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Each year, Mycobacterium tuberculosis accounts for approximately 1.4 million deaths and is responsible for 9 million newly diagnosed cases of tuberculosis worldwide. Mycobacterium tuberculosis is spread from person-to-person via respiratory transmission, and has the potential to become resistant to many or all of the antibiotics currently used if antimycobacterial treatment is not promptly initiated. Therefore, rapid and accurate detection of Mycobacterium tuberculosis in patient specimens is of clinical and public health importance.
Conventional culture methods can generally detect Mycobacterium tuberculosis in 2 to 3 weeks, although up to 8 weeks of incubation may be required in some instances. Developed at Mayo Clinic, this rapid PCR assay detects Mycobacterium tuberculosis complex DNA directly from respiratory specimens and other specimens without waiting for growth in culture and, therefore, the results are available the same day the specimen is received in the laboratory. A mycobacterial culture should always be performed in addition to the PCR assay. The PCR assay is rapid but the culture has increased sensitivity over the PCR assay. The PCR assay targets a unique sequence within the katG gene, which is present in members of the Mycobacterium tuberculosis complex. In addition, the assay can detect genotypic resistance to isoniazid mediated by mutations in the katG target, when present.
Rapid detection of Mycobacterium tuberculosis complex DNA, preferred method
Detection of Mycobacterium tuberculosis, when used in conjunction with mycobacterial culture
A positive result indicates the presence of Mycobacterium tuberculosis complex DNA. Members of the Mycobacterium tuberculosis complex detected by this assay include Mycobacterium tuberculosis, Mycobacterium bovis, Mycobacterium bovis Bacillus Calmette-Guerin, Mycobacterium africanum, Mycobacterium canetti, and Mycobacterium microti. Other species within the Mycobacterium tuberculosis complex (eg, Mycobacterium bovis subspecies caprae and Mycobacterium pinnepedi) should, in theory, be detected using the primer and probe sequences in this assay, but they have not been tested. This assay method does not distinguish between the species of the Mycobacterium tuberculosis complex.
A negative result indicates the absence of detectable Mycobacterium tuberculosis complex DNA.
Isoniazid (INH) resistance mediated through a katG mutation will be reported when observed but lack of a katG mutation does not imply that the isolate is susceptible to INH. There are other genetic loci in addition to katG that can contribute to resistance for this drug.
This test should always be performed in conjunction with mycobacterial culture.
This rapid PCR assay detects Mycobacterium tuberculosis complex nucleic acid and, therefore, does not distinguish between viable, disease-related organisms and nucleic acid persisting from prior infection. Test results should be correlated with patient symptoms and clinical presentation before a definitive diagnosis is made.
A negative result does not rule out the presence of Mycobacterium tuberculosis complex or active disease because the organism may be present at levels below the limit of detection for this assay.
This test has not been studied for use with specimens from patients being treated with antituberculous agents and, therefore, should not be used to determine bacteriologic cure or to monitor response to therapy. It is not known how long the PCR assay can remain positive following treatment for Mycobacterium tuberculosis.
The sensitivity of this test with stool specimens is 80% and testing of additional stool specimens should be considered if the result from the first specimen is negative.
1. Iseman MD: A clinician’s guide to tuberculosis. Philadelphia, PA. Lippincott Williams and Wilkins, 2000
2. Centers for Disease Control and Prevention: Treatment of Tuberculosis, American Thoracic Society, CDC, and Infectious Diseases Society of America. MMWR Morb Mortal Weekly Rep 2003;52(No. RR-11):1-88