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Test Code STFRP Shiga Toxin, Molecular Detection, PCR, Feces

Reporting Name

Shiga Toxin PCR, F

Useful For

Sensitive, specific, and rapid detection of the presence of Shiga toxin-producing organisms such as Escherichia coli O157:H7 and Shigella dysenteriae type 1 in stool

Performing Laboratory

Mayo Medical Laboratories in Rochester

Specimen Type

Fecal


Necessary Information


Specimen source is required.



Specimen Required


The high sensitivity of amplification by PCR requires the specimen to be processed in an environment in which contamination of the specimen by shiga toxin DNA is unlikely.

 

Submit only 1 of the following specimens:

 

Supplies:

C and S Vial (T058)

Stool container, Small (Random), 4 oz Random (T288)

 

Preferred:

Specimen Type: Preserved stool

Container/Tube: Commercially available transport system specific for recovery of enteric pathogens from fecal specimens (15 mL of nonnutritive transport medium containing phenol red as a pH indicator, either Cary-Blair or Para-Pak C and S) (T058)

Specimen Volume: Representative portion of stool

Collection Instructions:

1. Collect fresh stool and submit in container with transport medium.

2. Place stool in preservative within 2 hours of collection.

 

Acceptable:

Specimen Type: Unpreserved stool

Container/Tube: Stool container (T288)

Specimen Volume: Representative portion of stool

Collection Instructions: Collect fresh stool and submit in container.


Specimen Minimum Volume

1 mL

Specimen Stability Information

Specimen Type Temperature Time
Fecal Ambient (preferred) 7 days
  Frozen  7 days
  Refrigerated  7 days

Reference Values

Not applicable

Day(s) and Time(s) Performed

Monday through Sunday

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

87798

LOINC Code Information

Test ID Test Order Name Order LOINC Value
STFRP Shiga Toxin PCR, F In Process

 

Result ID Test Result Name Result LOINC Value
SRC59 Specimen Source 31208-2
56052 Result 33764-2

Clinical Information

Shiga toxins (also known as Shiga-like toxins, Vero toxins, or Vero-like toxins) are encoded by some strains of Escherichia coli, most notably O157:H7. Shiga toxin can also be produced by other serogroups of enterohemorrhagic E coli (EHEC), as well as Shigella dysenteriae type 1. Generally, Shiga toxin-producing organisms cause bloody diarrhea, although this is not universal. Unlike some bacterial gastrointestinal infections, antimicrobial therapy is contraindicated, as antimicrobials may exacerbate disease. Treatment is primarily supportive (eg, hydration). A complication of infection by an organism producing Shiga toxin is hemolytic uremic syndrome (HUS). The percentage of people that develop HUS varies among outbreaks of E coli O157:H7, but generally ranges from 3% to 20%. HUS is characterized by a triad of findings: hemolytic anemia, thrombocytopenia, and kidney failure. Most people recover completely, however, some require permanent dialysis, and some die as a result of complications.

 

Several diagnostic methods available for the detection of EHEC lack sensitivity, are labor intensive, or have a long turnaround time. There are more than 160 serogroups of EHEC; the first serogroup to be associated with HUS was O157:H7. This is also the serogroup that is most commonly implicated in outbreaks. EHEC O157:H7 is detectable as nonfermenting colonies when cultured on sorbitol MacConkey (SMAC) agar, but the majority of non-O157:H7 Shiga toxin-producing E coli strains ferment sorbitol and, therefore, are undetectable by this method. The Vero cell line is susceptible to the Shiga toxin, but the assay can take up to 48 hours and is nonspecific. Commercial enzyme-linked immunosorbent assay (ELISA) antigen detection kits have a sensitivity of 90% when compared to culture, but an overnight enrichment step is necessary for adequate sensitivity. PCR detection of stx, the gene encoding Shiga toxin, directly from stool specimens is a sensitive and specific technique, providing same-day results. PCR assay identifies non-O157:H7 Shiga toxin-producing bacteria, extending the utility beyond strains identifiable on SMAC agar.

Interpretation

A positive PCR result indicates the likely presence of Shiga toxin-producing Escherichia coli in the specimen. Although Shigella dysenteriae serotype 1 may produce a positive result, it is extremely rare in the United States.

 

A negative result indicates the absence of detectable Shiga toxin DNA in the specimen, but does not rule out the presence of Shiga toxin-producing E coli, as false-negative results may occur due to inhibition of PCR, sequence variability underlying the primers and probes, or the presence of the Shiga toxin gene in quantities less than the limit of detection of the assay. Shiga toxins are encoded on mobile genetic elements and can theoretically be lost by their bacterial host.

Clinical Reference

1. Gould LH, Bopp C: Recommendations for diagnosis of Shiga toxin-producing Escherichia coli infection by clinical laboratories. MMWR Morb Mortal Wkly Rep 2009 Oct;16:v58

2. Grys TE, Sloan LM, Rosenblatt JE, Patel R: Rapid and sensitive detection of Shiga toxin-producing Escherichia coli from nonenriched stool specimens by real-time PCR in comparison to enzyme immunoassay and culture. J Clin Microbiol 2009;47:2008-2012

3. Grys TE, Patel R: Update on Shiga toxin-producing Escherichia coli. Mayo Clinic, Mayo Medical Laboratories Communique

4. Nyre LM, Kiemele DL, Zomok CD, et al: Clinical experience with rapid PCR for detection of Shiga toxin in stool. Abstract of the Annual Meeting of the American Society for Microbiology, 2010 General Meeting, San Diego, CA, May 23-27, 2010

Analytic Time

1 day

Reject Due To

Hemolysis

NA

Lipemia

NA

Icterus

NA

Other

Formed stool or stool in gel transport medium; ECOFIX preservative; formalin or PVA fixative

Method Name

Real-Time Polymerase Chain Reaction (PCR) Using LightCycler and Fluorescent Resonance Energy Transfer (FRET)

(PCR is utilized pursuant to a license agreement with Roche Molecular Systems, Inc.)

Testing Algorithm

See Laboratory Testing for Infectious Causes of Diarrhea in Special Instructions.

Forms

If not ordering electronically, complete, print, and send 1 of the following forms with the specimen:

Microbiology Test Request Form (T244) (http://www.mayomedicallaboratories.com/it-mmfiles/microbiology_test_request_form.pdf)

Gastroenterology and Hepatology Test Request Form (T728) (http://www.mayomedicallaboratories.com/it-mmfiles/gastroenterology-and-hepatology-test-request.pdf)