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A laboratory approach to diagnosis of Clostridioides difficile

A laboratory approach to diagnosis of Clostridioides difficile
  • The laboratory approach to diagnosis of C. difficile infection consists of NAAT, either alone or as part of an algorithm, as summarized above. In laboratories that use the above approach, the steps are frequently performed reflexively.
  • Only liquid stool samples from patients with ≥3 loose stools in 24 hours should be tested.
  • The GDH antigen test uses antibodies to test for the presence of the GDH enzyme, a protein present in all C. difficile isolates. It is a useful screening test with good sensitivity, rapid turnaround time, and low cost. However, specificity is poor since this assay does not test for toxin production and cannot distinguish between toxigenic and nontoxigenic strains of C. difficile.
  • The toxin test uses antibodies to detect the presence of C. difficile toxin A and/or toxin B; testing for both toxins is preferred. The toxin test has high specificity with rapid turnaround time and low cost. However, the sensitivity is low, so there is a high rate of false negatives.
  • The NAAT tests for the presence of toxigenic C. difficile organisms in stool by amplifying one or more genes specific to toxigenic strains; the critical gene is tcdB, which encodes for toxin B. This test is specific for toxigenic strains but does not test for active toxin production and also detects asymptomatic carriers of toxigenic C. difficile.
  • If both tests are done and are discordant (an indeterminate result), there are several possible explanations:
    • The patient has active toxigenic C. difficile infection (one or the other of the tests was a false negative).
    • The patient has nontoxigenic C. difficile colonization (the GDH test was positive, but the toxin test was a true negative).
    • The patient is an asymptomatic carrier of toxigenic C. difficile but does not have active disease (again, one or the other of tests was a false negative, perhaps related to the density of the organism in stool). Patients should only be tested if they are symptomatic to avoid detection of asymptomatic carriage.
IBD: inflammatory bowel disease; GDH: glutamate dehydrogenase; EIA: enzyme immunoassay; NAAT: nucleic acid amplification testing.
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