What confirms a positive Clostridioides difficile (Cdiff) infection in a patient?

Confirming a Positive C. difficile Infection

A positive C. difficile infection is confirmed by demonstrating the presence of C. difficile toxins in stool from a symptomatic patient with diarrhea (≥3 unformed stools in 24 hours), using either a two-step algorithm (GDH screening followed by toxin testing, or NAAT followed by toxin confirmation) or NAAT alone in appropriate clinical contexts. 1

Essential Clinical Prerequisites for Testing

Before any laboratory testing, the patient must meet specific clinical criteria:

• Diarrhea must be present: defined as ≥3 unformed stools in 24 hours with no obvious alternative explanation 2, 3
• Only test unformed stool specimens: testing formed stool results in false positives and unnecessary antibiotic therapy 2, 3
• Never test asymptomatic patients: this leads to detection of colonization rather than active infection 1, 4

Recommended Diagnostic Algorithms

Two-Step Algorithm (Preferred by Most Guidelines)

The most widely recommended approach uses a two-step testing strategy to balance sensitivity and specificity: 1

1. First step - GDH screening: GDH enzyme immunoassay detects the presence of C. difficile but cannot distinguish between toxigenic and non-toxigenic strains 1

   • If GDH negative: C. difficile infection ruled out
   • If GDH positive: proceed to second step

2. Second step - Toxin detection: Toxin A/B enzyme immunoassay confirms toxin production 1

   • If toxin positive: confirms active C. difficile infection
   • If toxin negative but GDH positive: may use NAAT as arbitration test 1

NAAT-Based Testing

Nucleic acid amplification tests (NAATs) offer excellent sensitivity (80-100%) and specificity (87-99%) but have important limitations: 1

• NAAT alone: detects toxin genes but cannot distinguish between colonization and active infection, potentially leading to overdiagnosis 1
• NAAT plus toxin confirmation: increasingly recommended to reduce false positives from colonization 1
• Clinical context is critical: NAAT results must be interpreted with recent antibiotic exposure, hospitalization history, fever, abdominal pain, and leukocytosis 2, 3

Performance Characteristics of Individual Tests

Toxin A/B EIA (Not Recommended Alone)

• Sensitivity: 32-98% (too variable and often low) 1
• Specificity: 84-100% 1
• Major limitation: insufficient sensitivity when used as standalone test 1, 3

GDH Screening

• Cannot confirm infection alone: only indicates presence of organism, not toxin production 1
• Requires confirmatory testing: approximately 20% of C. difficile strains are non-toxigenic 1

Cell Cytotoxicity Assay (Historical Gold Standard)

• Detects toxin B in stool: historically considered reference standard 5, 6
• Limitations: slow turnaround time, requires specialized laboratory facilities 1, 5
• Rarely used clinically: replaced by faster methods in most laboratories 1

Toxigenic Culture (Research Gold Standard)

• Most sensitive method: detects toxin-producing C. difficile 1, 5
• Major drawback: cannot distinguish colonization from active infection, as 7% of asymptomatic hospitalized patients are colonized 1
• Primary use: epidemiological typing and strain characterization 1

Critical Pitfalls to Avoid

Testing errors that lead to misdiagnosis:

• Never repeat testing after initial negative result unless clinical presentation changes significantly (new character of diarrhea or new supporting evidence) 1
• Do not test patients on laxatives within 24-48 hours 3
• Avoid testing asymptomatic patients post-treatment: test of cure is not recommended 4
• Do not rely on single toxin EIA alone: sensitivity is inadequate 1, 3

Special Testing Circumstances

For patients with ileus unable to produce stool:

• Perirectal swabs are acceptable: demonstrate 95.7% sensitivity, 100% specificity when tested by PCR 2
• Use same algorithms as stool: apply multistep testing protocols to perirectal swab specimens 2

Adjunctive Diagnostic Methods

CT imaging has limited diagnostic utility:

• Sensitivity only 52%: inadequate for screening purposes 1
• Specificity 93%: may help in severe-complicated cases 1
• Typical findings: colonic wall thickening >4mm, "accordion sign," peri-colonic stranding 1
• Best use: assessing severity in clinically suspected severe-complicated colitis, not for initial diagnosis 1

Endoscopy:

• Sensitivity 51-55%: insensitive for C. difficile infection diagnosis 4
• Specific finding: pseudomembranes confirm pseudomembranous colitis 4
• Primary indication: when immediate results needed or to evaluate for underlying inflammatory bowel disease 7

Likelihood Ratios for Clinical Decision-Making

NAAT-based testing provides high diagnostic confidence:

• Positive likelihood ratio: 46 (person with organism is 46 times more likely to test positive) 1
• Negative likelihood ratio: 0.05 (person with organism is 20 times less likely to test negative) 1
• Both ratios meet "high information value" thresholds (LR+ >10, LR- <0.1) 1