Classifying medical histories in US Medicare beneficiaries using fixed vs all-available look-back approaches.

PURPOSE: Evaluate use of fixed and all-available look-backs to identify eligibility criteria and confounders among Medicare beneficiaries.
METHODS: We identified outpatient visits (2007-2012) with recently documented (≤180 days) cardiovascular risk and classified patients according to whether the exposure (statin) was initiated within 14 days. We selected each beneficiary's first eligible visit (in each treatment group) that met criteria during the respective look-backs: continuous enrollment (1 or 3 years for fixed look-back; 180 days for all-available), no cancer history, and no statin claims. We estimated crude and standardized mortality ratio weighted hazard ratios (HRs) for the effect of statin initiation on incident 6-month cancer (a known null effect) and 2-year mortality, separately, adjusting for covariates assessed by using each look-back.
RESULTS: Analyzing short-term cancer, the estimated HR from the all-available approach (HR = 0.90, 95% CI: 0.83, 0.98) was less biased than the 1-year look-back (HR = 0.79, 95% CI: 0.73, 0.84), which included beneficiaries with prevalent cancer. The 3-year look-back (HR = 1.05, 95% CI: 0.90, 1.21) was somewhat less biased than the all-available estimate but less precise due the exclusion of a large proportion of observations without sufficient continuous enrollment (62.0% and 59.9% of initiators and non-initiators, respectively). All approaches produced similar estimates of the effect on all-cause mortality. Alternative look-backs did not differ in their ability to control confounding.
CONCLUSIONS: The all-available look-back performed nearly as well as the 3-year fixed, which produced the least biased point estimate. If 3-year look-backs are infeasible (eg, due to power/sample), all-available look-backs may be preferable to short (1-year) fixed look-backs.