Understanding Discordant Results in C. difficile Testing
In C. difficile infection (CDI) testing, "discordant" refers to conflicting results between different diagnostic tests, which creates challenges in determining whether a patient truly has CDI or is merely colonized with C. difficile.
Causes of Discordant Results
- Discordant results typically occur in two-step testing algorithms when the first test (e.g., GDH or NAAT) is positive but the second test (usually toxin EIA) is negative 1
- This pattern creates uncertainty about whether the patient has active CDI or is simply colonized with C. difficile without active infection 1
- Two-step testing approaches inherently generate discrepant results due to differences in what each test detects (organism presence vs. toxin production) 1
Clinical Significance
- When patients test positive for C. difficile organism (by GDH or NAAT) but negative for toxins, CDI cannot be differentiated from asymptomatic colonization 1
- Discordant results underscore the need for improved diagnostic methods and access to reference tests 1
- Patients with discordant results may have different clinical outcomes than those with concordant positive results (positive for both organism and toxin) 1
Management Implications
- If cohorting is required for infection control, patients with CDI who are discordant for other multidrug-resistant organisms (like MRSA or VRE) should not be cohorted together 1
- Discordant results emphasize the importance of clinical correlation - diagnosis should be based on clinical signs and symptoms in combination with laboratory tests 1
- The European Society of Clinical Microbiology and Infectious Diseases (ESCMID) recommends that interpretation of C. difficile laboratory results should be done in the clinical context, considering the background prevalence of C. difficile in the institution 1
Testing Recommendations
- A two-step protocol is recommended for CDI testing: first step using GDH, toxin EIA, or molecular test for TcdB; second step using a method to detect free fecal toxins or GDH/toxin genes (depending on first test) 1
- Samples with negative first test results can be reported as negative, but positive first test results require additional testing 1
- Testing should only be performed on unformed stools from symptomatic patients with clinically significant diarrhea (≥3 loose stools in 24h) with no obvious alternative explanation 1
Addressing Discordant Results
- Clinical assessment remains crucial - patients should be managed based on symptom severity, risk factors, and overall clinical presentation 1
- Discordant results increase the time needed to achieve a confirmed diagnosis, emphasizing the need to manage suspected CDI patients on clinical grounds 1
- Isolation of patients with suspected CDI should be implemented promptly to limit C. difficile transmission, even before confirmatory testing is complete 1
Understanding discordant results is essential for proper patient management and infection control in healthcare settings, as misinterpretation can lead to both under-treatment of true infections and unnecessary treatment of colonized patients without active disease.