What is the usual interpretation of a Cytotoxin A and B assay for Clostridioides difficile infection?

Interpretation of Cytotoxin A and B Assay for Clostridioides difficile

A positive cytotoxin A and B assay indicates the presence of free toxins in stool and has high specificity (98-100%) for active C. difficile infection, while a negative result effectively rules out CDI due to excellent negative predictive value (98-99%), though the test should not be used as a stand-alone diagnostic method in endemic settings. 1

Test Characteristics and Clinical Interpretation

Positive Result Interpretation

• A positive toxin A/B assay strongly suggests active CDI when the clinical context is appropriate (diarrhea ≥3 unformed stools in 24 hours with risk factors), with positive predictive values ranging from 94-100% in high-quality assays like Immunocard tox A/B and Tox A/B Quik Chek 1

• The specificity of toxin A/B detection is excellent (97-100% for most assays), meaning false positives are rare when proper neutralization testing is performed 1, 2

• However, positive predictive value drops significantly in low-prevalence settings (5-10% CDI prevalence), ranging from only 28-77% for many assays, which is why confirmatory testing or two-step algorithms are recommended 1

Negative Result Interpretation

• A negative toxin A/B assay effectively excludes CDI with negative predictive values of 98-99% at endemic prevalence rates (5-10%), making it highly reliable for ruling out infection 1

• The high negative predictive value means CDI can be confidently excluded when the test is negative, and no further testing is typically needed 1

• Repeated sample submission during the same episode is not recommended in endemic situations but may be useful during outbreaks 1

Critical Limitations and Pitfalls

Sensitivity Concerns

• Toxin A/B assays have suboptimal sensitivity (48-96% depending on the specific assay) compared to reference standards, meaning they can miss true cases of CDI 1

• When compared to toxigenic culture as the reference standard, all toxin assays showed unacceptably low sensitivity except molecular tests 1

• The sensitivity varies significantly between different commercial assays: Immunocard tox A/B showed 94% sensitivity while some assays like Ridascreen showed only 57% sensitivity 1

Toxin A-Negative, Toxin B-Positive Strains

• Assays detecting only toxin A will miss clinically significant toxin A-negative, toxin B-positive strains that are increasingly recognized worldwide and cause genuine disease 3

• This is why assays detecting both toxins A and B are essential for comprehensive detection 3

Recommended Two-Step Algorithm

Step 1: High-Sensitivity Screening

• Screen first with either GDH enzyme immunoassay (sensitivity 90.8%) or NAAT/PCR (sensitivity 91-92%) to capture all potential cases 1, 4

• If the screening test is negative, report as negative and no further testing is needed 1

Step 2: Confirmatory Toxin Detection

• For positive screening results, confirm with toxin A/B detection to distinguish active infection from asymptomatic colonization 1, 4

• If free fecal toxins are absent but GDH or toxin genes are present, CDI cannot be differentiated from asymptomatic colonization 1

• This two-step approach achieves optimal diagnostic performance with sensitivity of 91% and specificity of 98% 5

Clinical Context Requirements

• Diagnosis must combine laboratory results with clinical symptoms including diarrhea, abdominal pain, and appropriate risk factors (recent antibiotics, hospitalization, advanced age) 1, 5

• Testing should only be performed on symptomatic patients with clinically significant diarrhea to avoid detecting asymptomatic colonization 5

• The interpretation must account for institutional CDI prevalence, as this dramatically affects positive predictive value 1

Comparison to Reference Standards

• The cell culture cytotoxicity assay (detecting toxin B through cytopathic effects on cultured cells) remains a reference standard with high specificity (95-100%) and clinical correlation 1, 4, 2

• Toxigenic culture is more analytically sensitive than cytotoxicity assay but may have lower diagnostic specificity for active disease 1

• Commercial toxin A/B assays trade some sensitivity for practical advantages: rapid turnaround time (1.5 hours vs 24-48 hours), ease of use, and no requirement for specialized cell culture facilities 1, 6, 7