Can I appeal a CO-100 or PR-100 denial?

Yes. Appealing is free and it is your legal right. For this code the odds are low: many denials are reversed when challenged with the right evidence. Fewer than 1 in 100 denials are ever appealed — insurers count on silence.

Who has to pay after a code 100 denial?

Usually your cost to pay. The letters in front of the number matter: CO (contractual obligation) usually means an in-network provider must absorb it, while PR (patient responsibility) means the plan says you owe it.

How long do I have to appeal?

It depends on your coverage: 180 days for most employer and marketplace plans, 120 days for Original Medicare, about 90 days for Medicaid (varies by state), and only 65 days for Medicare Advantage. Set your deadline clock before you do anything else.

Denial code 100 · CO-100 / PR-100 · translated

Denial code 100: what it means — and how to fight it

Denial code 100 (shown on your EOB as CO-100 or PR-100) means: “Payment made to patient/insured/responsible party.” In plain English: Not a denial — the insurer paid YOU instead of the provider. You now owe the provider that money.

low appealability Usually your cost to pay

What they're really saying

“Payment made to patient/insured/responsible party.”

Not a denial — the insurer paid YOU instead of the provider. You now owe the provider that money.

Your odds

Check the math first — this is usually cost-sharing, not a judgment.

Fewer than 1 in 100 denied claims are ever appealed. When people do appeal, roughly 4 in 10 win the first round — and independent external review overturns 40–50% of what reaches it.

How to appeal a code 100 denial

Decode. You just did — this page is step one.
Set your clock. 180 days for most employer and marketplace plans, 120 for Original Medicare, ~90 for Medicaid, only 65 for Medicare Advantage.
Urgent? If this is care you still need, call and request an expedited appeal — they must decide within 72 hours.
Check your bank/mail for the insurer's check and pay the provider with it. Forward the payment to the provider promptly to avoid collections. Keep the EOB showing the amount.
Send it certified. Letter plus evidence, keep copies of everything.
Escalate. Denied again? Independent external review — a doctor who doesn't work for your insurer decides.

Do it in minutes, free

The AppealClock tool translates your denial, drafts your appeal letter, computes your exact deadline — and watches the clock so you don't have to. No account. Your medical details never leave your device.

Start your appeal & set your clock →

Common questions

What does denial code 100 mean?: Not a denial — the insurer paid YOU instead of the provider. You now owe the provider that money.
Can I appeal a CO-100 or PR-100 denial?: Yes. Appealing is free and it is your legal right. For this code the odds are low: many denials are reversed when challenged with the right evidence. Fewer than 1 in 100 denials are ever appealed — insurers count on silence.
Who has to pay after a code 100 denial?: Usually your cost to pay. The letters in front of the number matter: CO (contractual obligation) usually means an in-network provider must absorb it, while PR (patient responsibility) means the plan says you owe it.
How long do I have to appeal?: It depends on your coverage: 180 days for most employer and marketplace plans, 120 days for Original Medicare, about 90 days for Medicaid (varies by state), and only 65 days for Medicare Advantage. Set your deadline clock before you do anything else.