Development of Acquired Drug Resistance in Recurrent Tuberculosis Patients with Various Previous Treatment Outcomes

Setting: Chiang Rai province, Northern Thailand.

Objective: To study the probability of acquiring drug resistance to isoniazid (H) and rifampicin (R) on recurrence after treatment success, default and failure, among sputum smear-positive pulmonary tuberculosis (TB) patients treated with standardised short-course chemotherapy.

Design: Retrospective analysis of registration records of TB patients from May 1996 to December 2000 in Chiang Rai, where routine drug susceptibility testing (DST) is conducted for surveillance purposes. Patients registered twice or more were examined.

Results: Of 59 cases treated with HRZE/HR who underwent DST at the time of registration, 31 were fully susceptible to H and R at first registration, of whom four acquired drug resistance to H or R. Of 13 cases resistant to H or R at first registration, 11 became multidrug-resistant (MDR). The remaining 15 patients were original MDR cases. Among 28 MDR or H- or R-resistant cases, six reverted from resistant to susceptible.

Discussion: A high proportion of patients with H- or R-resistant TB became MDR after treatment with 2HRZE/HR. Using this regimen, MDR may increase in a population with a high prevalence of H or R resistance. We are unable to explain why some drug-resistant cases became drug-susceptible. Further investigation is necessary.