Setting: Tuberculosis clinic in México. Objective: Since the Xpert®MTB/RIF does not detect resistance to isoniazid, our objective was to emphasize the need for additional drug susceptibility testing. Design: A prospective study. All patients with an Xpert®MTB/RIF and a positive tuberculosis culture with drug susceptibility testing were included. Results: 70 patients were included. Forty-two (60%) had a history of previous treatment for TB. Fourteen patients (20%) had a strain resistant to isoniazid (H), twelve of them (85.7%) with a history of TB treatment in the past vs. 2 (7.1%) among new cases (p = 0.028). Four patients (5.7%) had resistance to rifampin (R); three of them were previously treated cases. Additionally, six patients with a negative Xpert test (8.6%) had a positive MGIT culture; three of them were resistant to H (the 3 were poly-resistant). Two patients with a positive Xpert®MTB/RIF test without R resistance were phenotypically multidrug-resistant. Conclusion: Isoniazid resistance is associated with overall increased treatment failure, relapse, and acquired multidrug resistance in patients treated with regimens containing only first-line tuberculosis drugs. It is urgent that national TB programs implement the necessary infrastructure to complement the Xpert®MTB/RIF results with DST either by phenotypic or genotypic methods.
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