Does a positive Clostridioides difficile antigen (glutamate dehydrogenase assay) indicate an active infection?

No, a positive GDH (glutamate dehydrogenase) antigen test alone does NOT indicate active C. difficile infection.

A positive GDH test only confirms the presence of C. difficile organism (both toxigenic and non-toxigenic strains) but cannot distinguish between active infection, colonization, or non-toxigenic strains—you must perform confirmatory toxin testing or molecular testing to diagnose actual CDI.

Why GDH Alone is Insufficient

The 2018 IDSA/SHEA guidelines explicitly state that GDH immunoassays detect a common antigen present in ALL C. difficile isolates, including non-toxigenic strains that do not cause disease 1. The test has:

• High sensitivity (91.1%) but low specificity (91.2%) 2
• Cannot differentiate between colonization and true infection
• Must be combined with another test in a multistep algorithm 1

The Correct Diagnostic Approach

The guidelines strongly recommend using GDH as a screening test only, followed by confirmatory testing 1:

Two-Step Algorithm (Recommended):

1. Screen with GDH (high sensitivity catches most cases)
2. If GDH positive → Perform toxin A/B immunoassay OR nucleic acid amplification test (NAAT)
3. If GDH negative → CDI effectively ruled out (100% negative predictive value) 3, 4

Three-Step Algorithm (Alternative):

1. GDH screening
2. Toxin testing
3. If discordant (GDH+/toxin-) → Arbitrate with NAAT

Clinical Significance of GDH+/Toxin- Results

This is a critical pitfall: 63% of GDH-positive/toxin-negative patients actually harbor toxigenic C. difficile 5. These patients:

• Have significantly worse outcomes, including higher in-hospital mortality 6
• Should NOT be dismissed as simple colonizers
• Require clinical correlation and often confirmatory NAAT testing 5

Key Clinical Caveats

• Never test asymptomatic patients - colonization rates are high, especially in hospitalized patients 1
• Never test infants ≤12 months - colonization is nearly universal without clinical significance 1
• Only test unformed stools from symptomatic patients with ≥3 unformed stools in 24 hours 1
• Do not repeat testing within 7 days of the same diarrheal episode 1
• Never perform test of cure - patients can remain colonized for months after successful treatment 1

Bottom Line for Clinical Practice

When you receive a positive GDH result, treat it as a screening positive that requires confirmation. The patient may have active CDI, may be colonized with toxigenic strains, or may harbor non-toxigenic C. difficile. Clinical correlation with symptoms plus confirmatory toxin or molecular testing is mandatory before initiating treatment 1, 7, 8.