Unattributed Medicare Beneficiaries Have Greater Social Disadvantage Than Those Linked to Primary Care Physicians

Introduction Attributing patients to physicians using claims data is a key design feature of most payment and delivery models. However, claims-based attribution makes model participants less representative by excluding beneficiaries who use less care. Methods We used the attribution methodology from the largest completed primary care model, Comprehensive Primary Care Plus (CPC+), to describe differences in attributable and unattributable Medicare beneficiaries living in CPC+ regions from 2016-2021. Results Unattributed beneficiaries were more male (56% vs 43%), eligible for the low-income drug subsidy (15% vs 12%), dual eligible (13% vs 10%), and non-White (22% vs 15%). Conclusion Tests of models that use claims-based attribution may fail to generate evidence relevant to these groups. Policymakers should consider attribution designs that could improve representativeness of model test populations.