Retention in care as a quality measure for opioid use disorder.

US health care systems are struggling to formulate quality metrics that are patient-centered and describe outcomes rather than processes. Addiction medicine is no exception. Of particular interest is the identification of quality metrics in opioid use disorder (OUD) treatment. The prevalence and lethality of OUD, together with concomitant efforts to increase provision of its care, makes well-designed and validated quality metrics especially important. One insightful approach has been to use the "cascade of care" model derived from human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) care. A core part of the cascade is "retention in care," a concept initially identified during the development of methadone-based OUD care. Not only is retention in care associated with improved morbidity and mortality, it also offers strategic approaches to improving care for OUD. This paper provides an introduction to retention in care and its implications for quality measurement.