Abstract:

Objective To assess the performance of GeneXpert MTB/RIF (GeneXpert) assay in the detection of Mycobacterium tuberculosis (MTB) and its rifampicin resistance, and further analyze the relationship between the false-positive rifampicin-resistant results of GeneXpert assay and the low bacterial load in the clinical samples. Methods From April 2016 to September 2017, a total of 2707 clinical samples (1945 sputum specimens and 762 alveolar lavage fluid specimens) of suspected pulmonary tuberculosis cases from Xi’an Chest Hospital were collected for GeneXpert detection, BACTEC MGIT 960 liquid culture and drug susceptibility test. Among them, GeneXpert detected rifampicin-resistant samples were further tested by GenoType MTBDRplus assay. Results The positive rate of GeneXpert assay in detecting MTB was 39.8% (1077/2707). Seven cases had “indeterminate” rifampicin resistance results and “Very low” bacterial load level. Considering MGIT 960 drug susceptibility test as the gold standard, the sensitivity and specificity of GeneXpert test in detecting rifampicin resistance was 89.4% (135/151) and 94.7% (721/761), respectively. The inconsistent results of two methods were found in 56 cases. Among them, 40 cases were rifampicin-resistant detected by GeneXpert but were rifampicin-sensitive detected by MGIT 960. Of the 40 patients, 10, 6, 14 and 10 showed a MTB load level of “Very low”, “Low”, “Medium” and “High”, respectively. Taking MGIT 960 test result as gold standard, the false-positive rates of rifampicin-resistance by GeneXpert in the four MTB load levels were 58.8% (10/17), 16.7% (6/36), 19.2% (14/73) and 20.4% (10/49), respectively; the difference was statistically significant (χ ²=13.981, P<0.05). Among the 40 cases, 30 cases with Low, Medium and High level of MTB load were rifampicin-resistant when detected by the MTBDRplus test, which was consistent with the GeneXpert results. As to the 10 cases with Very low level of MTB load, 2 cases were negative and 8 were positive by MTBDRplus test, and in the cases with positive results, 5 cases were rifampicin-sensitivity and 3 rifampicin-resistance. Conclusion The detection of MTB rifampicin-resistance with GeneXpert assay has a higher sensitivity and specificity. However, when the tested load of MTB is Very low, false drug resistant results may occur.

Key words: Mycobacterium tuberculosis, Rifampicin, Drug resistance, Laboratory techniques and procedures, Comparative study, False positive reactions