEST diagnostic step is:
- A Repeat stool EIA testing three times on separate specimens
- B Stool culture for C. difficile on cycloserine-cefoxitin-fructose agar (CCFA)
- C Stool nucleic acid amplification test (NAAT/PCR) for C. difficile toxin gene ✓
- D Serum procalcitonin and CRP to confirm antibiotic-associated colitis
Explanation
When clinical and endoscopic findings (pseudomembranes) strongly suggest C. difficile colitis but stool toxin EIA is negative, NAAT/PCR for C. difficile toxin genes (tcdA/tcdB) is the recommended next step. NAAT is 15–20% more sensitive than toxin EIA alone and remains the gold standard for diagnosis. Current guidelines recommend a two-step algorithm: GDH antigen or PCR as a sensitive screen, confirmed by toxin EIA for active disease distinction. Repeating EIA multiple times adds little yield and is not recommended. Culture on CCFA takes 48–72 hours and detects colonizers as well as pathogenic strains. Serum inflammatory markers are non-specific.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.