CO 252 denial code

Claim adjustment reason code 252. Often reported as CO 252.

Attachment or documentation required

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 252 means

The payer needs records or another attachment before it can decide the claim, so the service is held or denied until you send them.

This is the payer asking for paperwork. Something about the claim or line triggered a review, and the payer will not finish processing it until it sees supporting documents such as office notes, operative reports, invoices, or an itemized bill.

The code should always come with at least one remark code, and that remark code is what tells you which document is wanted. Some payers also send a separate letter or portal request with a deadline and an attachment control number.

With CO the provider carries the balance until the documents are received and reviewed. Do not move it to the patient.

Group codes seen with 252

On a remittance, 252 comes after a group code. These are the ones it is most often reported with:

• CO 252 (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 billed procedure or code combination is on the payer's prepayment review or documentation list.
• An unlisted, miscellaneous, or not otherwise classified code was billed without a description or invoice.
• A modifier such as 22 or 59 was used and the payer wants notes to support it.
• High dollar or unusual services triggered a medical records review.
• The payer needs an itemized bill, invoice, certificate of medical necessity, or other form for that service type.

What to do next

1. Read the remark codes on the line to identify which document the payer wants.
2. Check the payer portal and mail for a records request letter, its deadline, and any reference or attachment number.
3. Send only the records for the date of service on the claim, with the claim number and patient identifiers on every page.
4. Use the payer's attachment channel (portal upload, fax cover sheet, or electronic attachment) so the documents link to the claim.
5. Follow up after the usual review period, and if the payer denies after review, appeal with the same records.

Related codes

• 251 (reason code) Attachment received but incomplete
• 16 (reason code) Missing information or billing error
• 226 (reason code) Provider did not send requested info
• N706 (remark code) Documentation missing