Objective: We conducted a retrospective study to evaluate the performance of MeltPro assay for detecting ethambutol (EMB) susceptibility of Mycobacterium tuberculosis  (MTB) isolates in sputum specimens in Beijing, China.
Methods: Smear-positive TB patients undergoing MeltPro assay in the Beijing Chest Hospital between January 2019 and December 2019 were included. Phenotypic drug susceptibility testing (DST) was used as the reference standard to calculate the diagnostic accuracy of MeltPro assay for EMB resistance. Sanger sequencing of embB  gene was conducted to resolve the discrepancies between MeltPro assay and phenotypic DST.
Results: A total of 222 smear-positive patients were included in our analysis. The overall agreement rate between the two assays was 91.4%, with a kappa value of 0.78. Among 59 EMB-resistant TB cases diagnosed by DST, 49 were identified by MeltPro assay, demonstrating a sensitivity of 83.1%. In addition, 154 out of 163 EMB-susceptible patients diagnosed by DST were correctly detected with MeltPro assay, yielding a specificity of 93.9%. The probe frequency associated with the observed EMB-resistance was as follows: A (45/58), B (7/58), and D (6/58), and no EMB-resistance was associated with probe C. The presence of amino acid substitution was observed among all 9 cases with potentially “false-negative” results, including 7 with Met306Ile, 1 with Met306Val, 1 with Gly406Asp, respectively.
Conclusion: MeltPro assay is a promising diagnostic tool for the detection of EMB resistance in China. The specific amino acid substitution in embB  gene is the major reason for discrepancies between MeltPro assay and phenotypic DST.
Keywords: tuberculosis, ethambutol, molecular diagnostic, embB, China