Incident user cohorts for assessing medication cost-offsets

To develop and test incident drug user designs for assessing cost savings from statin use in diabetics.

Random 5 percent sample of Medicare beneficiaries, 2006-2008.

Seven-step incident user design to assess impact of statin initiation on subsequent Medicare spending: (1) unadjusted pre/post initiation test; (2) unadjusted difference-in-difference (DID) with comparison series; (3) adjusted DID; (4) propensity score (PS)-matched DID with static and dynamic baseline covariates; (5) PS-matched DID by drug adherence strata; (6) PS-matched DID for high adherers controlling for healthy adherer bias; and (7) replication for ACE-inhibitor/ARB initiators.

Subjects with prevalent diabetes and no statin use (January-June 2006) and statin initiation (July 2006-January 2008) compared to nonusers with a random "potential-initiation" month. Monthly Medicare spending tracked 24 months pre- and post-initiation.

Statistically significant savings in Medicare spending were observed beginning 7 months post-initiation for statins and 13 months post-initiation for ACEIs/ARBs. However, these savings were only observed for adherent patients in steps 5 and 6.

Drug initiator designs are more robust to confounding than prevalent user designs in assessing cost-offsets from drug use but still require other adjustments and sensitivity analysis to ensure proper inference.