Obtain the primary payer's EOB (paper or electronic 835 ERA): record the paid amount, contractual adjustment, patient responsibility, payment date, and the primary payer's own claim control number for each service line
In the secondary 837 claim, populate SBR segment in Loop 2000B: SBR01 payer responsibility sequence code '2' (secondary) or '3' (tertiary), SBR09 claim filing indicator code for the secondary payer type (e.g., 'CI' commercial, 'MB' Medicare Part B, 'MC' Medicaid)
Add Loop 2320 (other subscriber information) for the primary payer: OI03='Y' (benefits assigned), SBR01='P' (primary) in the nested NM1*PR name loop; include COB claim-level AMT segments: AMT*D (primary paid), AMT*EAF (remaining patient liability)
At each service line, add Loop 2430 (line adjudication information): SVD01 (primary payer ID), SVD02 (primary paid amount per line), SVD03 (procedure code composite matching the SV1), SVD05 (units); then CAS segments for each primary adjustment group/reason code/amount
Include DTP*573 (claim adjudication date) in both Loop 2320 and Loop 2430 to document when the primary adjudicated; many secondary payers reject COB claims missing adjudication date
Submit to the secondary payer; verify the secondary 835 ERA credits the COB correctly by confirming the paid amount plus primary paid equals or approaches the billed amount less contractual adjustments
Known gotchas
Medicare crossover claims for dual-eligible beneficiaries are often forwarded automatically by Medicare to Medicaid via the CMS crossover system — submitting a separate secondary claim to Medicaid for a crossover-eligible claim can cause a duplicate denial
The secondary payer applies its own fee schedule to determine its liability; the secondary paid amount may be less than the remaining patient balance after primary payment, leaving a residual patient responsibility
SVD03 must contain the same procedure code in the same format as SV101 on the primary claim line; a code format mismatch (e.g., HCPCS vs CPT qualifier difference) will cause the secondary to fail to match the line and deny with a COB coordination error
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