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Among persons with human immunodeficiency virus (HIV) infection, illegal drug use and hazardous alcohol use are hypothesized to be strong risk factors for failure to achieve or maintain a suppressed HIV viral load, but accurate quantification of this association is difficult because of challenges involved in measuring substance use as part of routine clinical care. We estimated the associations of recent cocaine use, opioid/heroin use, and hazardous alcohol use with unsuppressed viral load among 1,554 persons receiving care at the John G. Bartlett Specialty Practice (Baltimore, Maryland) between 2013 and 2017. We accounted for measurement error in substance use using Bayesian models and prior estimates of the sensitivity and specificity of 2 imperfect measures of substance use derived from a previous analysis in this cohort. The prevalence difference for unsuppressed viral load associated with recent cocaine use was 11.3% (95% credible interval (CrI): 6.4, 17.0); that associated with recent opioid/heroin use was 13.2% (95% CrI: 6.6, 20.7); and that associated with recent hazardous alcohol use was 8.5% (95% CrI: 3.2, 14.4). Failure to account for measurement error resulted in clinically meaningful underestimates of the prevalence difference. Time-varying substance use is prevalent and difficult to measure in routine care; here we demonstrate a method that improves the utility of imperfect data by accounting for measurement error., (© The Author(s) 2019. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Accurate, routine measurement of recent illicit substance use is challenging. The Johns Hopkins Human Immunodeficiency Virus Clinical Cohort (Baltimore, Maryland) collects 2 imperfect but routine measurements of recent substance use: medical record review and self-interview. We used Bayesian latent class modeling to estimate sensitivity and specificity of each measurement as well as prevalence of substance use among 2,064 patients engaged in care during 2007-2015. Sensitivity of medical record review was higher than sensitivity of self-interview for cocaine and heroin use; posterior estimates ranged from 44% to 76% for cocaine use and from 39% to 67% for heroin use, depending on model assumptions and priors. In contrast, sensitivity of self-interview was higher than sensitivity of medical record review for any alcohol use, hazardous alcohol use, and cigarette smoking. Posterior estimates of sensitivity of self-interview were generally above 80%, 85%, and 87% for each substance, respectively. Specificity was high for all measurements. From one model, we estimated prevalence of substance use in the cohort to be 12.5% for cocaine, 9.3% for heroin, 48.5% for alcohol, 21.4% for hazardous alcohol, and 55.4% for cigarettes. Prevalence estimates from other models were generally comparable. Measurement error of substance use is nontrivial and should be accounted for in subsequent analyses.