OA B13 denial code

Claim adjustment reason code B13. Often reported as OA B13 or CO B13.

Previously paid

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What B13 means

The payer says it already paid for this claim or service on an earlier remittance, so it is not paying again.

B13 is the payer's way of flagging a repeat. It believes payment for the same service was already issued, possibly on a different claim number or remittance.

It is close to a duplicate denial, but the emphasis is on the earlier payment. Find that payment before doing anything else, because it may have been posted to the wrong account or date.

Group codes seen with B13

On a remittance, B13 comes after a group code. These are the ones it is most often reported with:

• OA B13 (Other Adjustment): Used for adjustments that are neither the provider's contractual write-off nor the patient's share. It often appears with coordination of benefits, for example when another payer already paid.
• CO B13 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The same service was billed twice, for example after a resubmission instead of a corrected claim.
• The original payment was posted to the wrong patient, date or encounter.
• A corrected claim was sent without the frequency code that marks it as a replacement.

What to do next

1. Search your remittances for the earlier payment of the same service and date.
2. Correct any misposted payment so the account shows the original payment properly.
3. Send a corrected claim with the right frequency code if you meant to replace the original claim.
4. Appeal only if you can show the services are distinct and were not paid before.

Related codes

• 18 (reason code) Exact duplicate claim or service
• A0 (reason code) Money going back to the patient
• B20 (reason code) Service furnished by another provider