CO-166: Payer's Plan Responsibility Ended

What Does CO-166 Mean?

CO-166 means the payer has denied the claim as a contractual adjustment because its responsibility for processing claims under this plan has ended. Under CO, the provider cannot bill the patient for this adjustment — it is a write-off unless you can redirect the claim to the correct payer. The typical scenario is that the employer changed insurers, the plan was discontinued, or the patient's coverage terminated. Your recourse is to identify the correct current payer and resubmit, or appeal if the plan was terminated retroactively while the patient was still eligible.

When CARC 166 appears on a remittance, the payer is telling you that it is no longer responsible for processing claims under the patient's plan. This does not necessarily mean the service was not covered at the time it was rendered — it means the payer's administrative responsibility for adjudicating claims under that particular plan has ended. The plan may have terminated, the employer may have switched insurance carriers, or the patient may have transitioned to different coverage.

This code frequently appears after plan year transitions, employer insurance changes, and during periods when patients move between coverage types (employer to marketplace, COBRA election or lapse, aging into Medicare). The denial may also trigger when claims are submitted well after the date of service and the plan has since been replaced. In some cases, retroactive plan terminations create a gap where the provider rendered services believing the patient was covered, but the plan was terminated effective a prior date.

The key distinction with CARC 166 is that it is not a judgment about medical necessity, coding accuracy, or coverage policy — it is purely an administrative determination that this specific payer is no longer the right entity to process the claim. Your path forward depends on the situation: if the patient has new coverage, submit to the new payer; if the plan terminated retroactively while the patient was still eligible, appeal to the original payer; if no coverage exists, bill the patient directly.

Common Causes

How to Resolve

Verify the patient's coverage dates, identify the correct payer, and either resubmit to the replacement plan or appeal if coverage was active at the time of service.

1. Confirm the coverage termination date Verify with the payer exactly when the plan's claim processing responsibility ended and compare against the date of service.
2. Locate replacement coverage Contact the patient to identify their current insurance coverage and obtain the new payer information.
3. Resubmit to the correct payer Submit the claim to the patient's current payer if the new plan was active on the date of service.
4. Appeal retroactive terminations If the termination was retroactive and you have proof the patient was eligible at the time of service, appeal with documentation of active eligibility.

Common RARC Pairings

The RARC code tells you exactly what triggered the CO-166:

How to Prevent CO-166

• Verify patient insurance eligibility and coverage dates in real time before every visit
• Update insurance information at every patient encounter, especially near plan year transitions
• Submit claims promptly after the date of service to stay within plan processing windows
• Monitor for plan year transitions and employer insurance changes in your patient population
• Implement automated eligibility checks at scheduling and check-in

General Prevention

• Verify patient insurance eligibility and coverage dates in real time before every visit, especially for patients who have not been seen recently
• Update patient insurance information at every visit, including checking for plan changes, employer changes, and coverage termination dates
• Submit claims promptly after the date of service to avoid exceeding plan processing deadlines, particularly near plan year transitions
• Maintain awareness of plan transition periods (January for calendar-year plans, employer contract renewal dates) and prioritize claim filing during these periods
• Implement automated eligibility verification at scheduling, check-in, and pre-claim submission stages
• Track patients with COBRA or continuation coverage to monitor for lapses that could affect claim eligibility

Also Filed As

The same CARC 166 may appear with different Group Codes:

Related Denial Codes