The test-retest reliability of the frequency of multiple drug use in young drug users entering treatment.

Assessment of multiple drug use relies primarily on self-report. Several studies support the reliability of client self-reports of drug use but these studies have not involved assessment of the actual frequency of drug use. This test-retest reliability study assessed the frequency of drug use in a clinical sample of 103 multiple drug users, aged 16-25 years. At initial assessment, all participants completed the Drug Use History Form (DUHF) that inquired about the number of drug-using days and the daily frequency of use for 13 drug classes during four time intervals. The DUHF was readministered 2-4 weeks later. Reliability was assessed using Intra-class correlations (ICC's). The results indicated that clients do, in general, reliably report both the number of days of use and daily frequencies. The two frequency measures were not highly correlated. Reliability estimates declined over time but most markedly after 90 days, suggesting that assessments of drug use can be reliably extended beyond 30 days. Frequency estimates based solely on the number of days of use of a substance may be unreliable estimates of actual drug consumption, indicating limitations to this commonly used outcome measure.