CO 29 and N39011 Denials in Medicare Billing (2025 Guide)
Blog Outline
Introduction
When a claim is not received by Medicare within its strict filing window, it does not just get delayed, it gets denied outright. In Medicare billing, the two most common codes that signal a missed filing deadline are CO 29 and N39011. Both mean one thing: the claim was submitted too late to be paid under Medicare’s one-year claim filing limit.
Understanding CO 29 and N39011 Denial Codes
In Medicare billing, every denied claim is accompanied by Claim Adjustment Reason Codes (CARCs) and Remittance Advice Remark Codes (RARCs) that explain why payment was refused. Among these, CO 29 and N39011 are the two codes that specifically indicate a timely filing failure.
What Is Denial Code CO 29?
CO 29 denial appears when “The time limit for filing claim has expired.”
It is a Claim Adjustment Reason Code (CARC) primarily used for Medicare Part B professional claims, such as those submitted by physicians, therapists, and outpatient providers. When you see CO 29 on your Electronic Remittance Advice (ERA/835) or paper remittance:
- It means the claim was received by Medicare after the 12-month timely filing limit from the date of receipt, not the submission or postmark date.
Example:
A claim for a date of service (DOS) on June 10, 2024, received by Medicare on June 15, 2025, will automatically deny with CO 29 because it exceeds the one-year filing limit.
What Is Denial Code N39011?
N39011 is a Medicare Part A denial code used by MACs to indicate “Timely filing expired.”
This code applies to institutional providers — hospitals, skilled nursing facilities (SNFs), and home health agencies — that file inpatient or facility-based claims. It is the Part A equivalent of CO 29 and often appears together with related remark codes such as M290 or MA39, which indicate late or incomplete submissions.
Example:
A hospital discharge occurred on January 2, 2024, but the final claim was filed on January 15, 2025. Since the filing date exceeded 12 months from the discharge (“Through”) date, the MAC will issue N39011 and deny the claim.
Where CO-29 and N39011 Denials Appear in the ERA (835)
| Code Type | Example Code | Appears In | Meaning |
|---|---|---|---|
| CARC | CO 29 | 835 “Adjustment Reason” Segment | Claim filed after timely limit (Part B) |
| RARC | M290 / MA39 | 835 “Remark Code” Segment | Further explains why claim missed the filing window |
| Part A Reject Code | N39011 | MAC Return File or Remittance | Institutional claim exceeded 12-month rule |
Reasons of Denial CO 29 or N39011 Codes
While CO 29 and N39011 sound like technical codes, they often signal simple operational issues that went unnoticed until it was too late. In almost every case, the root cause is the same. Let’s break down the most common reasons this happens in 2025.
🕒 1. Claim Filed After the 12-Month Limit
The most direct cause. If the MAC receives the claim—even one day past the 12-month deadline—it automatically triggers CO 29 (Part B) or N39011 (Part A).
Example: DOS = May 1 2024 → Received May 2 2025 = Denied.
🔁 2. Returned-to-Provider (RTP) or Rejected Claims Not Resubmitted in Time
Unprocessable (RTP) claims do not preserve the original submission date. If they are not corrected and resubmitted within the same 12-month period, Medicare treats them as new late filings.
Tip: Always correct and re-submit RTPs within 10 business days.
📅 3. Incorrect or Delayed Patient Eligibility Updates
If Medicare coverage is added or corrected retroactively, any previously denied claim must still meet the timely filing limit—unless an approved retroactive eligibility exception is filed.
Example: A patient gains retroactive Part B entitlement six months after service; the provider must request a reopening (TOB xxQ) with eligibility proof if the 12-month window has passed.
🧾 4. MAC or Clearinghouse Rejection Not Followed Up
Sometimes the claim leaves the practice management system but never reaches Medicare because of a clearinghouse or EDI rejection. If the billing team doesn’t monitor daily acknowledgments (999/277CA files), the claim might remain unsubmitted until the window closes.
🩺 5. Provider Enrollment or PTAN Delays
If the rendering provider’s NPI/PTAN was not active at the time of claim submission, Medicare automatically rejects the claim. If credentialing takes too long, the filing deadline can expire before re-submission—producing CO 29 or N39011.
🌩️ 6. Administrative or System Errors (MAC/CMS)
Rare but recognized by CMS. If a system outage or processing error prevented timely submission, providers may later file under the administrative error exception in IOM § 70.7 with documented proof.
Claim Lifecycle Path to Denial Code CO 29 or Code N39011 Denial
DOS → Claim created → Submitted to clearinghouse
EDI Rejected (RTP) or Not Transmitted
No Resubmission within 12 Months
MAC Receives Late Claim
Denial Issued: CO 29 (Part B) or N39011 (Part A)
Examples of CO 29 and N39011 Denials
Example 1: Part B Outpatient Claim Denied with CO 29
Scenario of Claim denied with CO 29 Code
A physical therapist submits a claim for services rendered on June 1, 2024. Due to staff turnover, the claim is not transmitted through the clearinghouse until June 10, 2025.
Result
The MAC receives the claim 9 days past the 12-month window. The claim automatically denies with CO 29.
Resolution
Since no CMS exception applies, the claim cannot be appealed or reopened. The loss caused with CO 29 denial is final.
Example 2: Institutional Claim Denied with N39011 (Part A)
Claim denied with N39011 Scenario
A hospital discharges a patient on January 2, 2024, but delays its final bill while awaiting documentation. The claim is submitted on February 5, 2025.
Result
Because the submission occurred over 13 months after the “Through” date, the MAC issues denial code N39011 – Timely filing expired.
Resolution
The hospital may only file a reopening (TOB xxQ) if it can prove a valid CMS exception, such as a system error or retroactive beneficiary entitlement. Otherwise, the denial stands final.
How to Fix CO 29 and N39011 Denials (Medicare 2025)
When you receive a CO 29 (Part B) or N39011 (Part A) denial, it means your claim was filed after Medicare’s 12-month deadline and has been automatically disallowed for payment. However, under CMS IOM § 70.7, certain exceptions permit a reopening if specific conditions are met. Here’s exactly how to handle it.
Confirm the Denial Type and Claim Timeline
Check the ERA/835 remittance advice or MAC portal for the denial message:
- CO 29: “Time limit for filing claim has expired.” (Part B)
- N39011: “Timely filing expired.” (Part A)
Then verify the Date of Service (DOS) or Through date against the claim’s receipt date. If the claim was received after 12 months, proceed only if a CMS exception applies.
Determine If a Valid CMS Exception Exists
Under CMS Pub 100-04 § 70.7, only a few circumstances qualify for a reopening:
| Exception | Proof Required |
|---|---|
| Administrative error by CMS or MAC | Written documentation of system or processing failure |
| Retroactive Medicare entitlement | SSA or CMS entitlement letter showing effective date |
| Retroactive coverage or enrollment | Proof of coverage correction issued after DOS |
| Natural disaster or public emergency | FEMA or MAC-issued disaster notice |
If none of these apply, the denial is final and unpayable.
File a Reopening Request (TOB xxQ)
A reopening is not the same as a standard appeal — it’s a special process used only for timely filing exceptions.
How to submit
- Identify the denied claim number and DOS.
- Prepare a new Type of Bill (TOB) xxQ for Part A (or the reopening form for Part B).
- Attach supporting documentation proving the exception (e.g., eligibility letter, MAC memo, disaster declaration).
- Submit via MAC portal, EDI, or paper as instructed on your MAC’s website.
- Retain confirmation of submission for audit purposes.
Example
A retroactive entitlement letter dated March 2025 shows Part B coverage effective April 2024. The claim for April 2024 services denied with CO 29 can be reopened with this proof.
Track the Reopening Status
MACs generally process reopening requests within 30–60 days. You can monitor progress through the MAC’s provider portal or inquiry system (e.g., Noridian Direct Data Entry, CGS Portal, FCSO SPOT, etc.).
If the reopening is approved, payment is reissued. If denied, the MAC will cite the reason (e.g., insufficient documentation, invalid exception).
Financial and Compliance Impact of CO 29 and N39011 Denials
When Medicare denies a claim for timely filing, the result is not just administrative frustration — it is permanent revenue loss. Unlike coding or documentation errors, late-filed claims cannot be appealed, adjusted, or corrected. Every CO 29 or N39011 denial represents lost reimbursement that can never be recovered unless a valid CMS exception applies.
1. Direct Revenue Loss
If your practice averages $150 per Medicare encounter, losing just 20 late claims per year equals $3,000 in unrecoverable revenue. For hospitals or large groups, these losses can reach tens of thousands annually.
In 2025, data from MAC educational reports show that timely filing denials remain one of the top five preventable causes of Medicare revenue leakage — particularly among outpatient clinics, rehab centers, and home health providers.
2. Compliance Risk
Medicare expects providers to demonstrate internal controls that ensure compliance with the 12-month filing rule. Repeated CO 29/N39011 denials can trigger:
- MAC data reviews,
- Targeted Probe and Educate (TPE) audits, or
- Revenue integrity investigations into systemic claim management issues.
Lack of a timely filing log or reopening documentation can also raise program-integrity concerns during audits.
3. Operational Indicators of Risk
| Metric | Healthy Benchmark | High-Risk Indicator |
|---|---|---|
| % of claims denied for CO 29/N39011 | < 0.2 % | > 1 % |
| Average days from DOS to initial submission | ≤ 45 days | ≥ 90 days |
| % of RTPs resubmitted within 30 days | ≥ 95 % | ≤ 80 % |
4. Strategic Benefit of Automation and Outsourcing
Practices that integrate claim-aging automation or outsourced RCM support experience far fewer timely filing denials. At MedStates, our Medicare billing specialists use:
- DOS-based aging dashboards that flag claims 300 days after service,
- Automatic RTP tracking, and
- MAC-specific reopening workflows that preserve compliance.
This not only prevents denials but also protects provider revenue and audit standing under Medicare’s 2025 compliance standards
Conclusion
CO 29 and N39011 denials are not just another category of claim rejections — they’re final indicators of missed Medicare deadlines. Once the 12-month filing limit under 42 CFR § 424.44 passes, payment eligibility ends unless a valid CMS-approved exception applies.
At MedStates, our Medicare billing specialists help providers eliminate timely filing issues through:
- Real-time submission tracking and aging dashboards,
- Daily RTP and rejection reconciliation, and
- Expert reopening support under CMS § 70.7 exceptions.
Our goal is simple — to ensure that no claim expires before payment.
If your organization faces recurring CO 29 or N39011 denials, or needs help tightening timely filing compliance, our team can audit your Medicare claim workflow and build preventive systems that stop denials before they happen.
👉 Contact MedStates today to safeguard your Medicare reimbursements and maintain full compliance with 2025 CMS filing rules
