Skip to main content

CO-164 Denial Code: Attachment Not Received in Time

CO-164 means the claim referenced an attachment or other documentation, and the payer did not receive it within its required time frame. The documents may have arrived, but too late to be used for the original adjudication.

Quick facts

Code
CO-164 (CARC 164)
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 the late documentation. The amount is not billable to the patient.
Official description
Attachment/other documentation referenced on the claim was not received in a timely fashion.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-164 means

CARC 164 reads “Attachment/other documentation referenced on the claim was not received in a timely fashion.” The claim told the payer that documentation would follow, usually through the PWK segment on an electronic claim or box 19 on paper. The payer gives a window for that documentation to arrive. When the window closes without a match, the claim is denied with CO-164.

The key difference from CO-163 is timing: with 164 the problem is lateness. In practice both can occur when documents arrived but were not matched in time.

Example: a claim for a service needing an invoice is sent with a PWK reference. Staff intend to fax the invoice later but it goes out two weeks after the claim. The payer’s window has passed, and the claim comes back CO-164.

Common causes

  • Attachments batched and sent days or weeks after the claims.
  • Waiting on outside documents, such as operative reports, invoices, or certificates, before sending.
  • Payer mailroom or fax indexing delays when the attachment control number was unclear.
  • Staff turnover or unclear ownership of the attachment queue.
  • Choosing mail when the payer accepts faster electronic or portal submission.

How to fix it

  1. Confirm the timeline. Compare the claim receipt date, the date you sent the documentation, and the payer’s stated window.
  2. Check the timely filing limit for the claim itself. Recovery depends on having time left.
  3. Resubmit with the attachment properly linked, or use the payer’s portal or reconsideration route for late documentation.
  4. If you sent the documents on time, appeal with fax confirmations, upload receipts, or delivery tracking.
  5. Write off only when both the attachment and claim filing windows have closed and no appeal applies.

How to prevent it

  • Hold the claim until the document is ready when an attachment is certain to be needed, as long as the filing limit allows.
  • Send attachments the same day as the claim using electronic attachments or the portal.
  • Track PWK claims in a work queue with due dates based on each payer’s window.
  • Record send dates for every attachment so you can prove timeliness. Related filing issues are explained in timely filing denials.

Remark codes that may appear with CO-164

  • M127 (Missing patient medical record for this service.): The medical record was the missing piece the payer was waiting for.
  • N706 (Missing documentation.): Confirms the payer still considers documentation missing.
  • CO-163 (Attachment/other documentation referenced on the claim was not received.): The referenced attachment never arrived at all, rather than arriving late.
  • CO-252 (An attachment/other documentation is required to adjudicate this claim/service.): Documentation was required but the claim did not reference it.
  • CO-29 (The time limit for filing has expired.): The claim itself was filed past the timely filing limit, which is a separate deadline.

CO-164 FAQ

How long do payers wait for claim attachments?

It varies by payer and is set in its provider manual or companion guide. Some wait only a short period after the claim is received, so send attachments the same day whenever possible.

Can a CO-164 claim still be paid?

Often yes, if the underlying claim is still within the timely filing limit. Resubmit with the documentation or follow the payer's reconsideration process, including proof of when you sent the documents.

Is CO-164 the same as timely filing?

No. CO-164 is about the attachment deadline. Timely filing (CO-29) is about when the claim was submitted. Missing the attachment window, though, can push the claim close to the filing limit.

Are electronic attachments faster than faxing?

Usually. Where a payer supports electronic attachments or portal uploads, documents are linked to the claim more reliably than a fax that must be indexed by hand, which reduces the chance of missing the window.