CO-166 denial code
Services submitted after the payer's responsibility for processing claims ended
How to fix it
Identify the payer responsible for the period and rebill.
How to prevent it
Verify plan termination dates, particularly for patients moving between plans mid-year.
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 patient moved from one Medicare Advantage plan to another in January. A claim for a February service is submitted to the previous plan out of habit and returns CO-166.
The plan's responsibility for processing claims ended when the patient's enrolment did. It is not denying coverage so much as declining jurisdiction, which is a different thing and points somewhere else.
Identify the payer responsible for the service period and submit there. The filing deadline with the correct payer has been running from the date of service throughout, so this is worth doing quickly rather than appealing to the wrong plan.
What sits behind it
Enrolment transitions concentrate this denial into predictable windows. January is the largest as annual elections take effect, but Medicare Advantage special enrolment periods, Medicaid managed care reassignments and employer plan changes all produce mid-year transitions with the same effect.
The trap is that eligibility responses sometimes lag the transition, so a verification performed in early January can return the previous plan as active. Where a payer identity looks stale, checking the effective dates in the response rather than the active flag catches it.
Retroactive enrolment complicates it further, since a plan can be assigned responsibility for dates already past. Claims denied by one payer during that period frequently become payable by another once the retroactive assignment is visible, so these are worth rechecking rather than writing off.
Related codes
Terms used here — Eligibility Verification · Timely Filing · Coordination of Benefits
How we handle it — Eligibility Verification · AR Management · Denial Management
Primary sources
The rules behind CO-166, 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-166
No. Timely filing means the claim came too late to the correct payer. This means the payer is not responsible for the period at all, so no appeal to that plan will succeed. The claim needs redirecting rather than defending.
Verify eligibility for the specific date of service rather than for today, and read the effective dates in the response rather than the active flag. Eligibility systems sometimes lag enrolment transitions, particularly in January.
Yes, and it is worth rechecking before writing off. A plan can be assigned responsibility for dates already past, and claims denied during that window frequently become payable by the newly responsible payer once the assignment appears in the eligibility record.
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