Substance use assessment: comparing self-reports with objective data in a research setting.

Accurate assessment of substance use is essential in public health, clinical, and research settings. While self-reports are widely used, they are prone to biases such as social desirability and recall errors. Objective biological measures, such as hair toxicology, offer a longer detection window and may improve data validity. This study examined the concordance between self-reported substance use and hair toxicology results, with a particular focus on cannabis. It also explored the prevalence and predictors of underreporting, and the relationship between cannabis use patterns and THC detection in hair. Data were collected from 75 adult participants. Self-reported substance use was assessed via questionnaire, and hair samples were analyzed using liquid chromatography-mass spectrometry (LC-MS/MS) to detect drug use over the prior 3 months. Underreporting was defined as a negative self-report with a positive hair test. Results showed that 21.3% of participants underreported use of at least one substance. While group differences were not statistically significant, moderate-to-large effect sizes were observed, these effect sizes are descriptive in nature and may reflect possible discordance for substances such as MDMA and cocaine. No sociodemographic factors or alcohol use patterns significantly predicted underreporting. However, self-reported cannabis use frequency and quantity were significant predictors of THC detection, while years of use and time since last use were not. These findings highlight the limitations of relying solely on self-reports and emphasize the value of integrating objective biological measures. A combined approach improves the accuracy of substance use assessment and helps address underreporting biases, particularly in settings where legal or social pressures may influence disclosure.