Reliability and validity of the Opiate Treatment Index among a sample of opioid users in the United Kingdom

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Abstract
To validate the Opiate Treatment Index (OTI) for use in the United Kingdom, 72 subjects attending two methadone clinics in London completed confidential interviews with two researchers at two points in time, 1–2 weeks apart. About half the interviews were conducted by the same interviewer on both occasions, and the remaining by different interviewers. These research designs enabled an estimation of the test‐retest and inter‐rater reliability of the instrument. Subjects' responses were validated against key worker reports and self‐reported drug use against urinalysis results. The majority of the respondents were males (65%) and their mean age was 34.2 years (range 21–52; SD 8.5). Test‐retest reliability coefficients were high for all the scales (range 0.77‐0.86) in both the same and different interviewer situations. The internal reliability of the scales was generally high (Cronbach's α: range 0.34‐0.93). The correlation with key workers' reports was high with the percentage agreement on the items ranging 74–100 (mean 87). The percentage agreement of self‐reported drug use with urinalysis results was high (range 69–95; mean 89). These findings are similar to those obtained among Australian subjects and suggest a high cross‐cultural validity for the OTI. Minor changes to the OTI have been incorporated into a modified version for use in the UK.