HCC V28 Explained: What Actually Changed from V24

If you maintain anything that touches risk adjustment, the CMS-HCC model version is effectively a breaking dependency upgrade — and the industry is mid-migration from V24 to V28. Here's the changelog view.

• The HCC map was reorganized and renumbered — V24 HCC numbers do not line up with V28.
• The count of payment HCCs changed and a number of categories were constrained or removed.
• Coefficients were re-estimated, so the same member can produce a different RAF under V28 vs V24.
• CMS is phasing V28 in over multiple payment years — both models are live simultaneously during the blend.

What this breaks in practice

1. Hard-coded HCC numbers. Any mapping table keyed on V24 HCC IDs is wrong under V28. Treat the model version as an explicit input through your whole pipeline, not a constant.
2. Trend comparisons. Year-over-year RAF deltas during the phase-in are partly model artifact, not real population change. If you don't separate "blend shift" from "true shift," your dashboards lie.
3. Conditions that used to pay. Some diagnoses that mapped to a paying category under V24 don't under V28. Documentation behavior that was fine before can quietly under-capture now.

The migration checklist

• Load both coefficient sets; compute V24 and V28 in parallel during the blend years.
• Recompute historical members under V28 to get a clean apples-to-apples baseline.
• Flag members whose RAF moves materially between models — those are where documentation and care-gap workflows need attention. If you want a structured look at how V24 and V28 compare before you build the diff logic, that breakdown is a useful reference.
• Re-verify phase-in percentages and category counts against the current CMS Rate Announcement for the actual payment year — don't trust last year's numbers.

A side-by-side of the category remap and the phase-in schedule, plus a tool that computes a member under both models, is here: HCC V28. It's the fastest way to see, concretely, how a real member shifts under the V28 model before you trust your own implementation.

Why it's worth getting right

This isn't a cosmetic version bump. RAF drives Medicare Advantage funding; a quietly wrong model version means systematically wrong risk scores for real patients. Accuracy under the correct model — not score inflation — is the entire point.

Not coding, billing, or clinical advice. CMS-HCC specifications change by payment year — confirm against the current CMS Rate Announcement.

Written by Chin Ramamoorthi — CEO, VBC Risk Analytics. He has 20+ years across provider- and payer-side healthcare IT, including leading V24-to-V28 transition work on both the payer and provider side.
Reviewed by the VBC Risk Analytics Risk Adjustment & Coding Team.
Last updated: June 2026