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%CI=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.