Classification of drug-resistant tuberculosis in an epidemic area

Background Traditionally, patients with drug-resistant tuberculosis are cla ssified as having acquired drug-resistant or primary drug-resistant disease on the basis of a history of previous tuberculosis treatment. Only cases o f primary drug resistance are assumed to be due to transmission of drug-res istant strains. Methods This descriptive study of 63 patients with drug-resistant tuberculo sis assessed the relative contribution of transmission of drug-resistant st rains in a high-incidence community of Cape Town, South Africa, by restrict ion-fragment length polymorphism (RFLP). The RFLP results were compared wit h the results obtained by traditional classification methods. Findings According to RFLP definitions, 52% (33 cases) of drug-resistant tu berculosis was caused by transmission of a drug-resistant strain. The propo rtion of cases due to transmission was higher for multidrug-resistant (64%; 29 cases) than for single-drug-resistant (no cases) tuberculosis. By the c linical classification, only 18 (29%) patients were classified as having pr imary drug-resistant tuberculosis (implying transmission). The clinical cla ssification 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 pati ents who were initially infected with or reinfected with a drug-resistant s train. This definition could lead to misinterpretation of surveillance stud ies, incorrect evaluation of tuberculosis programmes, and delayed diagnosis and treatment of patients with multidrug-resistant disease. The clinical t erm acquired drug resistance should be replaced with the term "drug resista nce in previously treated cases", which includes cases with drug resistance due to true acquisition as well as that due to transmitted drug-resistant strains.