Three-year retention in methadone opioid agonist treatment: A survival analysis of clients by dose, area deprivation, and availability of alcohol and cannabis outlets

Objective: To determine the effect of clinical, socio-demographic, and contextual characteristics on treatment retention in an opioid treatment program (OTP). Methods: A retrospective longitudinal review of 851 clients who received methadone at the only state-funded OTP in Spokane County, Washington between 2015 and 2017. A time variable (the number of days in treatment) and a status indicator (to distinguish between clients who dropped out or censored) worked together to define retention in treatment. Our hypothesized covariates included: area deprivation, distance to the OTP, availability of cannabis retail outlets, availability of on-premise and off-premise alcohol outlets, methadone dosage, age, gender, race, and years on treatment. Cox regression within the family of survival analysis was used to model time-to-event data in the presence of censored cases. Results: The median duration of retention was 394 (95%CI = 324-464) days. In the multivariable Cox regression, factors predicting treatment retention were area deprivation (HR = 1.79, 95%CI = 1.02-3.15, p = 0.04), age (HR = 0.99, 95%Cl = 0.98-.99, p = 0.008), dosage of methadone (HR = 0.98, 95%CI = 0.98-0.98, p < 0.001), and the number of years on treatment (HR = 1.12, 95%CI = 1.06-1.18, p < 0.001). Conclusions: The findings of this study showed age and methadone dosage were protective factors and area deprivation and years on treatment were risk factors for treatment retention. After dichotomizing methadone dosage, a unique finding of this study was that higher dosage of methadone did not lead to increasingly smaller HRs for dropping out of treatment. Considering that opioid use disorder is a chronic condition, efforts need to be made to target factors associated with retention.