What is the recommended method for testing for Clostridioides difficile (C. diff) infection?

Testing for Clostridioides difficile Infection (CDI)

A two-step testing algorithm is recommended for diagnosing C. difficile infection, rather than using a single test alone, to optimize sensitivity and specificity while minimizing false results. 1

Sample Collection and Selection

• Testing should only be performed on unformed (liquid or semi-formed) stool samples from symptomatic patients, as testing formed stool can lead to false positive results 1
• CDI testing should be performed on stool samples from all patients with infectious diarrhea who have negative tests for common enteropathogens 1
• All patients with diarrhea who have been hospitalized for more than 72 hours should be tested for CDI, regardless of whether the physician specifically requested the test 1
• For patients with severe CDI complicated by ileus who cannot produce stool specimens, perirectal swabs may provide an acceptable alternative with high sensitivity (95.7%) and specificity (100%) 1

Diagnostic Testing Methods

Reference Standard Methods

• Cell Culture Cytotoxicity Neutralization Assay (CCNA):

  • Traditionally considered a reference standard for detecting C. difficile toxins directly in stool 1
  • Involves applying stool filtrate to cell culture and observing for cytopathic effect 1
  • Highly sensitive and specific but cumbersome, time-consuming, and lacks standardization 1

• Toxigenic Culture (TC):

  • Culture on selective media (cycloserine-cefoxitin-fructose agar) followed by toxin testing of isolates 1
  • More sensitive than CCNA but time-consuming (requires 48+ hours) 1
  • Allows recovery of isolates for epidemiological typing and characterization 1

Rapid Diagnostic Methods

• Enzyme Immunoassays (EIAs):

  • For toxins A/B: Fast and inexpensive with high specificity but relatively low sensitivity; not recommended as a standalone test 1
  • For Glutamate Dehydrogenase (GDH): Highly sensitive screening test that detects an enzyme present in all C. difficile strains (both toxigenic and non-toxigenic) 1

• Nucleic Acid Amplification Tests (NAATs):

  • Detect C. difficile toxin genes (tcdA, tcdB) directly from stool 1
  • High sensitivity but may detect asymptomatic colonization 1
  • Should be used in patients with high suspicion for CDI or as part of a two-step algorithm 1

Recommended Testing Algorithms

Two-Step Testing Approach

1. First step (screening): Use one of the following 1:

   • GDH EIA (detects C. difficile presence)
   • NAAT (detects toxin genes)
   • Toxin A/B EIA (detects toxins directly)

2. Second step (confirmation): If the first test is positive, confirm with 1:

   • If GDH or NAAT positive → confirm with toxin A/B EIA
   • If toxin EIA positive → confirm with GDH or NAAT

This two-step approach increases the positive predictive value compared to single tests alone 1

Special Considerations

• No single test is suitable as a stand-alone test for confirming CDI in endemic settings due to limitations in sensitivity or specificity 1, 2
• Repeat testing after a first negative sample during the same diarrheal episode may be useful only in selected cases with ongoing clinical suspicion, particularly during an epidemic situation 1
• Clinical context is crucial for interpretation of test results, including recent antibiotic use, hospitalization history, and symptoms such as fever, abdominal pain, and leukocytosis 1

Common Pitfalls to Avoid

• Using a single test alone, especially toxin EIA, which has limited sensitivity 1
• Testing formed stool samples, which can lead to false positive results 1
• Testing for cure after treatment, which is not recommended as C. difficile and its toxins may persist after clinical resolution 1
• Failing to consider clinical context when interpreting test results, as positive tests may represent colonization rather than infection in some cases 1
• Delaying empiric isolation and treatment while awaiting test results in patients with high clinical suspicion 1

By following these evidence-based recommendations for C. difficile testing, clinicians can optimize diagnostic accuracy while minimizing unnecessary testing and treatment, ultimately improving patient outcomes and reducing healthcare costs.