Is PCR testing necessary for confirming Clostridioides difficile (C. difficile) infection when initial test results are mixed?

C. difficile Mixed Test Results: PCR Confirmation Strategy

When initial C. difficile test results are discordant (e.g., GDH positive/toxin negative, or other mixed results), PCR/NAAT testing is recommended as part of a multi-step algorithm to clarify whether active infection is present, though the result must be interpreted in clinical context rather than automatically triggering treatment. 1

Understanding Discordant Results

What causes mixed results:

• Two-step testing algorithms inherently generate discrepant results because different tests detect different targets—organism presence (GDH, PCR) versus toxin production (toxin EIA) 2
• When the first test is positive but the second test is negative, this creates diagnostic uncertainty about whether the patient has active CDI or is simply colonized 2
• Approximately 44-55% of PCR-positive patients will be toxin-negative, representing colonization rather than true infection 3

The Role of PCR in Mixed Results

PCR characteristics:

• PCR/NAAT detects C. difficile genetic material with very high sensitivity (93-94%) but cannot distinguish between active infection and harmless colonization 3
• PCR is most appropriately used as part of a multi-step algorithm rather than as a stand-alone test 1

When to use PCR for arbitration:

• The IDSA/SHEA 2018 guidelines recommend using NAAT to arbitrate discordant results in a three-step algorithm: GDH plus toxin, arbitrated by NAAT when results are discordant 1
• This approach provides results for approximately 85-92% of samples on the day of receipt, with only 8-15% requiring further testing 1

Clinical Interpretation of PCR Results

PCR-positive/Toxin-positive patients:

• These patients have true infection requiring treatment 3
• They experience significantly worse outcomes: 7.6% complication rate, 8.4% mortality, and longer duration of diarrhea 3

PCR-positive/Toxin-negative patients:

• These patients should generally NOT be treated, as they likely represent colonization or "excretors" rather than true infection 4
• They have minimal complications: 0% complication rate in the largest study, 0.6% mortality, and outcomes similar to patients without C. difficile 3
• Mortality in this group (9.7%) is not significantly different from controls (8.6%), compared to 16.6% in toxin-positive patients 4

Management Algorithm for Mixed Results

Step 1: Assess clinical criteria

• Diarrhea severity: ≥3 unformed stools in 24 hours that conform to container shape 3
• Recent antibiotic exposure (strong risk factor) 3
• Fever, significant leukocytosis, rising creatinine, or severe abdominal pain 3

Step 2: Apply testing algorithm

• If GDH positive/toxin negative: Perform PCR/NAAT for arbitration 1
• If PCR positive/toxin negative: Consider this colonization unless high-risk features present 4

Step 3: Treatment decision

• Do NOT treat PCR-positive/toxin-negative patients with low-risk features (minimal diarrhea, no fecal inflammation, alternative explanation for symptoms) 4
• Consider empiric treatment with oral vancomycin 125 mg four times daily only if high-risk features present: significant leukocytosis, rising creatinine, severe illness, or strong clinical suspicion despite negative toxin 3, 4

Critical Pitfalls to Avoid

Testing errors:

• Do NOT repeat testing within 7 days of the initial test during the same diarrheal episode—this increases false-positive results and has only 2% diagnostic yield 3
• Do NOT use PCR alone as a stand-alone test in endemic settings due to low positive predictive value 1

Treatment errors:

• Do NOT automatically treat all PCR-positive patients—this leads to unnecessary antibiotic exposure in colonized patients 3
• Do NOT perform test of cure, as >60% of successfully treated patients remain C. difficile positive 3

Infection Control Considerations

Regardless of test results:

• Implement contact precautions promptly for suspected CDI to limit transmission, even before confirmatory testing is complete 2
• PCR-positive/toxin-negative patients ("excretors") present an infection control risk and require isolation despite not needing treatment 4

When Clinical Suspicion Remains High Despite Negative Results

Additional options:

• Consider flexible sigmoidoscopy to visualize pseudomembranes 4
• Evaluate for alternative causes of diarrhea 4
• In epidemic settings (not endemic), multiple sample submission may be warranted due to lower negative predictive values 1