C. difficile Testing After Negative DNA Test
No, you do not need to check the antigen (GDH) if the C. difficile DNA test (NAAT) is negative. A negative NAAT result effectively rules out C. difficile infection and no additional testing is warranted during the same diarrheal episode.
Rationale for Not Pursuing Additional Testing
The American Society for Microbiology (ASM) provides high-quality evidence that NAAT-only testing is a recommended practice with substantial diagnostic accuracy (negative likelihood ratio of 0.05, meaning a person with C. difficile is 20 times less likely to test negative than someone without the organism) 1. This excellent negative predictive value means a negative NAAT effectively excludes the diagnosis.
Why Repeat or Additional Testing Is Not Recommended
Repeat testing within 7 days has minimal diagnostic yield. The IDSA and SHEA strongly recommend against repeat C. difficile testing within 7 days during the same diarrheal episode, as only 3% of patients convert from negative to positive within this timeframe 1, 2, 3.
Adding GDH testing after negative NAAT provides no clinical benefit. The two-step algorithm (GDH followed by NAAT) is designed to screen with GDH first, then confirm with NAAT 1. Reversing this sequence—performing GDH after a negative NAAT—has no established diagnostic utility and contradicts recommended testing algorithms 1.
NAAT detects the toxin gene with high sensitivity. With 46 studies demonstrating substantial pooled diagnostic accuracy, NAAT-only testing has a sensitivity that makes additional antigen testing redundant when the NAAT is negative 1.
What To Do Instead When NAAT Is Negative
If diarrhea persists despite a negative C. difficile NAAT, consider alternative diagnoses rather than pursuing additional C. difficile testing 3:
Evaluate for other enteric pathogens, particularly if there are risk factors such as recent travel, antibiotic exposure for other organisms, or outbreak settings 1, 2.
Consider non-infectious causes including medication-related diarrhea, inflammatory bowel disease, or post-infectious irritable bowel syndrome 3.
Assess for complications or severe disease only if clinical suspicion remains extremely high (e.g., pseudomembranous colitis on imaging, toxic megacolon), in which case flexible sigmoidoscopy may be considered 1.
Common Pitfalls to Avoid
Do not reflexively add toxin EIA testing after negative NAAT. This reverses the intended diagnostic algorithm and lacks evidence support 1.
Avoid empiric treatment based solely on clinical suspicion after negative testing unless the patient has severe/fulminant disease with high pretest probability 1, 4.
Do not perform "test of cure" after treatment, as more than 60% of successfully treated patients remain C. difficile positive 2, 3.
Testing should only be performed on unformed stools from symptomatic patients with no alternative explanation for diarrhea 1.