We evaluated the accuracy of a point-of-care test designed to measure adherence to isoniazid (INH) preventive therapy in a hospital setting in Rio de Janeiro, Brazil. Patients on treatment with daily INH and patients not receiving INH were included. Sensitivity and specificity of the test were 84%/98% at the first minute, and 95%/98% at the fifth minute, respectively. Among smokers, sensitivity and specificity was reduced (80%/89% at the fifth minute, respectively), but only 17% smoked. This test accurately detected INH metabolites 24h following directly observed INH intake, though sensitivity and specificity may be compromised by tobacco smoke exposure.