25475 Mycobacterium tuberculosis Complex, Molecular Detection, PCR (MTBRP)

This test should always be performed in conjunction with mycobacterial culture.

Specimen must arrive within 7 days of collection; if greater than 7 days of collection, the specimen will be rejected.

Specimen source is required. ​

​Body fluid: Only fresh, non-NALC/NaOH-digested body fluid is acceptable.

Gastric washing: Neutralize specimen within 4 hours of collection with 20 mg of sodium carbonate per 2 mL of gastric washing.

Stool: Only fresh, non-NALC/NaOH-digested stool is acceptable. 

Tissue: Only fresh - keep moist with sterile water or saline, non-NALC/NaOH-digested tissue is acceptable.

Urine: Collect a random urine specimen.

NALC/NaOH-digested respiratory: Submit digested specimen treated with NALC/NaOH. Clearly indicate on the container and order form that specimen is a digested specimen.

A mycobacterial culture must always be performed in addition to the polymerase chain reaction (PCR) test. If your facility is unable to perform mycobacterial culture, the Mycobacteria Culture test (CTB / Mycobacteria and Nocardia Culture, Varies) should be ordered. The overall sensitivity of the PCR from acid-fast smear positive specimens is approximately 96% compared to mycobacterial culture but sensitivity of the PCR from a smear negative specimen is lower and a negative result does not rule out M. tuberculosis complex.

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 M 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 M 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.

Each year, Mycobacterium tuberculosis accounts for more than a million deaths and is responsible for millions of newly diagnosed cases of tuberculosis worldwide. M tuberculosis is spread from person-to-person via respiratory transmission and has the potential to become resistant to many or all antibiotics currently used if antimycobacterial treatment is not promptly initiated. Therefore, rapid and accurate detection of M tuberculosis in patient specimens is of clinical and public health importance.

Conventional culture methods can generally detect M 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 polymerase chain reaction (PCR) assay detects M tuberculosis complex DNA directly from specimens without waiting for growth in culture and, therefore, the results are available rapidly after receipt in the laboratory. A mycobacterial culture must 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 M tuberculosis complex. In addition, the assay can detect genotypic resistance to isoniazid mediated by mutations in the katG target, when present.

A positive result indicates the presence of Mycobacterium tuberculosis complex DNA. Members of the M tuberculosis complex detected by this assay include M tuberculosis, Mycobacterium bovis, Mycobacterium bovis bacillus Calmette-Guerin, Mycobacterium africanum, Mycobacterium canettii, and Mycobacterium microti. Other species within the M tuberculosis complex (eg, Mycobacterium caprae, Mycobacterium pinnipedii, and Mycobacterium mungi) 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 M tuberculosis complex. If an isolate of M tuberculosis complex is already available, species identification can be performed; order TBSP / Mycobacterium tuberculosis Complex Species Identification, PCR, Varies.​

A negative result indicates the absence of detectable M tuberculosis complex DNA.

Isoniazid (INH) resistance mediated through a katG variant will be reported when observed but lack of a katG variant 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.