CO-100 denial code
Payment made to patient, insured or responsible party
How to fix it
Bill the patient for the amount paid to them, referencing the payer's payment. Check whether an assignment of benefits was on file.
How to prevent it
Obtain assignment of benefits at registration, and identify non-assigning plans at verification.
In practice
A remittance shows the claim allowed and paid in full, but no money arrives. CO-100 explains it: the payer sent the payment to the patient rather than the practice.
This happens where no assignment of benefits is on file, or where the plan does not accept assignment for out-of-network providers. The payer has discharged its obligation; the money exists and is simply in the wrong hands.
Bill the patient for the amount the payer sent them, referencing the payment date and amount from the remittance so the conversation is concrete. Patients who have already spent it are the hard case, which is why speed matters more here than in almost any other denial.
What sits behind it
Collection rates on these fall sharply with time. A patient contacted within a week of receiving a cheque they did not expect will usually forward it. The same patient contacted at ninety days has typically deposited and spent it, and the account behaves like any other aged self-pay balance.
Assignment of benefits obtained at registration prevents most instances, but not all. Some plans, particularly certain non-participating arrangements and a number of individual market products, will not honour assignment at all and always pay the member regardless of what is signed.
Identifying those plans at verification is the durable control. Where a payer is known not to accept assignment, the practice can decide in advance whether to collect at the time of service, which turns a difficult recovery into an ordinary payment.
Related codes
Terms used here — Patient Responsibility · Out-of-Network · Payment Posting
How we handle it — Patient Collections · Eligibility Verification · AR Management
Primary sources
The rules behind CO-100, at the bodies that publish them.
- Advance Beneficiary Notice of Noncoverage (ABN) (opens in a new tab)
Centers for Medicare & Medicaid Services — The form and the rules for issuing it. Whether a non-covered service can be billed to the patient usually turns on whether a valid ABN was obtained beforehand.
- No Surprises Act guidance (opens in a new tab)
Centers for Medicare & Medicaid Services — Balance billing restrictions, good faith estimates and the independent dispute resolution process — all of which change what a practice may bill a patient.
- Claim Adjustment Reason Codes (CARC) (opens in a new tab)
X12 — The authoritative, maintained CARC list. Our denial code lookup explains these in plain English; X12 is where the canonical definitions live.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-100
Within days. Recovery rates fall steeply once the payment has been deposited and spent, so a call in the first week that explains what arrived and why it belongs to the practice succeeds far more often than a statement sent at thirty or sixty days.
It prevents most cases but not all. Some plans will not honour assignment for non-participating providers and pay the member regardless of what is signed, so identifying those payers at verification matters as much as collecting the form.
Be specific: name the payer, the payment date and the amount, all of which appear on the remittance. A concrete reference to a cheque or deposit they can verify is far more effective than a general statement that a balance is owed.
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