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Clostridium difficile Toxin/Glutamate Dehydrogenase (GDH) with Reflex to PCR

Test code(s) 91664

This is an outdated version of this FAQ. It was effective 03/24/2015 to 03/03/2017.

The current version is available here.

Question 1. What is the significance of C difficile in the healthcare setting?

C difficile is an opportunistic, toxin-producing bacterial pathogen of the gastrointestinal tract that is the most common cause of healthcare-associated diarrhea. It causes 25% to 30% of antibiotic-associated diarrhea and >95% of pseudomembranous colitis cases. Incidence has almost doubled since 1996, and the mortality rate is rising owing to emergence of a hypervirulent strain (ribotype 027/NAP1/toxinotype III).

Disease-causing C difficile strains produce 1 or both of 2 toxins: toxin A is an enterotoxin and toxin B is a cytotoxin. Other strains produce neither toxin and are thought to be avirulent.

Question 2. What is the new test being introduced for C difficile testing?

This new test is a rapid, membrane enzyme immunoassay that simultaneously detects C difficile glutamate dehydrogenase (GDH) antigen and toxins A and B in a single reaction well. It is a manual cartridge-based test that requires no instrumentation. GDH antigen is a metabolic enzyme expressed at high levels by all strains of C difficile and some other Clostridium species.

Question 3. What is the reason for introducing this new test?

Studies have shown that EIA testing for C difficile toxins A and B lacks diagnostic sensitivity; some studies show sensitivity as low as 31%. For diagnosis of C difficile infection, it is recommended to use either 1) nucleic acid amplification testing (NAAT) or 2) a combination of GDH antigen and toxin detection with NAAT confirmation of discordant results. The C difficile toxin A and B EIA assay has been discontinued by Quest Diagnostics due to its relatively low sensitivity.

Question 4. Are there any changes to specimen collection requirements?

Specimen collection and transport instructions remain the same as the previous C difficile EIA toxin A/B test with one exception: formed stools are no longer accepted for the new GDH/toxin test.

Question 5. How will the reflex testing work?

If both the toxin A and B test and the GDH test are positive, then the sample will be considered positive for toxigenic

C difficile infection. If both the toxin and GDH tests are negative, then the sample will be considered negative for toxigenic

C difficile infection (Figure). Samples with discordant results (ie, GDH-positive but toxin-negative or GDH-negative but toxin-positive) will be reflexed (at additional charge and additional CPT code) for confirmation with C difficile PCR. The C difficile PCR test targets the toxin B gene (tcdB). According to our validation studies, the above discordant results may occur about 6% of the time.

Question 7. Should we test neonates using this assay?

Up to 50% of neonates may be colonized with toxigenic C difficile. While testing for C difficile in this population will be performed, results should be interpreted with caution.

Question 8. How are C difficile infections treated?

For decades, the drugs of choice were either vancomycin or metronidazole. However, the recurrence rate associated with these antibiotics is 20% to 30%, which is unacceptability high. Fidaxomicin is an alternative that has a lower relapse rate than vancomycin. There is no strong evidence that probiotics aid in the treatment of C difficile disease.


  1. American Society for Microbiology. A practical guidance document for the laboratory detection of toxigenic Clostridium difficile. 2010. http://www.alere.com/content/dam/alere/docs/guidelines/2010_ASM_guidelines_dated_9.21.10.pdf. Accessed February 10, 2014.
  2. Cohen, SH, Gerding DG, Johnson, SN, et al. 2010. Clinical practice guidelines for Clostridium difficile infection in adults: 2010 Update by the Society for Healthcare Epidemiology of America (SHEA) and the Infectious Diseases Society of America (IDSA). Infect Control Hosp Epidemiol. 2010;31(5):431-455.
  3. Cornely OA, Miller MA, Louie TJ, et al. Treatment of first recurrence of Clostridium difficile infection: fidaxomicin versus vancomycin. Clin Infect Dis. 2012;55(suppl 2):S154-S160.
  4. Swindells J, Brenwald N, Reading N, et al. Evaluation of diagnostic tests for Clostridium difficile infection. J Clin Microbiol. 2010;48:606-608.
  5. Surawicz CM, Brandt LJ, Binion DG, et al. Guidelines for diagnosis, treatment, and prevention of Clostridium difficile infections. Am J Gastroenterol. 2013;108:478–498.
  6. Versalovic J, Carroll KC, Funke G, et al. Manual of Clinical Microbiology. 10th ed. Washington D.C: ASM Press; 2011.
This FAQ is provided for informational purposes only and is not intended as medical advice. A clinician’s test selection and interpretation, diagnosis, and patient management decisions should be based on his/her education, clinical expertise, and assessment of the patient.
Document FAQS.136 Version: 1
Version 1 effective 03/24/2015 to present