Offering our clients state-of-the-art testing is part of CPL’s ongoing commitment to excellence.

Effective February 27, 2024, Clinical Pathology Laboratories (CPL) will update the rejection criteria for C. difficile Toxin A/B with C. difficile GDH Reflex Toxin/PCR to include formed stools. As a reminder, the reflex testing follows the recommendations of the Infectious Diseases Society of America (IDSA) and the Society for Healthcare Epidemiology of America (SHEA). The algorithm offers a sensitive, specific, and cost-effective approach for the diagnosis and treatment of C. difficile.


The reflex algorithm includes:

  • C. difficile Glutamate Dehydrogenase (GDH) Antigen, an Enzyme Immunoassay (EIA), is used as a screening test to detect C. difficile antigen. GDH Antigen is a metabolic enzyme produced in high amounts by toxigenic and nontoxigenic strains of C. difficile
    • GDH negative results indicate the sample is negative for C. difficile
    • GDH positive results indicate the presence of C. difficile but do not specifically indicate toxin production
  • Toxin A and B by EIA is performed to detect toxigenic C. difficile
    • Toxin positive results indicate the sample is positive for toxigenic C. difficile
    • Toxin negative results require discriminatory testing by Polymerase Chain Reaction (PCR) as the toxin may degrade in stool specimens. Approximately 8% of specimens will require PCR testing

Note: Reflex testing is performed at additional charge(s) and CPT code(s)

C difficile testing algorithm

Sensitivity and Specificity for C. difficile testing

Toxin A/B by EIA 32-99% 84-100%
Toxin by PCR 73-100% 91-100%
GDH Antigen + Toxin A/B by EIA 41-92% 94-100%
GDH + Toxin A/B by EIA + Toxin by PCR 68-100% 97-100%
C. difficile Testing Methodology Sensitivity Specificity


C. difficile GDH Reflex Toxin/PCR

Order Unit Code/Test Name: 6335 - C. difficile GDH Reflex Toxin/PCR
Test Method: Enzyme Immunoassay (EIA)
Specimen Requirements: 10 mL fresh stool. Refrigerate. If specimen cannot be delivered to lab within 72 hours, freeze.
Specimen Rejection Criteria: Will reject if:
  • Specimen submitted at room temperature
  • Formed stools
  • Specimen submitted in:
    • Preservative
    • Tissue paper or diapers
    • Transport media (e.g., Cary Blair or C&S)
    • Leaking containers
Transport Temperature: Refrigerated (2-8 °C)
Stability (collection to initiation of testing): Room Temperature Unacceptable
Refrigerated (2-8 °C) 3 Days
Frozen (≤ -20 °C) 1 Month
Performed: Sunday-Saturday AM Shift
Analytic Time: 1 Day Note: Analytic time may be longer if PCR testing is required
Reference Range: Negative
Interpretive Information:
  • Testing should be limited to patients with 3 or more unformed stool specimens in 24 or fewer consecutive hours unless obstruction or ileus is suspected.
  • Some patients may be asymptomatic carriers of C. difficile; therefore, a positive result must be correlated with clinical conditions.
  • Reassessment of a negative result is not recommended due to the high sensitivity and specificity of the testing algorithm.
  • Do not perform a test of cure on patients whose symptoms have resolved. Patients who recover may be carriers of C. difficile and shed C. difficile spores for a long time. Testing does not distinguish viable from nonviable organisms.
  • The performance characteristics of C. difficile Toxin A/B have not been thoroughly established in the pediatric population.
CPT Code: GDH - 87449
Toxin A and B - 87324
Toxin PCR - 87493

Clinical Practice Guidelines for Clostridium difficile Infection in Adults: 2010 Update by the Society for Healthcare Epidemiology of America (SHEA) and the
1. Infectious Diseases Society of America (IDSA).
2. Laboratory Diagnosis of Clostridium difficile Infections: There is Light at the End of the Colon, Brecher et al, Clinical Infectious Diseases, 2013, 57 (8): 1175.
(Summarizes multiple studies)