CMS Blends HCC V28 and V24 Models During Multi-Year Transition, Complicating RAF Calculations

The Centers for Medicare & Medicaid Services (CMS) is phasing in the HCC V28 risk adjustment model gradually, blending it with the existing V24 model over multiple payment years rather than switching immediately. During this transition, a plan's Risk Adjustment Factor (RAF) is calculated as a weighted combination of both models, with V28's weight increasing each payment year until it fully replaces V24. Analysts are cautioned that year-over-year RAF changes must be decomposed into model-shift effects versus genuine population or documentation shifts to avoid misleading reporting. Both coefficient sets must be computed per member for every year of the blend, and historical data should be re-scored under V28 to establish an accurate baseline. The precise blend weights for each payment year are published in the CMS Rate Announcement and must be verified annually rather than assumed from prior years.