Since 2019, the Medicare Shared Savings Program (MSSP) has allowed accountable care organizations (ACOs) in the Basic or Enhanced tracks to choose either retrospectively or prospectively attributed ACO patient populations. Seriously ill patients with high needs, costs, and functional limitations have a high risk of death and may benefit from ACO care management strategies. Little is known about how ACOs’ choice of attribution method affects ACO attribution, and thus incentives for care, for seriously ill Medicare beneficiaries.
Attribution methods in MSSP have evolved over time. Since program inception, CMS required MSSP ACOs to use retrospective assignment, with the exception of the 8% of ACOs in Track 3 or the Next Generation Track, which both started in 2016. Beginning in 2019, the Pathways to Success MSSP rule allows ACOs an annual attribution choice: retrospective or prospective (potentially supplemented by voluntary patient attribution). Retrospective and prospective options both use claims-based algorithms to attribute patients to an ACO if a plurality of qualifying services over a 12-month period are received from a participating provider. In retrospective attribution, patients are assigned based on care provided during the contract year, and final attribution occurs at the end of the contract performance year (PY). In retrospective attribution, patients who received care from the ACO during the PY are captured in the ACO population. In prospective attribution, patients are assigned at the start of the contract PY based on claims in the prior calendar year, which allows ACOs to know their defined population and provide care management during the PY.