How to Diagnose C. diff
Diagnose C. difficile infection using a two-step laboratory algorithm combined with clinical criteria: test only patients with ≥3 unformed stools in 24 hours, start with either GDH screening or NAAT, and confirm positive results with toxin EIA to distinguish true infection from colonization.
Clinical Criteria Required Before Testing
Do not order laboratory tests unless the patient meets these criteria:
- ≥3 loose or unformed stools in 24 hours (Bristol Stool Scale types 5-7)
- Recent antibiotic exposure, hospitalization, or other risk factors
- No obvious alternative explanation (e.g., laxative use)
Critical pitfall: Never test formed stools or asymptomatic patients—this leads to false positives from colonization (up to 46% of C. difficile isolates are non-toxigenic) 1, 2.
Recommended Two-Step Diagnostic Algorithm
Step 1 - Initial screening (choose one):
- GDH enzyme immunoassay (high sensitivity, detects both toxigenic and non-toxigenic strains), OR
- NAAT/PCR for toxin genes (high sensitivity and specificity)
Step 2 - Confirmatory testing:
- If Step 1 positive → Perform toxin A/B EIA
- If toxin EIA positive → Diagnose CDI and treat
- If toxin EIA negative but high clinical suspicion → Consider cell cytotoxicity assay or treat empirically
This approach addresses the critical limitation that single-step NAAT testing increases detection of asymptomatic colonization, leading to overdiagnosis and unnecessary treatment 1, 3.
Why Two-Step Testing Matters
The 2019 WSES guidelines and recent evidence demonstrate that:
- NAAT alone detects 17% positivity vs. 12.3% with confirmatory testing
- Patients positive by both NAAT and toxin assay have significantly higher complication rates (39% vs. 3%) and mortality 3
- Single-step NAAT cannot distinguish active infection from colonization 1
The two-step algorithm improves positive predictive value from 0.28-0.77 to acceptable levels (0.71-1.00) in typical hospital populations 4.
Alternative Testing Approaches
For patients with ileus unable to produce stool:
- PCR testing of perirectal swabs (sensitivity 95.7%, specificity 100%) 1
Reference standard methods (rarely used clinically):
- Cell culture cytotoxicity assay (CCA) - gold standard but requires 24-48 hours
- Toxigenic culture - most sensitive but takes 48-96 hours 4
What NOT to Do
- Never perform "test of cure" after treatment—56% of successfully treated patients shed spores for up to 6 weeks 2
- Never repeat testing during the same diarrheal episode unless high clinical suspicion in epidemic settings 4
- Never use toxin EIA alone—sensitivity too low (32-98%) 1
- Never test asymptomatic patients or contacts 1, 2
Clinical Context Integration
Laboratory results must be interpreted with clinical findings:
- Leukocytosis >15,000 cells/μL suggests severe disease
- Elevated creatinine >1.5× baseline indicates severity
- Fever, abdominal pain, and imaging findings (colonic wall thickening, ascites) support diagnosis 1, 5
Endoscopic findings of pseudomembranous colitis are diagnostic even without positive stool tests 5, 2.
Practical Implementation
Most laboratories now use:
- GDH + toxin EIA algorithm: Screen with GDH, confirm positives with toxin EIA, resolve discordant results with NAAT or CCA 4, 1
- NAAT + toxin EIA algorithm: Screen with NAAT, confirm positives with toxin EIA in patients with lower pretest probability 3
The choice depends on local CDI prevalence, laboratory capabilities, and cost considerations. In high-suspicion patients with discordant testing (positive NAAT, negative toxin), treat empirically 6.