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CO-252 Denial Code: Attachment or Documentation Required

CO-252 means an attachment or other documentation is required to adjudicate the claim or service. The payer has not decided whether to pay yet; it needs records or another document first. A remark code identifies what to send.

Quick facts

Code
CO-252 (CARC 252)
Status
Active In use since September 30, 2012; last modified June 2, 2013.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider must supply the documentation. Until then, the amount is not payable and generally cannot be billed to the patient.
Official description
An attachment/other documentation is required to adjudicate this claim/service. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT).
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-252 means

CARC 252 says an attachment or other documentation is required to adjudicate this claim or service. X12 requires at least one remark code with it, and that remark tells you which document the payer wants: medical records, an itemized bill, a certificate of medical necessity, images, an invoice, or another item.

CO-252 sits early in the documentation chain. It typically appears when the payer first decides it needs more than the claim data, before any request has gone unanswered. If documentation is then not sent, sent wrong, or sent incomplete, the claim may move to CO-226, CO-250, or CO-251.

The usage note for CO-16 specifically excludes attachments, so CO-252 is the right code for documentation needs. If you see it, the claim data itself is usually fine.

Common causes

  • Service policy requires records for every claim, such as unlisted procedures, certain high-cost drugs, or specific DME items.
  • Unlisted or not-otherwise-classified codes that need a description, invoice, or operative report for pricing.
  • Claims above a dollar threshold that trigger itemized bill review.
  • Modifiers that require support, such as modifier 22 for increased procedural services.
  • Pre-payment review targeting a provider or service.
  • Claim indicated an attachment (PWK) that the payer has not yet linked.

How to fix it

  1. Read the remark code to identify the exact document.
  2. Pull complete, signed documentation for the date of service, plus anything the payer’s policy requires.
  3. Send it through the payer’s attachment channel with the claim number. For electronic claims, match the attachment control number to the PWK segment.
  4. Keep proof of submission, including the date, method, and tracking or confirmation number.
  5. Follow up if the claim is not released. Some payers reopen automatically; others need a reconsideration or a corrected claim with the attachment.
  6. Do not resubmit the same claim without the attachment, which can trigger a duplicate denial. See CO-18 duplicate claims.
  7. Do not bill the patient while the claim is pending documentation.

How to prevent it

  • Keep a list of services and payers that always require documentation, and attach it with the original claim.
  • For unlisted codes, include a clear description and supporting records up front.
  • Use the PWK segment and a consistent attachment control number system for electronic attachments.
  • Monitor claims awaiting documentation so none age past appeal or timely filing limits.
  • Flag attachment-required services before submission; a Claims Validator can catch claims that need documentation before they go out.

For the difference between missing data and missing documentation, see CO-16 missing or invalid information.

Specialty notes

DME suppliers commonly see CO-252 for missing certificates of medical necessity or detailed written orders. Surgical specialties see it with modifier 22 and unlisted procedures, where the operative report is the key document.

Remark codes that may appear with CO-252

  • M127 (Missing patient medical record for this service.): The payer needs the patient's medical record for the service.
  • N706 (Missing documentation.): Documentation is missing.
  • N26 (Missing itemized bill/statement.): An itemized bill or statement is required.
  • M60 (Missing Certificate of Medical Necessity.): A certificate of medical necessity is required, common with DME.
  • N40 (Missing radiology film(s)/image(s).): Radiology films or images are required.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Claim data is missing or invalid. X12 says CARC 16 should not be used for attachments, which is what 252 is for.
  • CO-250 (The attachment/other documentation that was received was the incorrect attachment/document.): A document arrived but was the wrong one.
  • CO-251 (The attachment/other documentation that was received was incomplete or deficient.): A document arrived but was incomplete or deficient.
  • CO-226 (Information requested from the Billing/Rendering Provider was not provided or not provided timely or was insufficient/incomplete.): The payer asked the provider for information and did not get it.
  • CO-163 / CO-164 (Attachment referenced on the claim not received, or not received timely.): The claim referred to an attachment that never arrived or arrived late.

CO-252 FAQ

Is CO-252 a denial?

It is an adjudication hold expressed as a denial or pend. The service has not been judged payable or unpayable; the payer is waiting on documentation. Once it arrives, the payer can decide.

What is the difference between CO-16 and CO-252?

CO-16 is for missing or invalid claim data and billing errors. CO-252 is specifically for attachments and other documentation. X12's usage note for CARC 16 says not to use it for attachments.

How do I send an attachment for CO-252?

Use the payer's preferred method: portal upload, electronic attachment linked by the 837 PWK segment and an attachment control number, or fax or mail with the payer's cover sheet. Always include the claim number.

How long should I wait after sending documentation?

Payer turnaround varies. Keep the submission receipt and follow up if the claim has not been reprocessed within the payer's published timeframe, or within about a month if none is published.