Every time an insurance payer processes a claim and pays something other than the full billed amount, it has to say why. The standardized vehicle for that explanation is the remittance advice — and the codes embedded in it are CARCs and RARCs. Understanding them is the difference between accepting a denial and knowing whether to fight it.
CARC vs. RARC — the difference
CARC
Claim Adjustment Reason Code
Always paired with a dollar amount. It explains every line of the financial reconciliation — why the payer paid more or less than billed. Maintained by the X12 standards organization and required under HIPAA for electronic remittances.
RARC
Remittance Advice Remark Code
Provides context without a dollar adjustment of its own. A RARC might reference a coverage policy, specify what records are needed, or indicate the next step in the process. Some Medicare remittances include only a RARC with no associated CARC — the remark effectively IS the explanation.
The four adjustment groups
Every CARC starts with a two-letter prefix that determines the most important thing about any denial: who's responsible for the adjusted amount, and whether the patient can be billed.
The provider agreed to write off this amount under its contract with the payer or under law. The patient cannot be billed for a CO adjustment — attempting to do so is a contract violation. Most denial codes that generate appeals are CO codes.
Examples
- CO-50: Not medically necessary
- CO-97: Bundled per NCCI edits
- CO-45: Charge exceeds allowed amount
When you see this group: Investigate whether the denial is clinically correct. If not, appeal using the clinical record and the applicable coverage criteria.
The amount the patient owes — deductible, coinsurance, copay, or any other cost-sharing the plan requires. PR adjustments are generally not appealable by the provider, though the patient may dispute the amount if the plan processed the claim incorrectly.
Examples
- PR-1: Deductible
- PR-2: Coinsurance
- PR-3: Copay
When you see this group: Verify the cost-sharing was calculated correctly against the patient's plan document (Summary Plan Description or Evidence of Coverage). If the deductible was already met, that's an error worth contesting.
Adjustments that don't fit neatly into CO or PR — including coordination of benefits with a secondary payer, government program adjustments, and Medicare sequestration reductions. Context matters here more than with CO or PR.
Examples
- OA-23: Payment adjusted due to prior payer's adjudication
- OA-18: Duplicate claim
When you see this group: Identify the specific code and the context. COB claims (OA-23) often resolve by submitting the primary payer's EOB to the secondary. Sequestration reductions (OA-237 under Medicare) are generally not appealable.
Adjustments the payer makes that are neither contractual nor the patient's responsibility — for example, when a claim is adjusted because of a payer error or because recovery is being applied against a prior overpayment.
Examples
- PI-204: Service not covered under this plan
When you see this group: PI-204 is a coverage denial, not a medical necessity denial. The right appeal is a coverage argument: the plan document, Certificate of Coverage, or SPD should be reviewed for benefit language.
Common CARC codes at a glance
The full X12 list has hundreds of codes, but a small set accounts for the majority of denials in most practices.
| Code | Meaning | Typically appealable? |
|---|---|---|
| CO-50 | Not medically necessary | Yes |
| CO-97 | Bundled with another procedure (NCCI) | Yes |
| CO-45 | Charge exceeds contractual/allowed amount | Depends |
| CO-4 | Service billed with incorrect modifier | Yes |
| CO-16 | Claim lacks required information | Yes |
| CO-29 | Claim submitted past timely filing limit | Yes |
| CO-109 | Claim for eligible service under a different plan | Yes |
| PR-1 | Patient deductible | No |
| PR-2 | Patient coinsurance | No |
| OA-23 | Coordination of benefits — prior payer adjudicated | Depends |
The complete CARC list is maintained by the X12 standards organization.
How to use a CARC code to find your appeal argument
The code is the entry point, not the answer. Here's how to move from code to argument:
- 01Read the CARC group prefix first.CO means the patient can't be billed; PR means patient owes; OA is context-dependent.
- 02Look up the specific code. CO-50 is medical necessity; CO-97 is NCCI bundling; CO-29 is timely filing. Each has a different evidentiary standard.
- 03Read the RARC alongside it. The remark often specifies which coverage policy applied, what records are missing, or what the payer is actually looking for.
- 04Match to the governing regulation. Medical necessity appeals go to 29 USC 1133 or 45 CFR 147.136. NCCI edits go to the CMS NCCI Policy Manual. Timely filing goes to your proof-of-transmission documentation.
Turn the denial code into a signed appeal.
Upload the denied EOB and Merits builds the cited letter — the right argument for the right code, anchored to the source that governs your plan type.