Medicare Card MBI Format: How the 11 Characters Work

The Medicare card MBI format is 11 characters of numbers and uppercase letters in a fixed pattern. Positions 1, 4, 7, 10 and 11 are numbers, positions 2, 5, 8 and 9 are letters, and positions 3 and 6 can be either. S, L, O, I, B and Z never appear, and the printed dashes are not part of it.

Medicare Beneficiary Identifier character positions

The Medicare Beneficiary Identifier, or MBI, is the number printed on a Medicare card. It has 11 characters, each with a fixed type: a number, a letter, or either. Position 1 is a number from 1 to 9, never 0. Every other number position allows 0 to 9. The characters are randomly generated and carry no hidden meaning.

The rules come from the CMS guide to understanding the MBI format. It calls the characters "non-intelligent", meaning they do not encode a birth date, a state or any other fact about the person. It gives the sample 1EG4-TE5-MK73 and says the dashes are not used as part of the MBI and are not entered into computer systems or file formats.

This table walks the CMS sample through each position. The sample is the one CMS publishes for illustration, not a real person's number.

CMS adds that lowercase letters typed into its systems are converted to uppercase, and that the MBI fits on forms the same way the old number did, with no space needed for dashes.

Which letters can never appear in an MBI?

Six letters can never appear in an MBI: S, L, O, I, B and Z. That leaves 20 allowed letters: A, C, D, E, F, G, H, J, K, M, N, P, Q, R, T, U, V, W, X and Y. The exclusion applies to every letter position, including positions 3 and 6.

The CMS guide gives one reason for the exclusion: it helps the characters be easier to read. CMS does not list which pairs it had in mind, so treat the rule as a fact about the format rather than a guide to which character was meant.

For intake, the rule works as a quick check. An MBI keyed with an O, an I or an S anywhere is wrong, whatever the card seems to show. The likely fix is the digit it resembles, but confirm it against the card rather than guessing.

How is the MBI different from the old Medicare HICN?

The MBI replaced the Health Insurance Claim Number, or HICN, which was based on a Social Security number. The MBI is randomly generated, and each person gets their own, so spouses or dependents who may have had similar HICNs now have different MBIs. The transition period ended on December 31, 2019.

The CMS new Medicare card page says CMS removed Social Security number-based HICNs from Medicare cards and that Medicare now uses MBIs for transactions such as billing and claim status. It allows the HICN for fee-for-service claim status queries only when the earliest date of service is before January 1, 2020.

Old cards on file are the practical problem. If a patient record still holds a HICN, replace it. The CMS page on getting MBIs lists three ways: ask the patient for their card, look it up in the Medicare Administrative Contractor's secure portal, or check the remittance advice for older claims sent with a valid HICN.

An MBI can also change. The CMS page for providers and office managers says an eligibility error code AAA 72, "invalid member ID", may mean the patient's MBI changed, and that the new one is available through the contractor's lookup tool.

Medicare Advantage cards vs the original Medicare card

A patient enrolled in a Medicare Advantage plan has two cards: the Medicare card with the MBI, and a card from the plan with its own member ID. For services, the plan's card is the one to use. Recording only the MBI for a Medicare Advantage patient leaves billing without the number the plan issued for that patient.

Medicare's own guidance to patients is direct. The Medicare.gov page on your Medicare card tells people in a Medicare Advantage plan that they will use the plan's card to get services, not their Medicare card, and to keep the Medicare card in case they switch plans. The CMS page for health and drug plans urges plans to keep their own identifiers and not to include MBIs on members' ID cards.

So the intake question is not only "do you have Medicare?" but "do you have a Medicare health plan card too?" A plan card's member ID follows the plan's own layout, like any commercial card; the guide to where the member ID sits on an insurance card covers reading it.

How can intake software validate an MBI?

Intake software can validate an MBI with a pattern check against the CMS rules: strip dashes and spaces, convert to uppercase, confirm the length is 11, and match each character to its allowed set. The check catches typos, such as a zero in position 1 or an O anywhere, before a claim goes out.

Here is the pattern as a regular expression, built from the CMS position table, with the 20 allowed letters written as a character class:

^[1-9][AC-HJKMNP-RT-Y][AC-HJKMNP-RT-Y0-9][0-9][AC-HJKMNP-RT-Y][AC-HJKMNP-RT-Y0-9][0-9][AC-HJKMNP-RT-Y]{2}[0-9]{2}$

Two fictional test values show what it catches. The CMS sample, 1EG4TE5MK73, passes. The made-up value 0EO4TE5MK7, which is deliberately invalid, fails three ways: it starts with 0, it has an O in position 3, and it is only ten characters long.

A pattern check proves the number is well formed, not that it belongs to the patient or is active. A valid-looking MBI can still belong to someone else or have been replaced. Only an eligibility check against Medicare confirms that.

Paper intake forms add a second risk: a handwritten MBI that is misread before any check runs. Schema Care reads the insurance section of an intake form and routes a field it scores below the customer's threshold to a reviewer, who sees it on the original page; the human review section explains how. Capturing the number from a card image is covered in the guides to insurance card OCR and insurance card scanners, and claim fields that carry the MBI are in the claims and medical bills category.