[A study of the value of three molecular diagnostic techniques in the diagnosis of tuberculosis]

To evaluate the diagnostic value of real-time fluorescent RNA isothermal amplification detection technology (simultaneous amplification and testing, SAT), Mycobacterium nucleic acid detection (PCR-fluorescence probe)method (TB-NTM-PCR) and Xpert MTB/RIF detection in the diagnosis of tuberculosis.

A total of 378 sputum specimens from pulmonary tuberculosis patients were collected between April to July 2014 in Xi'an Thoracic Tumor and Tuberculosis Hospital. The specimens were detected by 5 methods at the same time including acid-fast stain, SAT method, TB-NTM-PCR method, TB 960 rapid liquid culture and Xpert MTB/RIF. The sensitivity and specificity of SAT method, TB-NTM-PCR method and Xpert MTB/RIF were calculated according to the results of TB 960 rapid liquid culture and staining. The difference among all the 3 methods was analyzed by Chi-squared test.

The positive rate of SAT-TB,TB-NTM-PCR and Xpert MTB/RIF were 37.6% (142/378), 37.8% (143/378) and 53.4% (202/378), respectively. In specimens both positive for acid-fast stain and culture, the positive rate of SAT method was 84.6% (77/91), that of TB-NTM-PCR was 91.2% (83/91), and that of Xpert MTB/RIF was 96.7% (88/91), the difference being significant (P=0.018 2). In specimens negative for acid-fast stain but positive for culture, the positive rate of SAT method was 61.9% (60 /97), that of TB-NTM-PCR was 44.3% (43/97), and that of Xpert MTB/RIF was 80.4% (78/97), the difference being significant (P<0.000 1). In specimens both negative for acid-fast stain and culture, the positive rate of SAT method was 1.6% (3/185), that of TB-NTM-PCR was 6.5% (12/185), and that of Xpert MTB/RIF was 16.8% (31/185), the difference being significant (P=0.018). In specimens positive for acid-fast stain but negative for culture, the number of positive samples of SAT,TB-NTM-PCR and Xpert MTB/RIF were 3 (3/5), 5 (5/5),and 5 (5/5), respectively. With the result of TB 960 rapid liquid culture and staining as the reference, Xpert MTB/RIF showed the highest sensitivity of 87.6% (163/186), the minimum rate of missed diagnosis of 12.4% (24/193), and the highest negative predictive value of 88.5% (185/209); SAT-TB showed the highest specificity of 98.2% (214/218), the minimum rate of misdiagnosis of 1.8%(4/218), the highest positive predictive value of 97.2% (138/142). With the result of TB 960 rapid liquid culture as the reference, the sensitivity and the specificity of Xpert MTB/RIF were 95.52% (128/134) and 95.24% (20/21). The accordance rate of Xpert MTB/RIF and TB 960 rapid liquid culture was 95.48%(148/155).

The 3 molecular detection methods showed good results for the auxiliary diagnosis of tuberculosis. Xpert MTB/RIF had the best performance both in smear positive and negative specimens and it can detect rifampicin related rpoB gene mutations at the same time.