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Updated: Aug 14, 2026

Diagnosing Pulmonary Tuberculosis with the Xpert MTB/RIF Test
Published on: April 9, 2012
Burden of Mycobacterium tuberculosis in sputum samples can be reliably determined using a quantitative,
E A Sweet-Cordero1, H F Chambers, R Cicero-Sabido
1Division of Infectious Diseases, University of California, San Francisco, USA.
Objective:
To develop a rapid assay for quantitation of Mycobacterium tuberculosis in sputum samples using the competitive polymerase chain reaction (PCR) and a colorimetric microtiter well detection format.
Design:
The assay relies on the co-amplification of a 419 base pair (bp) pab fragment of M. tuberculosis together with a target template (pab/tet) made by splicing a fragment of tet excised from pbr322 between the 5' and 3' ends of the pab fragment to create a 380 bp hybrid template amplified with the same primers but readily distinguishable using probes specific for either pab or tet.
Results:
We demonstrate a good correlation between the results obtained using this assay and the results of quantitative culture.
Conclusion:
This assay provides quantitative information regarding M. tuberculosis burden in samples containing between 10(3) and 10(8) colony forming units/milliliter (CFU/ml).
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