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Lab Manual for UCSF Clinical Laboratories

Lab Manual for SFGH

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CLOSTRIDIUM DIFFICILE TOXIN B GENE ASSAY

Item Value
Test Update Information On June 21, 2011, the Microbiology Laboratory implemented a new molecular assay for C. difficile Toxin B Gene. Please refer to the Memo for more information.
Available Stat? No
Test code CDBG
Performed by? Microbiology
Sendout? No
Price range $$$
In House Availability Specimen may be sent 24 hours/7 days. Test performed twice per day, 7 days a week.
Principle A qualitative Real Time PCR method is used to detect Clostridium difficile toxin B gene sequences.
Interpretation INTERPRETATION:
Specimens with Clostridium difficile toxin B gene sequence detected by PCR are reported as: POSITIVE for Clostridium difficile toxin B gene.

Toxin B gene negative specimens are reported as: Negative for Clostridium difficile toxin B gene.

If internal controls or amplification steps do not give acceptable results, the assay results will be reported as: INVALID. Presence or absence of Clostridium difficile target DNA cannot be determined. Re-submit if clinically indicated.

CLINICAL SIGNIFICANCE:
Clostridium difficile is the cause of 15-25% of cases of antibiotic-associated diarrhea and about 95-100% of cases of antibiotic-associated pseudomembranous colitis, a life-threatening condition. Most infected patients have more than 3 episodes of watery, foul-smelling stools per day. Other clinical features of C. difficile infection (CDI) include fever, increased WBC, and abdominal cramps. A major risk factor for CDI is use of antibiotics such as clindamycin, broad-spectrum cephalosporins or penicillin-based antibiotics such as amoxicillin or ampicillin that can alter the bowel bacterial flora and allow increased growth of C. difficile. If the organism then produces toxins, the bowel wall can be severely damaged, resulting in diarrhea. Other risk factors for CDI include advanced age, recent major surgery, and use of medications that suppress the immune system. C. difficile also produces spores that can contaminate the hospital environment, with resultant spread of the infection to other patients unless contact precautions are implemented. Since CDI can lead to a life-threatening condition and have implications for infection control, diagnosis with a sensitive and specific test is very important.

TEST PERFORMANCE:
In the SFGH Clinical Microbiology Laboratory, a comparison of the GeneXpert PCR assay with the enzyme immunoassay was performed on 70 patient stool specimens sent for C. difficile testing using the C. difficile Cytotoxin Cell assay as the "gold standard" test. In this study, the PCR assay was 100% sensitive and 95% specific with a positive predictive value of 77% and negative predictive value of 100%. These performance characteristics are comparable to studies in the literature where the GeneXpert C. difficile PCR assay reportedly demonstrated sensitivities of 90-95% and specificities of 95-98%.

Container type sterile, screw-capped container, without preservatives or diluents
Sample type Liquid or unformed stool. NOTE: Stool must conform to shape of the container.
Min. Volume 10 mL
Normal range Negative for Clostridium difficile toxin B gene.
Synonyms Antibiotic associated colitis;
Stability Deliver to Microbiology Lab (2M33) immediately after collection or hold at 4°C until delivery.
Causes for rejection 1. Stool that is formed (i.e., specimen does not conform to the shape of the container).
2. Stool in Parapak for C&S (Cary Blair transport media).
3. Stool in SAF for O&P.
4. Stool in 10% formalin, merthiolate formalin, polyvinyl alcohol or in any other preservative.
5. Leaking specimens.
6. Specimens greater than 24 hours old.
7. Viral culturettes or other swabs.
8. Stool from a patient less than two years of age.
9. Stool mixed with urine.
10. Stool containing Vagisil or zinc oxide; these are interfering substances.
11. Fluid obtained at colonoscopy.
12. Specimens submitted within 14 days of a prior POSITIVE result.
13. Specimens submitted within 4 days of a prior NEGATIVE result.
Additional information Requisition: Microbiology (Toxin Assay: Clostridium difficile)
References 1. Xpert C.difficile Product Insert. Cepheid, Sunnyvale, CA. 300-7871. Rev. A. July 2009.

2. GeneXpert Dx System Operator Manual. Version 4.0. Cepheid, Sunnyvale, CA. 900-7607, Rev. C.

3. Theilman,NM and KH Wilson. Antibiotic-Associated Colitis, Chapter 96, pp 1375-1387. In Mandell,GL, Benett,JE, and Dolin,R, Eds. Principles and Practice of Infectious Diseases, 7th Edition, Churchill Livingston, Inc., NY.
CPT coding 87493
Last Updated 8/13/2012 4:34:16 PM
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