Microbial diagnostics · USMLE Step 1

A patient has profuse watery diarrhea after antibiotic use. Stool ELISA for toxins A and B is negative. Which next test is most appropriate?

  1. Stool culture for C. difficile
  2. CT scan of abdomen and pelvis
  3. NAAT for C. difficile toxin genes
  4. Serum toxin PCR assay
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Correct answer: NAAT for C. difficile toxin genes

NAAT detects toxin genes with high sensitivity. It is often used when ELISA results are negative but suspicion remains high.

Difficulty: Medium Question 8 of 20

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