Words versus Strands: Reliability and Stability of Concordance Rates of Self-Reported and Hair-Analyzed Substance Use of Young Adults over Time.

Introduction
Population-level substance use research primarily relies on self-reports, which often underestimate actual use. Hair analyses offer a more objective estimate; however, longitudinal studies examining concordance are lacking. Previous studies showed that specific psychological and behavioral characteristics are associated with a higher likelihood of underreporting substance use, but the longitudinal stability of these associations remains unclear. We compared the prevalence of illegal and non-medical prescription substance use assessed with self-reports and hair analyses and predicted underreporting across two time points.Methods
Data were drawn from a community cohort study. At the first time point, the sample with self-report and hair analysis comprised 1,002 participants (Mage = 20.6 [SD = 0.38] years, 50.2% female), of which 761 (Mage = 24.5 [SD = 0.38] years, 48.3% female) also provided hair at the second time point. We compared substance use 3-month prevalence rates assessed by self-reports and hair analyses for the most frequent substances cannabis/tetrahydrocannabinol (THC), amphetamines, Ecstasy/3,4-methylenedioxymethamphetamine (MDMA), cocaine, ketamine, codeine, and opioid painkillers. Binary logistic regressions were conducted to test behavioral and psychological predictors of underreporting.Results
Self-reported past-year prevalence rates of non-medical substance use were high, specifically for cannabis (56% prevalence rate at age 20/49% at age 24), Ecstasy (13%/14%), codeine (13%/11%), cocaine (12%/13%), and opioid painkillers (4%/11%). Comparing self-report and hair-analysis 3-month prevalence rates over time, consistent underreporting (similar underreporting rates between time points and investigation of false negatives) was observed for daily cannabis (22%/23%), Ecstasy/MDMA (41%/52%), cocaine (30%/60%), ketamine (61%/72%), and codeine use (48%/51%). Underreporting of Ecstasy/MDMA, cocaine, ketamine, and opioid painkillers significantly increased. Contrarily, weekly to daily cannabis (31%/18%), amphetamine (95%/11%), and opioid painkiller use (12%/66%) were overreported. Hair analysis-derived 3-month prevalence rates of cocaine (9%/23%) and ketamine (2%/6%) strongly increased over time, while decreasing for codeine (11%/8%). Balanced accuracies were higher for hair analysis compared to self-reports for daily cannabis, Ecstasy/MDMA, cocaine, ketamine, and codeine but lower for weekly to daily cannabis and amphetamines, while fairly similar for opioid painkillers. Accuracy metrics were largely stable for cannabis measures but partially varied over time for other substances, which was likely driven by the large changes in underreporting. False negative reports were associated across both time points, indicating an intra-individual consistency of underreporting. At both time points, delinquency and attention-deficit hyperactivity disorder symptoms were associated with an increased likelihood of accurately reporting cocaine use, while internalizing symptoms increased the likelihood of accurately reporting codeine use.Conclusion
Consistent and changeable underreporting emphasizes the importance of objective substance use assessments, specifically for studies investigating cocaine, Ecstasy/MDMA, ketamine, and codeine.