An examination of Alzheimer's disease case definitions using Medicare claims and survey data

The prevalence and expenditure estimates of Alzheimer's disease (AD) from studies using one data source to define cases vary widely. The objectives of this study were to assess agreement between AD case definitions classified with Medicare claims and survey data and to provide insight into causes of widely varied expenditure estimates.

Data were obtained from the 1999-2004 Medicare Current Beneficiary Survey linked with Medicare claims (n = 57,669). Individuals with AD were identified by survey, diagnosis, use of an AD prescription medicine, or some combination thereof. We also explored how much health care and drug expenditures vary by AD case definition.

The prevalence of AD differed significantly by case definition. Using survey report alone yielded more cases (n = 1,994 or 3.46%) than diagnosis codes alone (n = 1,589 or 2.76%) or Alzheimer's medication use alone (n = 1,160 or 2.01%). Agreement between case definitions was low, with kappa coefficients ranging from 0.37 to 0.40. Per capita health expenditures ranged from $16,547 to $24,937, and drug expenditures ranged from $2,303 to $3,519, depending on how AD was defined.

Different information sources yield widely varied prevalence and expenditure estimates. Although claims data provided a more objective means for identifying AD cases, survey report identified more cases, and pharmacy data also are an important source for case ascertainment. Using any single source will underestimate the prevalence and associated cost of AD. The wide range of AD cases identified by using different data sources demands caution interpreting cost-of-illness studies using single data sources.