CO-29 Denial Code: When a Timely Filing Denial Can Be Overturned

Medicare’s claims manual is blunt about it. A determination that a claim was not filed on time “is not subject to appeal.” Palmetto GBA, one of the Medicare contractors, says it more plainly still: “Providers should not submit appeal requests on untimely claims.”

Billing offices send them anyway, and then wonder why nothing comes back.

That is the first thing to know about CO-29. Whether you can do anything about it depends on who sent it, which dates they counted, and what you can prove. Answer those in that order, before anyone writes an appeal.

What CO-29 is telling you

CO-29 means the payer believes your claim reached it after the filing limit. The CO group code puts the loss on the provider. If the denial stands, the balance is written off, and it cannot be moved to the patient. The CO-45 post explains how group codes work.

Most denials are worked on their merits. A CO-97 turns on whether a bundling edit applied, and that can be argued. CO-29 has no merits to argue. The only questions are dates, and proof of dates.

Question one: who sent it

The same code leads to very different places depending on the payer.

The four Medicare exceptions

Original Medicare gives you twelve months, or one calendar year, from the date of service, measured to the date it received the claim. Only four situations extend that window. Each one extends it through the last day of the sixth month after a specific event.

When one of these applies, the route is not an appeal. CGS, the contractor whose list of proof is used above, describes it as a reopening request or an adjusted claim, sent with the documents attached.

When none applies, stop. A redetermination request on an untimely claim has nowhere to go, and every hour spent on it is an hour not spent on a denial you can win.

Question two: was it actually late

Before you accept a CO-29 from any payer, check its arithmetic. Most wrong ones come down to which dates it used.

• The start date. Medicare counts from the date of service on the claim, using the From date. Institutional claims that span several dates use the Through date instead. Other payers may count from discharge, and their manual says which.
• The end date. What counts is the date the payer received the claim. A claim rejected at the clearinghouse never arrived, so it never stopped the clock.
• Corrected claims. Check whether the payer measured your corrected claim from the original date of service or gives corrected claims a window of their own. Payers differ, and a corrected claim denied as late may have been judged against the wrong rule.
• Secondary claims. Many payers measure a secondary claim from the date of the primary payer’s remittance rather than the date of service. A slow primary should not count against you there.
• Coverage granted later. Eligibility that arrived after the fact is a reason many payers recognize, Medicare included.

Question three: can you prove it

For every payer except Original Medicare, a CO-29 on a claim that really was on time is won with one document: proof that the payer received it. For electronic claims that is the payer’s 277CA acknowledgment, not the 999 from your clearinghouse.

The timely filing post explains the difference between those two and how to lay out the appeal. The short version: state the dates, attach the proof, do the arithmetic for the reviewer, and attach nothing else. Clinical notes do not help a timely filing appeal.

When it stands

Some CO-29s are simply correct. The claim went in late and no exception applies. Write it off, and spend the time finding out where the days went.

They usually go to the same few places:

• Clearinghouse rejections nobody worked
• Claims held for a missing authorization or referral
• Claims held while a new clinician’s payer enrollment was pending
• Eligibility checked after the visit instead of before
• Secondary claims waiting on a slow primary

Each of those is a queue with no deadline on it. Give it one. A weekly report of claims within thirty days of their filing limit takes an afternoon to build and pays for itself the first time it catches one. The same claims are what push up your days in AR.

The order to work a CO-29

• Identify the payer type. If it is Original Medicare, go straight to the four exceptions.
• Check the dates the payer used against the claim, the corrected claim and any primary remittance.
• Pull your proof of receipt. The 277CA, or the delivery record for a paper claim.
• Appeal or request the exception with that proof and nothing else.
• If it stands, write it off and fix the queue it came from.

Where CO-29 sits on the remittance, next to its group code and remark codes, is covered in how to read an ERA.

Sources

• CMS Transmittal 12909, time limits for filing Medicare fee for service claims, October 2024.
• CGS Medicare, Part A timely filing.
• Palmetto GBA, Medicare claim timeliness requirements and criteria for a timeliness extension, January 2026.
• Your contract and each payer’s current provider manual, which set every limit outside Original Medicare.

Reference information only. Not billing advice. Filing limits, exceptions and appeal rights vary by payer, by plan and by contract. Verify against the payer’s own rules before you file or appeal.