Claim adjustment group codes: CO, PR, OA, PI explained
Every adjustment on an 835 (the CAS segment) has a group code saying who carries the amount, and a reason code (CARC) saying why. They're read together: CO-45, PR-1, OA-23.
| Code | Group |
|---|---|
| CO | Contractual obligation (write off; don't bill the patient) |
| PR | Patient responsibility (may bill the patient) |
| OA | Other adjustment |
| PI | Payer-initiated reduction |
| CR | Correction / reversal |
Common combinations
- CO-45 — charge exceeds the contracted fee schedule: written off under your contract.
- PR-1 / PR-2 / PR-3 — deductible, coinsurance and copay: bill the patient (or the secondary payer).
- OA-23 — impact of prior payer adjudication, common on secondary claims.
- CO-97 — payment included in another service; often a bundling issue to review.
Group and reason summaries here are general descriptions. Your contract and the payer's policies decide what you can bill the patient — check with the payer when in doubt. The official code lists are maintained by X12 at x12.org/codes.
Last reviewed 2026-10-27. Code lists and payer policies change — confirm with X12 (x12.org/codes), the payer and your clearinghouse. Not billing, coding or legal advice.