THE PATTERN OF DRUG RESISTANCE AMONG NEWLY DIAGNOSED AND OLD CASES OF PULMONARY TUBERCULOSIS IN NRITLD

Background – Resistance to antituberculosis agents is an important obstacle in the treatment and control of tuberculosis in the world. The present study was undertaken to assess the pattern of resistance at a referral center in Tehran, Iran. Methods –The tests of pretreatment susceptibility to isoniazid (INH), rifampin (RIF), ethambutol (EMB), and streptomycin (STM) were performed by standard proportional method for the isolated bacilli from 273 smear- and culture-positive pulmonary tuberculosis patients (both old = 86/273 and new = 187/273) between September 1996 and March 2000, and the results were classified in three Groups: I) newly diagnosed without any history of treatment; II) patients with a history of treatment with one course; and III) patients with a history of treatment for two or more courses supposed to be MDR cases. The results were collected for each drug individually, and different combinations of two, three, and four medications. Results – Resistance was significantly higher in Group III compared with Groups II and I; the resistance in Group II was also significantly higher than that in Group I. We observed a high rate of primary resistance to INH and STM in Groups I (15%) and II (22.5%), respectively. A high rate of MDR (INH and RIF resistance) was found in Groups II (22.5%) and III (62%). Conclusion – The duration of bacilli exposure to antituberculosis agents in the past is a major factor in developing resistance. Due to high rates of primary resistance especially to STM in our area, we recommend a more conservative approach with six drugs for treatment of the patients whose initial four-drug regimens have failed (Group II).