Timely Filing Limit in Medical Billing: Medicare's 12 Months, Payer Windows and CO-29
A timely filing limit is the maximum time a payer allows between the date of service and the day it receives the claim. A claim that arrives after the limit is denied, and in almost every case the balance cannot be billed to the patient either.
Most practices lose a claim this way at some point. This article explains how to keep it rare, and how to find the cause when it happens.
Where you see it
| Place | What it shows |
|---|---|
| Payer contract or provider manual | The window for that payer, usually stated in days from the date of service |
| Medicare regulation 42 CFR 424.44 and the Medicare Claims Processing Manual, chapter 1, section 70 | The 12-month rule, which date the limit is counted from, and the four exceptions |
| CMS-1500 box 24A, 837P segment DTP*472 | The date of service the limit is counted from, line by line |
| CMS-1500 box 19 | Where a MAC asks you to note the reason when you request a filing exception |
| 277CA acknowledgment or clearinghouse acceptance report | The payer’s receipt date and claim number: your proof of filing |
| Remittance advice | CARC 29, group code CO, often with RARC N211 on Medicare; see the group and reason code columns on our annotated remittance |
Medicare’s rule
Medicare fee-for-service claims must be received by the Medicare Administrative Contractor no later than 12 months, one calendar year, after the date of service. The details matter:
- Which date starts the count. For professional claims (CMS-1500 and 837P) with a date span on a line, the line item From date is used. For institutional claims with span dates, the Through date is used. A service on February 29 must be filed by February 28 of the following year.
- Which date ends it. The receipt date, not the date you sent it. The receipt date is stamped on the claim record when the MAC receives it and stays there permanently.
- A rejected claim was never filed. A submission that fails the MAC’s front-end edits for completeness or validity is not accepted into the claims system and does not count toward timely filing. The same is true of claims that go to the wrong contractor. This is the most common cause of timely filing losses: the claim was sent in time, rejected, and never re-sent.
- The denial cannot be appealed. A timely filing denial is not an initial determination, so there are no appeal rights. The remittance usually says so with remark code N211.
- The patient cannot be billed. When the provider is responsible for the late claim, the provider may not charge the beneficiary for the services, other than the deductible and coinsurance that would have applied if Medicare had paid.
The exceptions
The regulation allows four extensions, and the MAC decides whether you qualify:
| Exception | What it covers | How long you get |
|---|---|---|
| Administrative error | Misinformation, delay or mistake by Medicare, a contractor or an agent of the Department caused the late filing | Through the last day of the sixth month after the error was corrected; MACs will not consider errors more than four years old |
| Retroactive Medicare entitlement | The patient was not entitled to Medicare on the date of service and later received entitlement retroactive to or before that date | Set by the MAC from the notification date; supporting documentation required |
| Retroactive entitlement with a Medicaid recoupment | The patient was a Medicaid-only patient, later became dually eligible retroactively, and the state Medicaid agency recouped its payment six months or more after the service | Set by the MAC from the recoupment |
| Retroactive disenrollment from a Medicare Advantage plan or PACE organization | The plan recouped its payment six months or more after the service because the patient was disenrolled retroactively | Set by the MAC from the recoupment |
Each is a request to the MAC with documentation, not an appeal. MACs publish their own instructions; Noridian, for example, asks for a comment in box 19 and the supporting letter.
Commercial and Medicaid windows
There is no single commercial rule. Contracted windows commonly run from 90 days to a year from the date of service, and out-of-network claims sometimes get longer. State Medicaid programs set their own limits, again commonly between 90 days and a year, and managed Medicaid plans may differ from the state’s fee-for-service rule. Treat the contract as the source of truth and load the number into your billing system per payer; set the daily routine by the shortest window among your payers.
Secondary claims and the filing limit
For a secondary claim you cannot file until the primary has adjudicated, so many commercial plans count their window from the date of the primary payer’s remittance. Medicare does not. Its exceptions list does not include waiting for a primary payer, which means a Medicare secondary claim must still reach the MAC within 12 months of the date of service. If a commercial primary is slow, follow up on the primary with that date in mind.
A worked example
A freestanding radiation oncology center bills Medicare for weekly treatment management, CPT 77427, with a date of service of March 10, 2026. The claim is sent on March 12 and rejected by the MAC’s front end for an invalid Medicare Beneficiary Identifier. The rejection stays in a clearinghouse queue that no one is assigned to work. The error is found during a cleanup in March 2027 and the corrected claim is received on March 20, 2027.
| Line | Billed | Allowed | CO-29 | Patient responsibility | Paid |
|---|---|---|---|---|---|
| 77427 | 240.00 | 0.00 | 240.00 | 0.00 | 0.00 |
Figures are illustrative. The entire charge is a write-off: no appeal rights, no patient balance. Had the rejection been worked in March 2026, the same claim would have paid in full. The only exception that could apply here is administrative error, and a clearinghouse queue is the practice’s error, not Medicare’s.
What to do
- Work rejections every day. A rejected claim has not been filed. Clear the rejection queue to zero every morning.
- Load each payer’s limit into the billing system and run an aging report of unbilled and unaccepted charges against it, not only an aging of claims already out. Iridium Suite’s accounts receivable work lists age open claims by payer, which is where a charge near its filing limit should appear before the limit passes.
- Keep the proof. Archive 277CA acknowledgments and acceptance reports with the payer’s claim number. For a commercial CO-29, send that acknowledgment with a reconsideration; most contracts honor documented receipt.
- Recognize the codes. CARC 29 with group CO is the timely filing denial. On Medicare it comes with N211 (you may not appeal). Check whether one of the four exceptions applies; if not, write the balance off and do not bill the patient.
- Request exceptions promptly. Retroactive entitlement cases are common in oncology, where patients qualify for Medicare on disability mid-treatment. Send the MAC the entitlement letter and the claim as soon as the notice arrives.
Related terms
- Claim adjustment group and reason codes
- Remark codes
- Coordination of benefits
- Crossover claims
- HICN, MBI and ICN
Further reading: Medicare Claims Processing Manual, chapter 1, section 70 (PDF); MLN web-based training on the CMS-1500: timely filing; Noridian JE Part B: timely filing; X12 claim adjustment reason codes.
Frequently asked
What is the timely filing limit for Medicare?
Medicare fee-for-service claims must be received by the Medicare Administrative Contractor no later than 12 months, or one calendar year, after the date of service. For professional claims with a date span, the limit is counted from the line item From date. A claim received later is denied, and the denial cannot be appealed.
What does denial code CO-29 mean?
Claim adjustment reason code 29 means the time limit for filing has expired. With group code CO the balance is a contractual write-off, not patient responsibility. On Medicare remittances it usually arrives with remark code N211, which says the decision cannot be appealed.
What counts as proof of timely filing?
The payer's own acknowledgment that it received the claim: the 277CA claim acknowledgment or the acceptance report from your clearinghouse showing the payer's receipt date and claim number. A clearinghouse rejection does not count, because a rejected claim was never received as a claim.
Does the timely filing limit restart for a secondary claim?
It depends on the payer. Many commercial plans count the secondary window from the primary payer's remittance date, but Medicare does not list waiting for a primary payer among its exceptions, so a Medicare secondary claim still has to arrive within 12 months of the date of service.