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CO-100 Denial Code: Payment Made to Patient

CO-100 means the payer issued the payment for this claim to the patient, insured, or responsible party rather than to you. The claim was paid, but you need to collect that amount from the patient, typically because assignment of benefits was not accepted or recorded.

Code: CO-100 (CARC 100)
Status: Active In use since January 1, 1995; last modified May 1, 2018.
Code set: Claim Adjustment Reason Codes (CARC)
Group codes: CO (Contractual Obligation): Commonly used by payers to report that the payment went to the patient. The amount is still owed for the service, so it is collected from the patient rather than written off.
PR (Patient Responsibility): Signals directly that the patient is responsible for the amount because they received the payment.
OA (Other Adjustment): Some payers report it as an other adjustment because it explains where the money went rather than reducing liability.
Official description: Payment made to patient/insured/responsible party.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified: September 24, 2026 against the official X12 list.

What CO-100 means

CARC 100 says payment made to patient/insured/responsible party. The payer processed and paid the claim, but the check or electronic payment went to the patient or policyholder. The ERA or EOB you receive reports the payment so you know what happened, but no money comes to you.

Although this code often arrives with a CO group, it is not a write-off. You provided the service, the payer paid for it, and the patient now holds the money. The amount should move to the patient’s balance.

Example: an out-of-network practice submits a claim without assignment of benefits. The plan pays the member directly and sends the practice a remittance showing CO-100. The practice bills the patient for the paid amount plus any cost-sharing shown on the remittance.

Common causes

Assignment of benefits was not indicated. On the CMS-1500 this is box 13 (insured’s signature for payment to the provider) and box 27 (accept assignment).
The provider is out of network and the plan pays non-participating providers’ claims to the member.
The patient submitted the claim themselves, or a duplicate of it, and the payer paid them first.
Enrollment or payment setup issues meant the payer could not pay the provider and defaulted to the member.
The plan’s rules or state law direct payment to the member in certain situations.

How to fix it

Confirm who received the payment and how much, using the remittance or by calling the payer.
Move the amount to patient responsibility in your system. Do not post it as a contractual adjustment.
Bill the patient promptly, explaining that the insurer sent them the payment for your services. Include a copy of the remittance detail if helpful.
If assignment was signed but not reported, ask the payer whether it will reissue payment to you. Most will not reverse a payment already sent to the member, but policies vary.
Fix the claim setup for future claims: signature on file in box 13, accept assignment in box 27, and correct billing provider enrollment.

How to prevent it

Collect a signed assignment of benefits at intake and record it in the patient account so it transmits on every claim.
For out-of-network plans known to pay members directly, collect payment at the time of service or set up a payment arrangement.
Confirm electronic payment enrollment and banking details with each payer so the payer can pay you.
Watch for CO-100 patterns by payer. A cluster can reveal a missing signature-on-file indicator in your claim file.

Specialty notes

Out-of-network behavioral health and therapy practices see this code often, because many plans pay members directly for non-participating care. Setting clear financial expectations at intake helps the patient understand that the insurer’s payment belongs to the practice.

CO-293 (Payment made to employer.): Payment made to the employer rather than the provider.
CO-294 (Payment made to attorney.): Payment made to the patient's attorney.
CO-111 (Not covered unless the provider accepts assignment.): Not covered unless the provider accepts assignment, a related assignment issue.
CO-A0 (Patient refund amount.): Patient refund amount, a different patient-payment adjustment.

CO-100 FAQ

Why did the payer pay the patient instead of me?

Usually because the claim did not indicate assignment of benefits, you are out of network with a plan that pays members directly, or the plan's rules send non-participating payments to the member.

Is CO-100 a denial?

No. The service was paid. The payment just went to the wrong party from your perspective, so the amount becomes a patient balance.

Should I write off a CO-100 amount?

No. Transfer it to the patient's balance and bill them for the amount the payer sent them, along with any cost-sharing.

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