Skip to main content

CO-251 Denial Code: Attachment Incomplete or Deficient

CO-251 means the attachment or documentation the payer received was incomplete or deficient, and the necessary information is still needed to process the claim. The right type of document arrived, but it did not contain everything the payer requires.

Quick facts

Code
CO-251 (CARC 251)
Status
Active In use since September 30, 2012; last modified June 1, 2014.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider must supply complete documentation. The amount generally cannot be billed to the patient.
Official description
The attachment/other documentation that was received was incomplete or deficient. The necessary information is still needed to process the claim. 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-251 means

CARC 251 says the attachment or other documentation that was received was incomplete or deficient. The necessary information is still needed to process the claim. Unlike CO-250, the payer got the right kind of document. It just did not contain enough to decide the claim.

A deficient record usually falls short on a specific element: a missing signature, a progress note without the plan of care it depends on, an order that does not cover the date of service, or a record that stops before the service in question. X12 requires a remark code, and reading it closely usually reveals the gap.

Common causes

  • Unsigned or undated notes, or signatures that do not meet the payer’s signature requirements.
  • Partial records, such as a single note when the payer needs the full episode or supporting orders.
  • Missing elements required by policy, such as a certificate of medical necessity, plan of care, or test results.
  • Illegible scans or pages cut off during upload.
  • Records for the wrong date range, missing the date of service.

How to fix it

  1. Read the remark code and letter to see what is missing.
  2. Compare the payer’s policy requirements with what you sent. For Medicare, the relevant LCD or policy article often lists required documentation elements.
  3. Complete the package. Add missing pages, signature attestations where permitted, orders, and plans of care. Do not alter original records; use proper late-entry or attestation processes where the payer accepts them.
  4. Send it through the correct channel with the claim number or attachment control number.
  5. Follow the payer’s reopening, reconsideration, or appeal process, and track it until the claim is reprocessed.

How to prevent it

  • Use documentation checklists for services that routinely need records, based on the payer’s policy.
  • Check signatures, dates, and legibility before sending records.
  • Send the full relevant record rather than a single note when in doubt.
  • Train providers on what each common policy requires in the note.
  • Watch patterns; if CO-251 keeps hitting one service, audit documentation for that service. See preventable causes of medical claim denials.

Remark codes that may appear with CO-251

  • N705 (Incomplete/invalid documentation.): The documentation was incomplete or invalid.
  • N730 (Incomplete/invalid patient medical/dental record for this service.): The medical or dental record for the service was incomplete or invalid.
  • M127 (Missing patient medical record for this service.): The medical record for this service is still missing.
  • CO-250 (The attachment/other documentation that was received was the incorrect attachment/document.): The wrong document was received, rather than an incomplete one.
  • CO-252 (An attachment/other documentation is required to adjudicate this claim/service.): An attachment is required but none has been received.
  • CO-226 (Information requested from the Billing/Rendering Provider was not provided or not provided timely or was insufficient/incomplete.): Information requested from the provider was not provided, was late, or was incomplete.
  • CO-164 (Attachment/other documentation referenced on the claim was not received in a timely fashion.): The attachment referenced on the claim arrived too late.

CO-251 FAQ

What makes documentation deficient?

Common problems include missing signatures or dates, missing pages, notes that do not cover the date of service, or records that lack the elements the payer's policy requires, such as orders or plans of care.

Is CO-251 a medical necessity denial?

Not exactly. CO-251 says the documentation is incomplete, so the payer cannot yet make a decision. If the payer reviewed complete records and disagreed, it would more likely use a medical necessity code such as CO-50.

How do I respond to CO-251?

Identify what was missing from the remark code or letter, send a complete package with the claim reference, and follow the payer's process for reopening or reconsidering the claim.

Can I add a late signature to fix CO-251?

Some payers, including Medicare, accept signature attestation statements for certain missing signatures, within their rules. Never back-date or alter the original note.