What are the methods for testing Clostridioides difficile (C. diff)?

From the Guidelines

The most effective approach for testing C. diff involves using a nucleic acid amplification test (NAAT) alone or as part of a multi-step algorithm, including glutamate dehydrogenase (GDH) and toxin enzyme immunoassays (EIAs), to optimize diagnostic accuracy. This recommendation is based on the latest evidence from the 2019 systematic review and meta-analysis published in Clinical Microbiology Reviews 1. The study evaluated the diagnostic accuracy of various laboratory testing algorithms, including NAATs, and found that using NAAT alone or in combination with GDH or GDH plus toxin EIA is a recommended best practice for detecting C. difficile in adults.

Key Testing Methods

• Nucleic acid amplification tests (NAATs) to detect C. diff DNA
• Enzyme immunoassays (EIAs) to identify C. diff toxins A and B
• Glutamate dehydrogenase (GDH) antigen testing to detect a common enzyme produced by C. diff

Specimen Collection and Testing Criteria

• Fresh stool sample in a clean container, sent to the laboratory promptly
• Liquid or soft stools preferred, as formed stools are unlikely to contain the pathogen
• Testing should only be performed on patients with clinically significant diarrhea (three or more loose stools in 24 hours) and risk factors such as recent antibiotic use or healthcare facility exposure

Repeat Testing and Stool Culture

• Repeat testing within 7 days is not recommended if the initial test is negative, as this rarely changes management and increases false positive results
• Stool culture for C. diff is highly sensitive but rarely used in clinical practice due to its complexity and long turnaround time, as noted in the 2018 clinical practice guidelines by the Infectious Diseases Society of America (IDSA) and Society for Healthcare Epidemiology of America (SHEA) 1

The use of NAAT alone or in combination with other tests provides a high degree of diagnostic accuracy, which is essential for appropriate treatment and infection control measures. By following these guidelines, clinicians can optimize their testing strategies and improve patient outcomes.

From the Research

Testing Methods for C. difficile

There are several methods for testing C. difficile, including:

• Nucleic acid amplification tests (NAAT) targeting the C. difficile toxin B gene 2
• Enzyme immunoassays (EIAs) that detect the presence of C. difficile toxins 2
• Cell cytotoxicity neutralization assay (CCNA) 2
• Toxigenic culture 2, 3
• Single-molecule array (SIMOA) technology for detecting CD toxins 2
• Glutamate dehydrogenase (GDH) assay 3

Diagnostic Algorithms

Several diagnostic algorithms have been proposed, including:

• A 2-step algorithm combining GDH and NAAT for the detection of C. difficile in stool specimens 3
• A multistep algorithmic testing approach in which a sensitive NAAT is followed by a specific toxin A and toxin B EIA 4
• The use of NAAT quantitation as a predictor of toxin presence in C. difficile infection 4

Test Performance

The performance of different diagnostic tests for C. difficile has been evaluated, with results showing:

• NAAT has high sensitivity but suboptimal specificity due to asymptomatic colonization 2
• EIAs have high specificity but low sensitivity 2
• SIMOA CD toxin EIAs have higher sensitivity than conventional CD toxin EIAs while maintaining excellent specificity 2
• The GDH-NAAT algorithm has high specificity, increased sensitivity, and is cost-effective 3

Clinical Outcomes

The impact of NAAT on clinical outcomes in patients with C. difficile infection has been investigated, with findings showing:

• The introduction of NAAT can improve clinical outcomes in CDI patients 5
• Predictors of CDI-related complications among NAAT-positive/EIA-negative patients include baseline severe disease, baseline fulminant colitis, and fever of >38.5°C 6