Influence of medication choice and comorbid diabetes: the cost of bipolar disorder in a privately insured US population.

BACKGROUND: Bipolar disorder is the most expensive mental disorder for US employer health plans. No published studies have examined the impact of comorbid diabetes on the cost of treating bipolar disorder. The objectives of this work were to determine the direct costs incurred by patients with bipolar disorder in a US managed care plan, and to examine the influence (1) of drug therapy regimen on bipolar-related costs, and (2) of diabetes on bipolar-related and all-cause costs.
METHODS: A retrospective analysis of claims in a US private insurance database from January 1, 1999 through December 31, 2002 was performed. The database included at least 4.7 million enrollees each year. Diagnosis codes were used to identify patients with bipolar disorder; patients with diabetes were identified using diagnosis codes and medication use.
RESULTS: From 1999-2002, treated bipolar disorder was identified in 262 (33.9) [mean (standard deviation)] cases per 100,000 enrollees. Among patients with bipolar disorder in this cohort, between 6.3 and 7.4% were treated for diabetes each year. Among patients with newly treated bipolar disorder, 61.8% received initial therapy with only mood stabilizers, 24.3% received only atypical antipsychotics, and 13.9% received both. Mean all-cause cost for patients with bipolar disorder was US$2,690 in the 6 months before the first bipolar-related claim, and US$6,826 in the following year. Of the latter cost, bipolar-related cost was US$1,272. Patients with comorbid diabetes had much higher all-cause cost (US$11,317) than those without diabetes in the year following the first bipolar-related claim, but only slightly higher bipolar-related cost (US$1,349). Among newly treated bipolar disorder patients, all-cause and bipolar-related cost in the year after diagnosis was lowest in patients receiving only mood stabilizers. Ordinary least squares regression analysis found that treatment with mood stabilizers only was associated with 41% lower bipolar-related cost than treatment with atypical antipsychotics only (P < .001). Significant individual associations were also found between bipolar-related cost and bipolar disorder I diagnosis, severe bipolar disorder and comorbid personality disorders (P < .001 for each) but not comorbid diabetes (P = .27).
CONCLUSIONS: These results suggest that patients with bipolar disorder who receive only mood stabilizer therapy incur lower bipolar-related and all-cause cost than those receiving only atypical antipsychotics. In contrast to that for all-cause cost, comorbid diabetes had little impact on direct costs related to treating bipolar disorder itself.