HCC V28 Transition Timeline & Phase-In Impact

V28 didn't replace V24 overnight. CMS blends the two models over multiple payment years, which means for the duration of the transition your RAF is a weighted mix of two models. If your pipeline treats the switch as a flag flip, your numbers are wrong.

The blend, conceptually

blended_RAF = w_v28 * RAF_v28 + w_v24 * RAF_v24
# w_v28 increases each payment year until V28 is 100%

The exact weights per payment year are set in the CMS Rate Announcement.

Re-read it for the actual year — do not hard-code last year's split.

What that does to your data

• YoY RAF deltas are contaminated. Part of any year-over-year change is just w_v28 increasing. Decompose every trend into model shift vs. true population/documentation shift, or your dashboards mislead.
• Two engines, not one. You must compute both RAF_v24 and RAF_v28 for every member through the entire blend, then weight. Retire V24 only when its weight reaches zero.
• Category drift. Some conditions that paid under V24 don't under V28; documentation that was sufficient before can under-capture now. Re-baseline historical members under the V28 model to see your real exposure. A category-by-category view of the V24-vs-V28 differences helps you spot which families to watch.

Minimal ops checklist

1. Load both coefficient/crosswalk sets, keyed by payment year.
2. Compute V24 + V28 per member; apply the year's blend weights.
3. Report model-shift and true-shift as separate series.
4. Re-score history under V28 for a clean baseline.
5. Re-verify weights and category counts against the current CMS Rate Announcement.

A side-by-side of the remap — plus a tool that computes a member under both models, so you can see the blend's effect on a real case — is here: HCC V28.

The point of all this is accuracy under the correct blended model for the payment year — not chasing a higher number.

Not coding, billing, or clinical advice. Phase-in weights 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: July 2026