Role of Fecal Immunoassay Testing in Diagnosing C. difficile Infection
Fecal immunoassay testing should be used as part of a two-step algorithm for diagnosing Clostridioides difficile infection (CDI), with enzyme immunoassays (EIAs) for glutamate dehydrogenase (GDH) as an initial screening test followed by toxin A/B detection for confirmation. 1
Diagnostic Approach for C. difficile
When to Test
- Test only unformed stool samples from symptomatic patients with ≥3 unformed stools in 24 hours 1, 2
- Consider testing in:
- Patients >2 years of age with diarrhea following antimicrobial use
- Patients with healthcare-associated diarrhea
- Patients with persistent diarrhea without an identified cause 1
Optimal Specimen Collection
- Fresh diarrheal stool sample (takes the shape of the container) is the preferred specimen 1
- For patients with suspected severe CDI complicated by ileus who cannot produce stool specimens, perirectal swabs may be used (sensitivity 95.7%, specificity 100%) 1
- A single stool specimen is sufficient; multiple specimens do not increase diagnostic yield 1
Recommended Testing Algorithms
Two-Step Algorithm (Preferred Approach)
First step: GDH screening test (high sensitivity)
- GDH is an enzyme produced by C. difficile in large amounts
- Detects presence of C. difficile but doesn't differentiate toxigenic from non-toxigenic strains
Second step: Toxin A/B EIA (high specificity)
- If GDH positive, confirm with toxin detection
- Detects actual toxins produced by C. difficile
- Fast, convenient, and inexpensive but with variable sensitivity (32-98%) 1
Interpretation of Results
- Negative GDH: Report as negative for CDI 2
- Positive GDH + positive toxin: Report as positive for CDI 2
- Positive GDH + negative toxin: Cannot differentiate between infection and colonization 2
Alternative Testing Methods
Nucleic Acid Amplification Tests (NAATs)
- Highly sensitive (80-100%) and specific (87-99%) for detecting toxigenic C. difficile 1
- Can be used as a standard diagnostic test for CDI
- May be used alone in patients with high clinical suspicion or as part of a two-step algorithm 1
Important Limitations of NAATs
- Cannot distinguish between active infection and asymptomatic colonization
- May lead to overdiagnosis and overtreatment when used alone 1
- Should be performed only in patients with high suspicion for CDI 1
Reference Methods (Not Routinely Used)
- Toxigenic culture (TC): Gold standard but slow and labor-intensive
- Cell cytotoxicity neutralization assay (CCNA): Detects fecal toxins but slow and labor-intensive 1
Pitfalls and Caveats
Testing asymptomatic patients: Do not test formed stool as this can result in false positives and unnecessary treatment 1
Repeat testing: Generally not recommended during the same diarrheal episode unless there is ongoing clinical suspicion during an epidemic or high clinical suspicion during endemic situations 1
Test interpretation: A positive bacterial culture or NAAT without toxin detection may indicate colonization rather than infection 2
Clinical correlation: Laboratory results must be interpreted in the context of clinical presentation, as no single test is perfect 1, 2
Public health reporting: All specimens that test positive by culture-independent diagnostic testing should be cultured in clinical or public health laboratories to ensure outbreaks are detected 1
Summary of Test Characteristics
| Test | Sensitivity | Specificity | Advantages | Disadvantages |
|---|---|---|---|---|
| GDH EIA | High | Moderate | Excellent screening test | Cannot differentiate toxigenic strains |
| Toxin A/B EIA | 32-98% | 84-100% | Fast, inexpensive | Variable sensitivity |
| NAATs | 80-100% | 87-99% | High sensitivity, rapid results | May detect colonization, not just infection |
| Toxigenic culture | High | High | Gold standard | Slow, labor-intensive |
| CCNA | High | High | Reference method | Slow, labor-intensive |
By implementing appropriate testing algorithms and correctly interpreting results in the clinical context, fecal immunoassay testing plays a crucial role in accurately diagnosing C. difficile infection while minimizing both underdiagnosis and overdiagnosis.