Connecticut County and Planning Region Lookup

The Connecticut geography break

Anyone who has tried to calculate a Medicare Advantage penetration rate for Connecticut during the past two years has encountered the same mismatch. The numerator comes from CMS data organized around eight legacy counties. The denominator comes from current Census data organized around nine planning regions. The two geographies do not nest, so there is no defensible one-to-one county crosswalk. Analyses that join them directly may omit Connecticut or produce incorrect rates.

CMS has now announced the transition. Beginning January 1, 2028, CMS will use Connecticut's nine planning regions instead of the eight legacy counties for Medicare Advantage and Part D operations. SSA has established nine new state/county codes to support the change.

This is the right decision, but it creates a second transition problem that should be addressed before 2028. With patient-level files, the reliable method is to assign each beneficiary's residential address to a Connecticut town, then attach both the legacy county and the current planning region through a town-level crosswalk. ZIP-code-only mappings can be ambiguous because ZIP codes cross municipal boundaries. This public tool demonstrates the method for individual, non-sensitive addresses. FastHSR can implement the same logic securely for batch data in the appropriate data-use environment.