# MBI lookup for CMS eligibility checks

> When patients don’t know their MBI, you can use Stedi's eligibility check APIs to perform an MBI lookup using their Social Security Number (SSN) instead.

Published Feb 21, 2025

Product: Eligibility checks

A Medicare Beneficiary Identifier (MBI) is a unique, randomly-generated identifier assigned to individuals enrolled in Medicare. You must include the patient’s MBI in every eligibility check you submit to the Centers for Medicare and Medicaid Services ([payer ID: CMS](https://www.stedi.com/healthcare/network/cms)). When patients don’t know their MBI, you can use Stedi's eligibility check APIs to perform an MBI lookup using their Social Security Number (SSN) instead.

To perform an MBI lookup:

1.  Construct an eligibility check request with the patient’s first name, last name, date of birth, and Social Security Number (SSN).

2.  Set the `tradingPartnerServiceId` to `MBILU`. This is a special payer ID that tells Stedi to perform an MBI lookup for the patient in addition to a standard eligibility check.

3.  Stedi returns a complete benefits response from CMS with the patient's MBI in the `subscriber` object for future reference.

Visit our [Medicare Beneficiary Identifier (MBI) lookup](https://www.stedi.com/docs/healthcare/mbi-lookup) docs for complete details.