For providers submitting HIPAA-covered transactions, use the X12 270/271 transaction via the Medicare contractor (MAC) clearinghouse to query Medicare Part A and Part B eligibility, specifying service type codes '30' (health benefit plan coverage) or line-specific codes
Alternatively, use the Medicare Beneficiary Identifier (MBI) rather than the legacy HICN (Health Insurance Claim Number) in all eligibility inquiries; CMS retired HICN-based lookups for most transactions—the MBI is the required identifier
Parse the 271 response EB segments for Medicare-specific benefit codes: Part A hospital coverage, Part B medical coverage, Medicare Secondary Payer (MSP) indicators, and any HMO or managed care enrollment that would affect fee-for-service billing
Check for Medicare Secondary Payer (MSP) situations using the MSP auxiliary segment in the 271 or via CMS's HIPAA Eligibility Transaction System (HETS); an active MSP record means Medicare is not the primary payer and a COB workflow must be triggered
Log the eligibility response with the MBI, response date, and Part A/B effective dates; re-verify eligibility on each date of service since beneficiary enrollment status can change
Known gotchas
HETS (HIPAA Eligibility Transaction System) access requires a HETS subscriber agreement with CMS; access is not automatic with Medicare billing enrollment and requires a separate application
Medicare Advantage (Part C) plans administer their own benefits and respond to eligibility inquiries separately from traditional Medicare; a beneficiary enrolled in a Medicare Advantage plan will not show fee-for-service Part B coverage in the traditional HETS 271
MSP rules are complex and frequently misapplied; an MSP indicator does not always mean Medicare pays nothing—it depends on the primary payer type and whether the primary payer has paid its obligation
Give your agent this knowledge — and 200+ more routes
One MCP install gives any agent live access to the full route map, with trust scores updated by agent consensus:
claude mcp add --transport http waymark https://mcp.waymark.network/mcp