Quick answer. Health insurers use standardized national codes to explain why each charge was paid, reduced, or denied. Whether they print on your Explanation of Benefits (EOB) varies by insurer: some EOBs show the codes (often in a “code” or “remarks” column with a glossary on the back), while others translate them into plain-language reasons — though the same codes always appear on the remittance your provider receives. When your EOB does show them, the two-letter prefix tells you who owes the money: PR (Patient Responsibility) is yours to pay; CO (Contractual Obligation) is the provider's write-off and should not be billed to you; OA and PI are other adjustments. The number after it gives the specific reason — for example, PR-1 is your deductible, CO-45 is the amount above your plan's allowed rate, and CO-50 is a “not medically necessary” denial you can appeal. You can look up any code free at x12.org.
You open your EOB. The dollar amounts make some sense, and the columns — billed, allowed, paid — are starting to look familiar. Depending on your insurer, the end of each line may carry a code like CO-45, PR-1, or CO-96 — or a plain-language reason that maps to the same system. Either way, these codes are the why of your claim: they state the exact reason your insurer paid what it paid and decided what you owe. Once you can read them, you can spot a fixable error in under a minute.
This is the code decoder. If you want the bigger-picture, line-by-line walkthrough of an entire EOB — billed vs. allowed, catching duplicate charges, what to save — start with Understanding Your EOB, then come back here for the codes.
The two code systems on every EOB
Insurers do not invent these codes. They use two standardized national lists maintained by an organization called X12:
- CARC — Claim Adjustment Reason Codes. The headline code. It explains why the insurer adjusted the charge from what the provider billed.
- RARC — Remittance Advice Remark Codes. The fine print. It adds detail to a CARC or carries an extra note (for example, an alert about your appeal rights).
These codes are standardized across the system, and they always appear on the electronic remittance your provider receives. Your patient EOB, however, is not standardized the same way: some insurers reprint the codes (look for a “code” or “remarks” column and a glossary on the back or last page), while others translate them into plain-language reasons. If your EOB shows only plain language, you can ask your insurer or your provider's billing office which code applied — the answer maps directly to the lists below. The full X12 registries are public and updated three times a year (March 1, July 1, November 1). You do not need to memorize numbers; the part that matters most is the two-letter prefix, because it tells you whether the charge is yours.
The four two-letter prefixes
Every CARC begins with a two-letter group code. These four cover almost everything you will see:
This part is yours to pay — deductible, coinsurance, copay, or a service your plan does not cover. PR is the only prefix that legitimately becomes your bill.
The provider must write this off under the contract they signed with your insurer. You should not be billed for a CO amount on an in-network claim. For example, CO-45 is the gap between the provider's sticker price and the negotiated rate — it disappears. (This is the in-network write-off; a provider with no contract with your plan may still bill you the difference.)
A catch-all for adjustments that do not fit the other groups, often used when another insurance is the primary payer (coordination of benefits).
The insurer reduced payment for a reason that is neither contractual nor your responsibility. Uncommon on most patient EOBs, but it can appear with Medicare and some commercial plans. (X12 also defines a fifth, rarely seen group code, CR, for corrections and reversals.)
A practical shortcut: if every adjustment line on your EOB starts with CO, you usually owe nothing extra. If a line starts with PR, that is what the provider will bill you.
Codes patients see most often
A handful of CARC numbers show up again and again. Here is what they actually mean — with the group code shown the way it most commonly appears.
| Code | What it means |
|---|---|
| CO-45 | Charge exceeds the allowed/contracted rate. The provider billed more than your plan's negotiated price; the difference is written off. Normal — not your bill. |
| CO-97 | The service is bundled — its payment is included in another procedure that was already adjudicated. Not a separate charge to you. |
| CO-16 | The claim lacks information or has a billing error. It never appears alone — a paired remark code (RARC) gives the real reason. Often a fix the provider's office can resubmit; worth a call. |
| CO-96 / PR-96 | Non-covered charge. If it is CO-96, the provider writes it off; if it is PR-96, it shifts to you. The prefix is everything. A paired remark code explains why. |
| CO-50 | Non-covered because the payer did not deem it “medically necessary.” This is a denial you can appeal — see below. |
| PR-1 / PR-2 / PR-3 | Deductible (PR-1), coinsurance (PR-2), and copayment (PR-3) — your standard cost-sharing. |
| 204 (often PR-204) | The service, drug, or equipment is not covered under your current benefit plan. Different from CO-50 (see the next section). |
When a CARC is paired with a RARC such as N130 (“consult your plan benefit documents”) or MA01 (a Medicare alert about your appeal rights — note Medicare's own deadlines, often 120 days, are shorter than the commercial 180-day floor), the remark code is pointing you to the next step.
Have your EOB in front of you?
The EOB Explainer reads your Explanation of Benefits and translates every code, every dollar, and every place a mistake could be hiding into plain language.
Open the EOB Explainer“Not medically necessary” vs. “not covered” — why it matters
Two denials look similar but call for completely different responses:
- CO-50 (not medically necessary) is a clinical decision about your situation — and it is appealable. The strongest response is a Letter of Medical Necessity from your clinician, attached to an appeal.
- 204 (not a covered benefit) is a plan-design decision — the service simply is not in your benefits. A medical-necessity appeal usually will not change it; the better paths are a formulary or benefit exception, an employer-plan exception (if your plan is self-funded), or cash and assistance options.
Telling these two apart before you act saves weeks of effort on the wrong pathway.
When the codes are worth fighting
Most EOB codes are routine. A few should make you stop and look closer:
- A PR charge on preventive care. Most ACA plans must cover in-network preventive services — recommended screenings, vaccines, annual wellness visits — with no cost-sharing, so a PR charge on one is often a coding error worth a call. Colorectal screening is a common trap: under federal guidance (polyp removal since 2013; a follow-up colonoscopy after a positive stool-based test since 2022), private plans must cover both with no cost-sharing — each counts as an integral part of the screening. If you were charged for one of those, it is usually a billing or coding error to dispute, not a charge you owe. (Medicare's rules differ and have been phasing this in.)
- CO-50 on care your doctor recommended. This is a formal “not medically necessary” denial, and you generally have the right to an internal appeal and, in many cases, an independent external review. The deadline is printed on the notice.
- Out-of-network charges you did not choose. If you got care at an in-network facility but were billed by an out-of-network anesthesiologist, radiologist, pathologist, or assistant surgeon — or received out-of-network emergency care — the federal No Surprises Act generally limits you to your in-network cost-share.
- A CO-16 that has been sitting for weeks. This is a claim the provider's office needs to fix and resubmit. It is not a denial — but if nobody resubmits, it can quietly turn into a balance owed.
- A PR amount that does not match your plan. If your Summary of Benefits says primary care is a $30 copay but the EOB shows PR-2 coinsurance of $180, something is likely coded wrong. Call.
How to look up any code in under a minute
- X12's official lists. The Claim Adjustment Reason Codes and Remittance Advice Remark Codes registries at x12.org are the source of truth (written for billers, but authoritative).
- The key printed on your EOB. Most EOBs include a short glossary of the codes used — usually on the back or last page — and for a denial, the insurer must tell you the code's meaning and your appeal rights.
- Your insurer's member services line. Ask: “What does code [X] mean on this claim, and do I have a right to appeal?” Write down the answer and the call reference number.
Tools that may help
- EOB Explainer — read your Explanation of Benefits and decode every line
- Understanding Your EOB — the full line-by-line guide to your statement
- Bill Audit — scan a medical bill for common errors
- Medical Necessity Letter Generator — the strongest response to a CO-50 denial
- Patient Appeal Generator — draft an appeal letter for a denied claim
Frequently asked questions
What is the difference between CARC and RARC codes?
A CARC (Claim Adjustment Reason Code) is the main code that tells you why your insurer adjusted, reduced, or denied a charge. A RARC (Remittance Advice Remark Code) adds detail — a more specific reason or an informational alert such as your appeal rights. Most EOB lines have one CARC and may carry one or more RARCs. Both are standardized national lists maintained by X12 and updated three times a year.
Does a CO code mean I have to pay it?
Usually the opposite. CO stands for Contractual Obligation: the provider agreed, by contract with your insurer, to write off that amount, and you should not be billed for it on an in-network claim. If a provider bills you for a CO amount on an in-network claim, that is generally not allowed.
Does a PR code always mean I have to pay?
Almost always. PR stands for Patient Responsibility — your deductible, coinsurance, copay, or a non-covered service. The exceptions are when a PR charge appears on care that should have been protected, such as ACA-required preventive care delivered in network, or out-of-network care covered by the No Surprises Act. In those cases the PR charge may be a coding error worth questioning.
Can I appeal a CO-50 denial?
Yes. CO-50 means the insurer decided the service was not medically necessary. Most plans allow an internal appeal, and many cases qualify for an independent external review afterward. The federal floor for filing an internal appeal is generally at least 180 days, but your denial notice states the deadline that applies to you. A Letter of Medical Necessity from your treating clinician is often the most important supporting document.
What is the difference between a CO-50 denial and a “not covered” (204) denial?
CO-50 says the service is not medically necessary for you — an appealable clinical decision, and a strong Letter of Medical Necessity is the usual response. Code 204 says the service is not a covered benefit under your plan at all — a plan-design decision, where a medical-necessity appeal usually will not change it and the better paths are a benefit or formulary exception, an employer-plan exception, or cash and assistance options.
Where can I find the official code lists?
X12 maintains the official lists at x12.org. The codes are also republished in Medicare's Claims Processing Manual and in each insurer's EOB glossary, usually on the back or final page. The lists are updated three times a year, on March 1, July 1, and November 1.
Sources
- X12. Claim Adjustment Reason Codes (CARC). x12.org
- X12. Remittance Advice Remark Codes (RARC). x12.org
- X12. Claim Adjustment Group Codes (CO, PR, OA, PI). x12.org
- CMS. Medicare Claims Processing Manual, Chapter 22 — Remittance Advice. cms.gov
- CMS. No Surprises: Understand Your Rights Against Surprise Medical Bills. cms.gov
- CMS. How to Read an Explanation of Benefits. cms.gov
- 45 CFR § 147.136 — internal claims and appeals (180-day federal floor). ecfr.gov
- KFF. Claims Denials and Appeals in ACA Marketplace Plans. kff.org