Home›Glossary›Medicare ID

Medicare ID

General

Medicare Beneficiary Identifier (MBI)

An 11-character alphanumeric code assigned to each Medicare beneficiary, replacing the older Health Insurance Claim Number that was based on the holder's SSN.

Definition

The Medicare Beneficiary Identifier (MBI), commonly called the Medicare ID or Medicare number, is an 11-character alphanumeric code assigned to each person enrolled in US Medicare. It replaced the older Health Insurance Claim Number (HICN), which was directly based on the beneficiary's Social Security Number (SSN) and printed on physical Medicare cards — a design that created serious identity-theft exposure whenever a card was lost or shared.

The transition to MBIs, mandated by MACRA and completed by 2019, decoupled Medicare identification entirely from the SSN, while keeping the number simple enough to read, say, and enter into claims systems.

Formula

The MBI has no checksum digit — it is validated entirely through positional format rules rather than a mathematical formula:

  • The format is 11 characters following the pattern: numeric-alpha-numeric-alpha-numeric-alpha-numeric-numeric-alpha-alpha-numeric (positions vary slightly by CMS documentation version, but the core rule is a fixed mix of digit and letter positions).
  • Only the letters A–Z excluding S, L, O, I, B, and Z are permitted, because those six letters are easily confused with the digits 5, 1, 0, 1, 8, and 2 respectively.
  • All letters are uppercase; no special characters or spaces are part of the stored value (spaces are sometimes added only for display).

A tool can only confirm structural plausibility — actual enrollment must be confirmed against CMS records.

Worked Example

A structurally valid but entirely fictional example: 1EG4-TE5-MK73, commonly displayed with hyphens grouping the characters for readability, though the underlying value is a continuous 11-character string.

Key Things to Know

  • No checksum, positional rules only: the Medicare ID Validator checks character-position patterns and excluded letters, not a computed check digit.
  • SSN-free by design: the MBI replaced the SSN-based HICN specifically to reduce identity-theft risk on physical Medicare cards.
  • Confusable letters excluded: S, L, O, I, B, and Z are never used, since they resemble digits when spoken or handwritten.
  • Identifies the patient, not the provider: an MBI identifies the Medicare beneficiary; the corresponding NPI identifies the provider delivering care on the same claim.
  • Can be reissued: unlike an SSN, an MBI can be changed if compromised, since it isn't derived from any other permanent ID.
  • Fully phased in since 2019: all Medicare transactions now require the MBI — the legacy SSN-based HICN is no longer accepted for claims.

Related Terms

SSNUS Social Security NumberNPINational Provider Identifier

Frequently Asked Questions

The old Health Insurance Claim Number (HICN) was based on the beneficiary's Social Security Number, which created a significant identity-theft risk since it appeared on physical Medicare cards. The Medicare Access and CHIP Reauthorization Act (MACRA) of 2015 mandated the switch to the SSN-free Medicare Beneficiary Identifier (MBI), completed across all cards by 2019.
No, the MBI has no mathematical checksum digit — validity is determined entirely by character-position rules (which positions must be numeric, which must be an allowed letter, and which letters are excluded). You can check whether an MBI follows this structure with the [Medicare ID Validator](/validators/medicare-id-validator/).
The MBI excludes the letters S, L, O, I, B, and Z from all its alphabetic positions, specifically because they are easily confused with the digits 5, 1, 0, 1, 8, and 2. This exclusion list is a deliberate design choice to reduce transcription errors when the ID is read aloud or handwritten.
No. A Medicare ID (MBI) identifies the patient who is a Medicare beneficiary, while an [NPI](/glossary/npi/) identifies the healthcare provider or organization delivering the service. Every Medicare claim references both — one for who is covered, one for who provided the care.
Yes, unlike a Social Security Number, an MBI can be reissued if it is compromised or exposed, since it isn't derived from any other permanent government identifier. Beneficiaries can request a new MBI from the Social Security Administration or CMS if they suspect their card or number has been misused.