XtracTB Assay, a Mycobacterium tuberculosis molecular screening test with sensitivity approaching culture

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Abstract
Nucleic acid amplification tests are increasingly used to diagnose tuberculosis (TB) due to their speed and sensitivity compared to sputum smear microscopy. However, these tests fail to equal culture’s sensitivity with sputum smear microscopy negative specimens and therefore cannot be used to rule out TB disease. For molecular tests to match culture’s sensitivity, they must detect ≤10 genomic copies of Mycobacterium tuberculosis (MTB) DNA, the limit of detection of culture, process ≥1 ml of sputum ensuring sufficient number of MTB are in the reaction, and efficiently remove sputum associated inhibitors from this large sample. Here we report the preliminary characterization of XtracTB Assay, a MTB testing protocol designed for inclusion in either an integrated point-of-care platform or a high throughput automated central laboratory system. The test combines DNA sequence specific sample prep to reduce the co-extraction of qPCR inhibitors with the amplification of two MTB specific loci (IS6110 and senX3-regX3) to increase test sensitivity and minimize the likelihood of false negatives. The analytical sensitivity of the XtracTB Assay was 5 genomic copies/ml of sputum rivaling that of culture. Furthermore, 142 valid test results yield clinical sensitivity of 94.9% (95% CI: 90.1–99.9) and specificity of 100% (95% CI: 90.0–100.0).

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