Skip to main content

CO-163 Denial Code: Referenced Attachment Not Received

CO-163 means the claim said documentation or an attachment was being sent, but the payer did not receive it. The fix is usually to send the attachment with the correct claim reference, or resubmit with the attachment included.

Quick facts

Code
CO-163 (CARC 163)
Status
Active In use since June 30, 2004; last modified June 2, 2013.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider is responsible for supplying the documentation. The amount is not billable to the patient while the attachment is outstanding.
Official description
Attachment/other documentation referenced on the claim was not received.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-163 means

CARC 163 reads “Attachment/other documentation referenced on the claim was not received.” Your claim told the payer that supporting paperwork was coming, typically through the PWK (paperwork) segment in the 837 or a note in box 19 of the CMS-1500. The payer waited and never got it, or could not match what it got to the claim.

It is a documentation logistics problem rather than a clinical judgment. Once the right documents arrive with the right reference, the claim can be adjudicated on its merits.

Example: a claim for a surgical procedure is sent with a PWK segment saying the operative report will be faxed, with attachment control number ABC123. The fax goes out without the control number on the cover sheet, so the payer cannot match it. After its waiting period, the payer denies the claim with CO-163.

Common causes

  • Attachment never sent after the claim was flagged with PWK.
  • Control number missing or mismatched between the claim and the fax or upload.
  • Sent to the wrong address, fax, or portal, or for the wrong payer.
  • Sent after the payer’s waiting period, which is often short and varies by payer.
  • Unreadable or incomplete documents, such as a partial fax.
  • PWK flagged by default in billing software when no attachment was intended.

How to fix it

  1. Check the claim’s PWK or box 19 entry to see what attachment and control number were referenced.
  2. Follow the payer’s instructions for late documentation: portal upload, fax with a specific cover sheet, or electronic attachment (275 transaction) where supported.
  3. If the payer closed the claim, resubmit it with the attachment correctly linked, or send a corrected claim (resubmission code 7) if the payer requires that path.
  4. If no attachment was needed, remove the PWK reference and resubmit.
  5. Keep proof of transmission (fax confirmations, upload receipts) in case you need to dispute the denial.

How to prevent it

  • Send attachments the same day the claim goes out, and use the payer’s preferred method.
  • Put the attachment control number on every page or cover sheet exactly as it appears on the claim.
  • Audit default PWK settings in your billing software.
  • Track open attachment claims until the payer confirms receipt. Documentation-driven denials are covered in preventable denials.

Remark codes that may appear with CO-163

  • M127 (Missing patient medical record for this service.): The payer specifically wants the medical record for this service.
  • N706 (Missing documentation.): Missing documentation. Tells you the payer still needs supporting documents.
  • MA130 (Your claim contains incomplete and/or invalid information, and no appeal rights are afforded because the claim is unprocessable.): The claim is treated as unprocessable, so a fresh submission may be needed rather than an appeal.
  • CO-252 (An attachment/other documentation is required to adjudicate this claim/service.): An attachment or documentation is required but was not referenced or sent. CO-163 means it was referenced but did not arrive.
  • CO-16 (Claim/service lacks information or has submission/billing error(s).): Missing or invalid claim data. Its usage note says not to use it for attachment issues.
  • CO-226 (Information requested from the Billing/Rendering Provider was not provided or not provided timely or was insufficient/incomplete.): Information requested from the billing provider was not provided or was insufficient.

CO-163 FAQ

What is the difference between CO-163 and CO-252?

With CO-163 the claim referenced an attachment (for example through the PWK segment) that never arrived. With CO-252 the payer needed an attachment that the claim did not indicate at all.

How do I link an attachment to a claim?

On electronic claims the PWK segment carries the attachment type, transmission method, and a control number. Put the same control number on the fax cover sheet or upload so the payer can match them.

Do I need to appeal CO-163?

Usually not. Many payers let you send the missing documentation through their portal or attachment process, or resubmit the claim with it. Check the payer's instructions for the correct route and time limit.