C. difficile Testing Order
Order a single diarrheal stool specimen for C. difficile testing using either nucleic acid amplification testing (NAAT) or a two-step algorithm (GDH antigen screening followed by toxin detection). 1
Specimen Requirements
- Collect one diarrheal stool sample that takes the shape of the container—this is the optimal specimen type 1
- Fresh stool is preferred for C. difficile toxin detection 1
- Do not order multiple specimens—repeat testing does not increase diagnostic yield and is not cost-effective 1, 2, 3
Recommended Testing Methods
The 2017 IDSA guidelines specify acceptable diagnostic approaches: 1
- NAAT (nucleic acid amplification testing) for toxigenic C. difficile strain detection
- Two-step algorithm: GDH antigen with or without toxin detection, followed by cytotoxin assay if needed
- Direct toxin detection alone is not recommended due to insufficient sensitivity 4, 3
The two-step approach (GDH screening followed by toxin confirmation, or NAAT followed by toxin confirmation) balances sensitivity and specificity while avoiding false positives from asymptomatic colonization. 4
When to Order Testing
Test only when specific criteria are met: 1
- Patients >2 years of age with diarrhea following antimicrobial use within the preceding 8-12 weeks
- Healthcare-associated diarrhea (symptoms developing >72 hours after hospital admission) 1, 4
- Persistent diarrhea without identified etiology even without recognized risk factors (weaker indication) 1
Critical Testing Restrictions
Do not test asymptomatic patients—asymptomatic colonization is common in hospitalized patients and long-term care residents, and testing without diarrhea leads to false-positive results that do not require treatment. 1, 5
Avoid testing in children <2 years of age unless other causes of diarrhea have been excluded, as asymptomatic colonization rates reach up to 70% in this age group. 6 By age 2-3 years, colonization rates fall to adult levels of 1-3%. 6
Common Pitfalls
- Never order repeat testing after an initial negative result—the sensitivity of a single properly collected specimen is adequate, and multiple specimens do not improve detection 1, 2
- Only test patients with active diarrhea (≥3 unformed stools in 24 hours)—testing formed stool or asymptomatic patients leads to detection of colonization rather than infection 4, 5
- Do not order "test of cure" after treatment—post-treatment testing in asymptomatic patients is not recommended 5