Who has to pay after a code B4 denial?

Usually the provider's problem — not yours. 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.

All denial codes · code B4

Denial code B4: what it means — and how to fight it

Denial code B4 (shown on your EOB as CO-B4 or PR-B4) means: “Late filing penalty.” In plain English: The provider sent the bill in after the plan's filing deadline. This is their deadline to meet, not yours.

high appealability Usually the provider's problem — not yours

What they're really saying

“Late filing penalty.”

The provider sent the bill in after the plan's filing deadline. This is their deadline to meet, not yours.

Your odds

Strong — this type of denial has real weaknesses.

You should not owe this — providers generally cannot bill patients for their own late filing.

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 B4 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.
Tell the billing office in writing that their late filing is not your bill. Tell the billing office in writing that a timely-filing penalty is a contractual write-off and ask for a zero balance. If they claim they filed on time, ask for proof of timely submission — a clearinghouse acceptance report or claim number with a date — and send it to the plan for reconsideration. If the delay happened because you gave them the wrong insurance information, the plan may still reprocess it once you explain, so say so up front.
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 B4 mean?: The provider sent the bill in after the plan's filing deadline. This is their deadline to meet, not yours.
Can I appeal a CO-B4 or PR-B4 denial?: Yes. Appealing is free and it is your legal right. For this code the odds are high: You should not owe this — providers generally cannot bill patients for their own late filing. Fewer than 1 in 100 denials are ever appealed — insurers count on silence.
Who has to pay after a code B4 denial?: Usually the provider's problem — not yours. 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.