Background: Traditionally, patients with drug-resistant tuberculosis are classified as having acquired drug-resistant or primary drug-resistant disease on the basis of a history of previous tuberculosis treatment. Only cases of primary drug resistance are assumed to be due to transmission of drug-resistant strains.
Methods: This descriptive study of 63 patients with drug-resistant tuberculosis assessed the relative contribution of transmission of drug-resistant strains in a high-incidence community of Cape Town, South Africa, by restriction-fragment length polymorphism (RFLP). The RFLP results were compared with the results obtained by traditional classification methods.
Findings: According to RFLP definitions, 52% (33 cases) of drug-resistant tuberculosis was caused by transmission of a drug-resistant strain. The proportion of cases due to transmission was higher for multidrug-resistant (64%; 29 cases) than for single-drug-resistant (no cases) tuberculosis. By the clinical classification, only 18 (29%) patients were classified as having primary drug-resistant tuberculosis (implying transmission). The clinical classification was thus misleading in 25 patients.
Interpretation: The term acquired drug resistance includes patients infected with strains that truly acquired drug resistance during treatment and patients who were initially infected with or reinfected with a drug-resistant strain. This definition could lead to misinterpretation of surveillance studies, incorrect evaluation of tuberculosis programmes, and delayed diagnosis and treatment of patients with multidrug-resistant disease. The clinical term acquired drug resistance should be replaced with the term "drug resistance in previously treated cases", which includes cases with drug resistance due to true acquisition as well as that due to transmitted drug-resistant strains.