Who has to pay after a code 55 denial?

Who owes depends on the details. 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 55 · CO-55 / PR-55 · translated

Denial code 55: what it means — and how to fight it

Denial code 55 (shown on your EOB as CO-55 or PR-55) means: “Procedure/treatment/drug is deemed experimental/investigational by the payer.” In plain English: They're calling your treatment experimental. External reviewers side with patients on these more than almost any other category.

high appealability Who owes depends on the details

What they're really saying

“Procedure/treatment/drug is deemed experimental/investigational by the payer.”

They're calling your treatment experimental. External reviewers side with patients on these more than almost any other category.

Your odds

Strong — this type of denial has real weaknesses.

Experimental denials have strong external-review overturn rates.

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 55 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.
Ask your doctor whether this treatment is standard of care for your diagnosis. Compile peer-reviewed literature, FDA status, and specialty-society guidelines. Physician letter framing this as standard of care for your condition. Push to external review — independent doctors apply real evidence.
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 55 mean?: They're calling your treatment experimental. External reviewers side with patients on these more than almost any other category.
Can I appeal a CO-55 or PR-55 denial?: Yes. Appealing is free and it is your legal right. For this code the odds are high: Experimental denials have strong external-review overturn rates. Fewer than 1 in 100 denials are ever appealed — insurers count on silence.
Who has to pay after a code 55 denial?: Who owes depends on the details. 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.