Objective: To estimate the prevalence of primary resistance to the drugs in the basic treatment regimen for tuberculosis in treatment-naïve patients with pulmonary tuberculosis and no known risk factors for such resistance, as well as to identify factors potentially associated with drug resistance. Methods: This was an exploratory cross-sectional study. We analyzed the medical records of the subjects enrolled in two clinical trials of treatments for drug-susceptible tuberculosis between November 1, 2004 and March 31, 2011 at the Prof. Newton Bethlem Outpatient Clinic of the Federal University of Rio de Janeiro Thoracic Diseases Institute, located in the city of Rio de Janeiro, Brazil. The inclusion criteria were being ≥ 18 years of age, testing positive for AFB in the first sputum sample, having a positive culture for Mycobacterium tuberculosis, having undergone drug susceptibility testing, and being treatment-naïve. Patients with a history of imprisonment or hospitalization were excluded, as were those who had been in contact with drug-resistant tuberculosis patients. Results: We included 209 patients. The overall prevalence of primary drug resistance was 16.3%. The overall prevalence of resistance to isoniazid and streptomycin was, respectively, 9.6% and 9.1%, compared with 5.8% and 6.8% for single-drug resistance to isoniazid and streptomycin, respectively. The prevalence of resistance to two or more drugs was 3.8%, and the prevalence of multidrug resistance was 0.5%. No statistically significant associations were found between the variables studied and drug susceptibility testing results. Conclusions: In this sample, the prevalence of primary drug resistance was high despite the absence of known risk factors.

Keywords: Tuberculosis, pulmonary; Tuberculosis, multidrug-resistant; Risk factors.