What is the recommended test order to rule out Clostridioides difficile (C. difficile) infection?

Recommended Testing Algorithm for C. difficile Infection

Use a multistep testing algorithm rather than any single test alone, with the specific approach depending on whether your institution has pre-agreed criteria for stool submission. 1

Two Distinct Testing Pathways Based on Institutional Criteria

Pathway 1: When Pre-Agreed Institutional Criteria Exist for Stool Submission

If your institution has strict criteria limiting testing to patients with ≥3 unformed stools in 24 hours, no laxative use within 48 hours, and no alternative explanation for diarrhea, then use NAAT alone OR a multistep algorithm (GDH plus toxin; GDH plus toxin arbitrated by NAAT; or NAAT plus toxin). 1

• This approach maximizes sensitivity when you've already filtered out inappropriate test requests clinically 1
• The pre-agreed criteria ensure you're testing the right population, reducing false positives from colonization 1

Pathway 2: When No Pre-Agreed Institutional Criteria Exist (Most Common Scenario)

Use a stool toxin test as part of a multistep algorithm (GDH plus toxin; GDH plus toxin arbitrated by NAAT; or NAAT plus toxin) rather than NAAT alone. 1

• This is the most practical recommendation for most institutions where stool submission isn't strictly controlled 1
• Never use NAAT alone as a stand-alone test because it cannot distinguish active infection from asymptomatic colonization, which occurs in 44-55% of NAAT-positive patients 2

Recommended Multistep Algorithm Details

Step 1: Initial Screening

• Perform GDH (glutamate dehydrogenase) immunoassay as the first screening test 1
• GDH detects the common antigen present in all C. difficile strains (both toxigenic and non-toxigenic) with high sensitivity 1

Step 2: Toxin Detection

• If GDH positive, perform toxin A/B enzyme immunoassay (EIA) 1
• Select a toxin EIA with sensitivity in the upper range reported in the literature, as approved stool EIA toxin tests vary widely in sensitivity 1

Step 3: Arbitration for Discordant Results

• If GDH positive but toxin negative, perform NAAT (nucleic acid amplification test) to arbitrate 2, 3
• This three-step approach provides results for approximately 85-92% of samples on the day of receipt 2

Clinical Significance of Test Results

GDH Positive + Toxin Positive

• These patients have true CDI requiring treatment 2
• Significantly worse outcomes: 7.6% complication rate, 8.4% mortality, longer duration of diarrhea 2

GDH Positive + Toxin Negative + NAAT Positive

• These patients likely represent colonization rather than active infection 4, 2
• Minimal complications: 0% complication rate in largest studies, 0.6% mortality, outcomes similar to patients without C. difficile 2
• Clinical correlation is essential—consider alternative causes of diarrhea 2
• May represent infection control risk as "excretors" but generally don't require treatment 2

Critical Pre-Test Requirements

Only test unformed stool samples from symptomatic patients meeting specific criteria: 1, 3

• ≥3 unformed stools in 24 hours that take the shape of the container 1, 2
• No laxative use within the previous 48 hours 1
• No obvious alternative explanation for diarrhea 3
• Laboratories should reject specimens that are not liquid or soft 1

Common Pitfalls to Avoid

• Never perform repeat testing within 7 days during the same diarrheal episode—this has only 2% diagnostic yield and increases false positives 1, 2
• Never test asymptomatic patients except for epidemiological studies 1
• Never test infants ≤12 months of age due to high prevalence of asymptomatic carriage 1
• Never use toxin EIA alone as a stand-alone test due to poor sensitivity 1, 3
• Never perform test of cure as >60% of successfully treated patients remain C. difficile positive 2

Special Populations

Children 1-2 Years Old

• Testing not routinely recommended unless other infectious or noninfectious causes excluded 1

Children ≥2 Years Old

• Test only those with prolonged or worsening diarrhea AND risk factors (inflammatory bowel disease, immunocompromising conditions) OR relevant exposures (healthcare contact, recent antibiotics) 1

Patients with Ileus

• For severe CDI complicated by ileus, perirectal swabs may provide an acceptable alternative with high sensitivity and specificity 3