Accounting for unobservable exposure time bias when using Medicare prescription drug data.
OBJECTIVE
To describe the prevalence and correlates of unobservable medication exposure time, and to recommend approaches for minimizing bias, in studies using Medicare Part D data..


METHODS
179,065 Medicare patients hospitalized for an AMI in 2007 or 2008.


METHODS
We compared two methods for creating medication exposure observation periods using acute care discharge vs. post-acute care discharge dates. We examined options for increasing cohort sizes by requiring different thresholds for observable days, or by using as a covariate, in the observation period. We calculated the extent and health status correlates of unobserved Medicare Part D exposure time and examined its association with receipt of beta-blockers.


RESULTS
39% of patients had unobservable time during the 30 day exposure assessment period following acute care; they were significantly older, had more comorbidity and longer acute care stays, had worse 1-year survival, and were significantly less likely to be classified as beta-blocker users. Using the alternative exposure assessment window, only 29% of the sample had unobservable time, and differences between groups were less pronounced. Significant gains in sample size can be obtained by restricting or controlling for the number of observable days required in the exposure assessment period.


CONCLUSIONS
Unobservable exposure time is common among Medicare Part D beneficiaries, and they are often in worse health. To retain patients with unobservable exposure time, we recommend stratifying patients on receipt of post-acute facility-based care, calculating and using observable days as a covariate and, when appropriate, using the discharge date from contiguous post-acute facility care for beginning the exposure assessment period.
University of Iowa College of Pharmacy-Department of Pharmacy Practice and Science.
Elizabeth A
EA
Cook
Buccaneer-A General Dynamics Company.
Kathleen M
KM
Schneider
University of Iowa College of Public Health-Department of Epidemiology.
Elizabeth
E
Chrischilles
University of Iowa College of Pharmacy-Department of Pharmacy Practice and Science.
John M
JM
Brooks
D000319:Adrenergic beta-Antagonists / Q000627:therapeutic use; D015982:Bias; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D054524:Medicare Part D / Q000706:statistics & numerical data*; D009203:Myocardial Infarction / Q000188:drug therapy; D010351:Patient Discharge / Q000706:statistics & numerical data; D055553:Prescription Drugs / Q000627:therapeutic use*; D013997:Time Factors; D014481:United States / Q000453:epidemiology
D000319:Adrenergic beta-Antagonists; D055553:Prescription Drugs
Administrative Data Uses; Epidemiology; Medicare; Pharmacy