Average daily dose, medication adherence, and healthcare costs among commercially-insured patients with fibromyalgia treated with duloxetine

Abstract

Objective:What is the relationship between average daily dose (ADD) of duloxetine, adherence to therapy, and healthcare costs among patients with fibromyalgia?Methods:Employing a retrospective cohort design, this study assessed administrative claims data for commercially-insured fibromyalgia patients who initiated duloxetine in 2006. Initiation was defined as a 90-day clean period without access to duloxetine. Five study cohorts were constructed based on the ADD of duloxetine, calculated from all prescriptions dispensed over the 12-month follow-up period: <30 mg, 30 mg, 31–59 mg, 60 mg, and >60 mg (duloxetine starting dose: 30 mg/day; target dose: 60 mg/day). Medication possession ration (MPR) was used to assess duloxetine adherence, with MPR ≥0.8 as ‘high’ adherence. Multivariate regressions were applied to examine the association between ADD, duloxetine adherence, and healthcare costs (total, inpatient, outpatient, and pharmacy), adjusting for differences in demographic and clinical characteristics between cohorts.Results:Of the 4869 patients with fibromyalgia, 3% had an ADD of <30 mg, 12% had 30 mg, 21% had 31–59 mg, 46% had 60 mg, and 18% had >60 mg. High adherence to duloxetine ranged from 16% to 41% across cohorts. Controlling for demographic and clinical characteristics, patients with an ADD of 31–59 mg or >60 mg were more likely to adhere to duloxetine therapy than those in the 60 mg cohort, while patients in the <30 mg cohort were less likely to adhere to duloxetine (all p < 0.05). Total costs were significantly higher for individuals in the 31–59 mg or >60 mg ADD cohorts ($1834 and $5766) than those maintaining an ADD of 60 mg, as were outpatient ($1167 and $2275) and pharmacy costs ($502 and $2983) (all p < 0.05). Patients with an ADD of 30 mg had significantly lower total (−$1770), inpatient (−$1524), and pharmacy (−$827) costs (all p < 0.05) than those with an ADD of 60 mg.Limitations:This study is subject to selection bias because some confounders might be unobserved in the administrative claims database. Indirect costs or over-the-counter medication costs were also not available.Conclusions:Fibromyalgia patients with an ADD of 60 mg of duloxetine had better adherence than patients with an ADD ≤30 mg. Duloxetine patients with an ADD of 30 mg (31–59 or >60 mg) had significantly lower (higher) healthcare costs than those maintaining an ADD of 60 mg.