CO-164CO group · Administrative

CO-164 denial code: attachment not received in time

CO-164 punishes a clock most practices never see: the one that starts when the payer asks for records, not when the service happened.

CO-164 fires when documentation the payer requested arrives after the deadline it set for that request. It is a separate and much shorter clock than the claim filing limit, and it runs from the request rather than from the date of service.

What CO-164 means

Records requests carry their own response windows, often measured in weeks, and they are frequently sent to an address or fax that the practice does not monitor daily. A request that sits unopened produces this denial without anyone ever deciding not to respond. The two things that prevent it are a single monitored intake for payer correspondence and a log that dates each request against its deadline.

Why CO-164 fires

  • The records request went to an address or fax the practice does not monitor.
  • Documentation was gathered and sent after the response window closed.
  • The request was never received and the payer treated silence as non-response.
  • The response was sent without the control number and was not matched in time.

Is CO-164 worth appealing?

Sometimes worth appealing

Where the request never reached you, say so and ask for the window to run from actual receipt. Where it did and the response was late, send the documentation with the appeal rather than waiting to be asked again.

How to resolve or appeal CO-164

  1. 1

    Establish when the request arrived

    If it went to a stale address on file, the payer's own record of where it sent it supports reopening the window.

  2. 2

    Send the documentation with the appeal

    An appeal that argues about the deadline without curing the underlying gap invites a second denial.

CO-164 — frequently asked

How long is the response window?
It varies by payer and is stated in the request itself — commonly a few weeks. It is unrelated to the claim filing limit, which is why practices are caught by it.
How do I stop this recurring?
One monitored intake for payer correspondence and a dated log. The failure is almost always operational rather than clinical.

Related guides

CO-163 · AdministrativeCO-251 · AdministrativeCO-252 · Administrative

Sources

Reason-code meanings are paraphrased from the X12 Claim Adjustment Reason Code list for plain-language reference; they are not reproduced verbatim. This is general information, not legal, coding, or medical advice — always confirm against the payer's remittance and policy.

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