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OBJECTIVE:: The objective of this study was to differentiate between 3 measures of antidepressant adherence with regard to the number of patients deemed adherent to therapy and the association between adherence and resource utilization.DESIGN AND SETTING:: The authors conducted a retrospective study of patients initiating selective serotonin reuptake inhibitor (SSRI) therapy for depression and/or anxiety between July 2001 and June 2002 in a large national managed care database.MAIN OUTCOME MEASURES:: Rates of 6-month SSRI adherence were measured by 3 different metrics: length of therapy (LOT), medication possession ratio (MPR), and combined MPR/LOT. Differences in resource utilization for each adherence metric were measured for patients deemed as 1) adherent, 2) nonadherent, 3) therapy changers, and 4) dose titraters.RESULTS:: There were 22,947 patients meeting study criteria. Although statistically different, 6-month adherence rates were numerically similar across all methods (LOT, 44.6%; MPR, 43.3%; and MPR/LOT, 42.9%, P < 0.001); approximately 57% of patients were nonadherent to therapy. Regardless of metric, the adherent cohort incurred the lowest yearly medical costs, followed by the nonadherent, titrate, and therapy change cohorts (P < 0.001 between adherent cohort and all other cohorts). The LOT method produced the greatest difference in yearly medical costs between adherent and nonadherent patients ($511) followed by MPR/LOT ($432) and MPR ($423). When antidepressant prescription costs were added to medical costs, patients requiring a therapy change and titrating therapy incurred higher costs than adherent patients, whereas nonadherent and adherent patients incurred similar costs.CONCLUSION:: Regardless of adherence metric, approximately 43% of patients were adherent to antidepressant therapy, and adherent patients were associated with the lowest yearly medical costs. [ABSTRACT FROM AUTHOR]