CO-138 denial code
Appeal procedures not followed or time limits not met
How to fix it
Check whether a further level of appeal remains available. Otherwise the determination stands.
How to prevent it
Track appeal deadlines separately from filing deadlines. They are shorter and start at the denial date.
The CO prefix marks this a contractual obligation. The balance is absorbed by the provider under the payer agreement and cannot be transferred to the patient.
In practice
A practice appeals a denial through the general correspondence address ninety days after the determination. The plan returns CO-138 — the appeal did not follow the required procedure and the time limit had passed.
Appeal rules are narrower than filing rules in both directions. The window is shorter, it runs from the denial date rather than the service date, and the channel is specified rather than optional.
Check whether a further level remains available, because a first-level appeal filed late does not always exhaust the process. Where nothing remains, the determination stands and the loss is procedural rather than substantive, which is the frustrating part.
What sits behind it
The two clocks are routinely confused. Timely filing runs from the date of service and is often generous. Appeal deadlines run from the date on the remittance and are frequently sixty or ninety days, which means an appeal can be late on a claim that was filed comfortably early.
Channel matters as much as timing. Plans specify appeal addresses, portals and forms, and correspondence sent to the claims address or a general enquiries line is often treated as never received. Following the published route is not bureaucratic pedantry; it is what makes the appeal exist.
Because these losses are entirely avoidable, the control is a tracked list rather than diligence. Recording every denial with its appeal deadline at the point it arrives, and working the list by deadline proximity, prevents a category of loss where the underlying claim may have been perfectly good.
Related codes
Terms used here — Appeal · Timely Filing · Denial
How we handle it — Denial Management · AR Management · Practice Analytics
Primary sources
The rules behind CO-138, at the bodies that publish them.
- Medicare timely filing requirements (opens in a new tab)
Centers for Medicare & Medicaid Services — The one-year filing limit and the narrow exceptions to it. Commercial payers set their own, usually shorter, limits by contract.
- Medicare claims appeals process (opens in a new tab)
Centers for Medicare & Medicaid Services — The five levels of appeal, what each requires and the deadline for each. Missing a level's deadline ends the appeal regardless of the claim's merits.
- Medicare Claims Processing Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — The operative manual for how Medicare claims must be coded, submitted, adjusted and appealed. When a payer policy and a vendor's advice disagree, this settles it.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-138
It runs from the denial date rather than the date of service and is usually much shorter, often sixty or ninety days. That means an appeal can be late on a claim that was submitted well within the filing window, which is why the two have to be tracked separately.
Frequently. Plans publish specific appeal addresses, portals and forms, and correspondence sent to the claims address or a general enquiries line is often treated as never received. Confirm the route before sending rather than after the deadline passes.
Sometimes. A late first-level appeal does not always exhaust the process, and higher levels or external review may remain depending on the plan and the product. Check what remains before writing the balance off, because the underlying claim may have been sound.
Find out what your denials are costing you
A free billing audit reviews your denial rate, AR aging and clean claim rate against industry benchmarks. Takes about two minutes to request. No sales pitch.
No setup fees · You pay when we collect · Pricing from 3% of net collections