Drug Use Monitoring in Australia (DUMA) participants are asked to answer a questionnaire about their use of drugs, as well as to supply a urine sample. This sample is tested for the presence of illicit drugs and compared with the self-reported answers in the questionnaire. In this report Kiah McGregor and Toni Makkai consider possible errors that may lead to inconsistent results between the urinalysis test and self-reported information, as well as reasons why under-reporting may occur, with a specific focus on sociodemographic factors.