Abstract:

Objective To analyze the relationship between GeneXpert MTB/RIF assay（Xpert）determination load and Mycobacterium tuberculosis（MTB） culture positive rate and rifampicin resistance phenotype. Methods From April 2016 to December 2019，the patients whose sputum Xpert determination results showed rifampicin resistance were enrolled from Wenzhou Central Hospital，and the differences between the positive rate of MTB and fasle positive rate for determining rifampicin resistance among patients with different Xpert determination loads were evaluated. Results Of the 183 patients，48，46，56 and 33 cases showed Xpert determination loads of high，medium，low and very low，respectively. Positive MTB culture was found in 80.87%（148/183） patients. The drug resistance rates of first-line anti-tuberculosis drugs were 79.05%（117/148）for isoniazid，81.08%（120/148） for rifampicin，27.03%（40/148） for ethambutol and 52.70%（78/148） for streptomycin. Totally，11 cases were fully sensitive to first-line anti-tuberculosis drugs. Among the 24 patients who were only resistant to 1 drug，the most common one was rifampicin（17 cases）. Totally，34 patients were resistant to 2 drugs，and rifampicin and isoniazid were the most common（25 cases）；53 patients were resistant to 3 drugs，among which rifampicin，isoniazid and streptomycin were the most common（40 cases）. The positive rates of MTB culture in patients with very low，low，medium and high Xpert determination loads were 48.48%，76.79%，93.48% and 95.83%，respectively. There was statistical significance in MTB culture's positive rates among patients with different Xpert determination loads（P<0.001）. The false positive rates of rifampicin phenotypes were 43.75%，23.26%，13.95% and 10.87% in patients with very low，low，medium and high Xpert determination loads，respectively. There was statistical significance in the false-positive rates of rifampicin phenotypes among patients with different Xpert determination loads（P=0.022）. Conclusions The determination of MTB with Xpert could reduce the missed diagnosis rate of culture-negative pulmonary tuberculosis. However，false-negative culture and false-positive rifampicin resistance are more likely to occur when the loads were low.

Key words: GeneXpert MTB/RIF assay, Mycobacterium tuberculosis, Rifampicin-resistance, Phenotype, Xpert determination load