Medicare Beneficiary Identifiers (MBIs): Is That a Zero or an O?

Published 08/13/2020

Medicare Beneficiary Identifiers (MBIs) are required on most Medicare transactions and must be used on claims, in the Interactive Voice Response (IVR) unit and the eServices portal, and when calling Customer Service. Knowing the format of an MBI number can help you successfully complete your transactions.

An MBI has 11 alpha-numeric characters. An MBI’s characters are “non-intelligent” so they don’t have any hidden or special meaning.

An MBI uses numbers 0–9 and all uppercase letters except for S, L, O, I, B and Z. Those letters are excluded to avoid confusion when differentiating some letters and numbers (for example, between the number “0” and the letter “O”).

  • The 1st, 4th, 7th, 10th and 11th characters will always be a number
  • The 2nd, 5th, 8th and 9th characters will always be a letter
  • The 3rd and 6th characters will be a letter or a number
  • The dashes aren’t used as part of the MBI. Do not use the dashes on claims, in the IVR or in the eServices portal.


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