Why CO-29 happens
CO-29 is a Contractual Obligation adjustment reason code for untimely filing. Payers set filing deadlines through plan contracts or regulatory requirements; once a window closes, the payer treats the claim as waived — a contractual right, not a clinical judgment. The CO prefix means the provider, as a participating plan member, agreed not to shift this write-off to the patient.
- —Late initial submission. The claim wasn't transmitted until after the filing window had already closed.
- —Incorrect payer on the first filing. The claim went to the wrong payer first; by the time it reached the correct payer, the window had closed.
- —Refiling after clearinghouse rejection. A front-end rejection reset the effective submission date. If the original rejection wasn't caught quickly, the corrected claim can arrive past the deadline.
- —Coordination of benefits delays. Waiting for a primary payer's EOB before billing secondary can push the secondary claim past its filing window, especially with short commercial plan windows.
Standard filing windows
These are the standard windows billing teams work with. The timely filing clock starts from the date of service, not the date of billing or claim generation.
| Payer type | Typical window | Note |
|---|---|---|
| Medicare Part A & B | 12 months from date of service | 42 CFR 424.44(a) |
| Medicare Advantage | 12 months (per CMS guidance) | Plan contracts may be shorter — verify with the plan. |
| Medicaid | Typically 90 days to 12 months | Varies by state — check state-specific rules. |
| TRICARE | 12 months from date of service | Per TRICARE claims filing requirements. |
| Commercial / ACA plans | 90–180 days from date of service | Per plan contract — check your payer contracts. |
Commercial plan windows vary significantly.A plan that says "180 days" in the provider manual may have a different window in the underlying contract. Pull the actual contract language for any payer where timely filing denials are recurring.
What counts as proof
The CO-29 appeal lives or dies on documentation. Four document types are accepted across most payers, in descending order of strength:
- 01
277 Claim Acknowledgment (277CA) — Accepted status
The strongest proof available. Shows the claim entered the payer's adjudication system with an accepted status. If you have this document dated within the filing window, this is what you lead with.
- 02
Clearinghouse submission report with timestamp
Acceptable at many payers. Shows the claim was transmitted by your clearinghouse on a specific date. Must be a system-generated report, not a screenshot of a portal view.
- 03
Certified mail receipt (USPS tracking)
For paper claims submitted by mail. The USPS tracking record showing delivery date constitutes proof of timely submission when the delivery date falls within the filing window.
- 04
Claims management system submission log
Must be time-stamped and system-generated — not a manually created log. Some payers accept a screenshot from your PM system; others require a formal audit report. Call the payer and ask what format they accept before you appeal.
A 277CA showing “accepted” is not the same as “paid.” It confirms the claim was received for adjudication — which is exactly what CO-29 proof requires. The payment determination happens later, through the 835 or EOB.
Working a CO-29 appeal
CO-29 appeals are procedural, not clinical. The argument is straightforward: the claim was submitted within the required window, and here is the evidence. The letter cites the 277CA date or clearinghouse timestamp, references the plan's filing window as stated in the contract or payer manual, and attaches the proof document.
If the initial submission was rejected at the clearinghouse — meaning it never reached the payer — note that distinction explicitly. A clearinghouse-level rejection restarts the effective submission date. Some payers will consider the original transmission date as evidence of intent; others will not. When the original 277CA shows a timely rejection date and the corrected resubmission is also within the window, include both documents. When only the resubmission falls within the window, the argument is harder and depends on payer policy.