My insurance denied a charge — why am I being billed for it?

Your Explanation of Benefits shows a line the plan did not pay, with a code next to it. The bill charges you for it anyway.

Whether you actually owe that money depends almost entirely on two letters most people never look at.

The two letters that decide it

Denial codes come with a group prefix, and the prefix — not the number — says who is responsible:

• CO — Contractual Obligation. The provider agreed to eat this as a condition of their contract with your plan. You do not owe it. Billing you for a CO amount is generally not permitted under that contract.
• PR — Patient Responsibility. This one is genuinely yours: a deductible, a copay, coinsurance, or a service your plan does not cover.
• PI — Payer Initiated Reduction. The plan decided not to pay, and it is also not the patient's.
• OA — Other Adjustment. Usually informational, often pointing at another payer.

The number after the prefix says why. The prefix says who pays. The same number means opposite things with different prefixes — CO-96 and PR-96 are both "non-covered charge", but one is the provider's problem and one is yours.

The ones that come up most

• CO-29 — the claim was filed too late. The provider missed the plan's filing deadline. That is squarely their mistake and the contract says they absorb it.
• CO-97 — bundled into another service. The charge is already included in something else that was paid. Billing it separately is the thing the bundling rule exists to prevent.
• CO-45 — charge exceeds the allowed amount. The classic write-off. This is the discount the provider agreed to.
• PR-1, PR-2, PR-3 — deductible, coinsurance, copay. These are yours, and they are the most common reason a bill is legitimately larger than you expected.

What to ask

If the code starts with CO or PI:

Please explain why [service] is being billed to me when the plan shows it as denied under [code], and please confirm whether you intend to take it up with the plan instead.

If they say it was a coding error on their end, ask them to resubmit the claim rather than bill you, and ask them to put the account on hold while they do.

A denial is not always the end

Even a PR denial can be appealed, and appeals succeed more often than people expect — particularly where the denial was for missing information, a prior authorisation the provider should have obtained, or a coding choice.

Your plan has to tell you how to appeal and give you a deadline. It is usually on the EOB itself, in small print near the bottom.

Worth knowing

A denial for no prior authorisation is worth pushing on. Getting prior authorisation is usually the provider's job, not yours, and many plans will not let a provider bill a patient for a service denied because the provider failed to obtain it.

This is general information, not legal, medical or financial advice, and not a statement about your particular bill. A difference between two documents often has an ordinary explanation, and the billing office is the one who can give it.

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