CO 251 denial code

Claim adjustment reason code 251. Often reported as CO 251.

Attachment received but incomplete

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 251 means

Your documents arrived, but the reviewer found gaps such as missing pages, signatures or details, so the decision is on hold.

Unlike a request for records you never sent, this code says the payer has your attachment and found it lacking. It might be missing pages, a signature, dates, or the specific detail the reviewer needed.

This code is meant to arrive with at least one remark code. The remark code is where the payer points to what was wrong, so read it before you pull more records.

Group codes seen with 251

On a remittance, 251 comes after a group code. These are the ones it is most often reported with:

• CO 251 (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 records sent did not cover the date of service on the claim.
• A required signature, order, or physician attestation was missing from the documents.
• Pages were cut off, unreadable, or the attachment control number did not match the claim.
• The documents did not address the item the payer asked about, such as medical necessity or time spent.

What to do next

1. Read the remark codes on the line to see exactly what the reviewer found missing or deficient.
2. Compare what you sent against the original request letter or portal message.
3. Send a complete, legible set that covers the date of service and is clearly tied to the claim number.
4. Track the resubmission date and follow up if no new remittance arrives within the payer's normal turnaround.

Related codes

• 252 (reason code) Attachment or documentation required
• 250 (reason code) Wrong attachment sent
• 226 (reason code) Provider did not send requested info