Medical costs after initiation of drug treatment for overactive bladder: effects of selection bias on cost estimates

Nicole M Nitz, Zhanna Jumadilova, Theodore Darkow, Jennifer R Frytak, Tamara Bavendam
American Journal of Managed Care 2005, 11 (4): S130-9

OBJECTIVES: To compare posttreatment medical costs for patients with overactive bladder (OAB) initiating treatment with oxybutynin chloride immediate release (oxybutynin IR), oxybutynin chloride extended release (oxybutynin ER), or tolterodine extended-release tartrate capsules (tolterodine ER).

METHODS: Data were drawn from administrative claims of enrollees aged 18 years and older of a large US health plan. OAB patients were identified if at least 1 claim with an International Statistical Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) code for OAB appeared in medical claims from January 1, 2001, to December 31, 2002. The index prescription was assigned as the first filled prescription of oxybutynin IR (n = 3052), oxybutynin ER (n = 4503), or tolterodine ER (n = 7027) during the subject identification period. Medical costs over the year after initiation were calculated as a function of the health plan and member liability. Independent variables were treatment cohort, sex, age group, geographic region, baseline costs, specific OAB diagnosis codes, and comorbid illnesses. To compare medical costs across treatment cohorts, multivariate regressions correcting for potential selection bias were used.

RESULTS: Multivariate analysis results revealed that costs for patients taking oxybutynin IR were 48% higher than costs for patients taking tolterodine ER (P = .026), and costs for patients taking oxybutynin ER were 191% higher than costs for patients taking tolterodine ER (P <.0001). Adjusted medical costs were dollar 7486 for patients taking oxybutynin IR and dollar 14 766 for patients taking oxybutynin ER compared with dollar 5074 for patients taking tolterodine ER.

CONCLUSION: Differences in medical costs that remained after adjusting for patient characteristics suggest that treatment with tolterodine ER may be associated with lower medical care utilization after initiation of therapy for OAB.

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