Who has to pay after a code B12 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 B12

Denial code B12: what it means — and how to fight it

Denial code B12 (shown on your EOB as CO-B12 or PR-B12) means: “Services not documented in patient's medical records.” In plain English: The plan looked at the chart and could not find the visit or service that was billed. Usually the notes are thin or were never sent.

high appealability Usually the provider's problem — not yours

What they're really saying

“Services not documented in patient's medical records.”

The plan looked at the chart and could not find the visit or service that was billed. Usually the notes are thin or were never sent.

Your odds

Strong — this type of denial has real weaknesses.

Often overturned once the complete record is actually submitted.

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 B12 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.
Get your own copy of the record for that date and compare it to the bill. This is a records problem before it is a coverage problem. Ask the provider to confirm what was sent and to resubmit the full chart for the date of service, including any late-signed or addended notes. Request your own copy of the record and read it against the bill. If the service genuinely is not in your record and you do not remember receiving it, treat this as a possible billing error or duplicate and dispute the charge outright.
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 B12 mean?: The plan looked at the chart and could not find the visit or service that was billed. Usually the notes are thin or were never sent.
Can I appeal a CO-B12 or PR-B12 denial?: Yes. Appealing is free and it is your legal right. For this code the odds are high: Often overturned once the complete record is actually submitted. Fewer than 1 in 100 denials are ever appealed — insurers count on silence.
Who has to pay after a code B12 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.