CARC and RARC codes: reading a remittance advice like a biller, not a clerk.
Every payment adjustment and every denial your practice ever receives is expressed on the remittance as a code, not a sentence. A clerk sees "denied" and moves on. A biller reads the code, knows the category before opening the chart, and knows in seconds whether the claim needs a correction, an appeal, or a deliberate write-off.
Key takeaways
- CARC states the category, RARC often adds the detail. Read both before deciding what to do with a denial.
- The code decides the fix. A submission error gets a corrected claim; a coverage dispute gets an appeal. Confusing the two wastes effort either way.
- X12 maintains the authoritative list. Don't rely on a memorized or saved list for anything beyond the handful of codes you see constantly.
- Not every denial means the payer is wrong. Some codes point to a claim that was correctly denied on the facts.
What CARC and RARC actually are
The 835 electronic remittance advice — the transaction a payer sends back after adjudicating a claim — doesn't communicate in prose. It communicates in codes, because the 835 format has to be machine-readable across every payer and every practice management system in the country, and a standardized code does that job in a way a free-text explanation never could. The Claim Adjustment Reason Code, or CARC, is the primary code on that remittance: it states the category of any adjustment between what was billed and what was paid, including a full denial. CARC codes are maintained by X12, the standards body responsible for the electronic data interchange formats used across US healthcare claims and remittances, and the current, authoritative list lives at x12.org/codes/claim-adjustment-reason-codes.
A Remittance Advice Remark Code, or RARC, frequently rides alongside the CARC on the same remittance line. Where the CARC states the category — timely filing, medical necessity, a bundled service — the RARC often supplies a level of detail the CARC alone doesn't carry, such as which specific documentation element was missing, or which specific policy the payer applied. Not every CARC arrives with a RARC attached, and not every RARC is equally informative, but where one is present it's worth reading before deciding how to work the denial, because it can turn a vague category into a specific, fixable instruction.
Both codes appear as segments within the ANSI X12 835 transaction, alongside a group code — CO for contractual obligation, PR for patient responsibility, and a handful of others — that indicates who is ultimately responsible for the adjusted amount. A CO group code paired with a CARC generally means the practice cannot bill the patient for that amount under its payer contract; a PR group code means the balance can typically move to patient responsibility. Reading the group code alongside the CARC is what tells a biller whether a denial is a write-off, a patient statement, or an appeal candidate, before any further investigation happens.
The seven codes worth knowing cold
Practices don't need to memorize the full X12 code set — it runs into the hundreds of active codes, most of which a given specialty or payer mix will rarely or never see. What's worth knowing cold is the small handful that show up constantly across almost any payer mix, because recognizing them instantly is what lets a denials team skip the investigation step and go straight to the fix.
| CARC | Meaning | Typical root cause | What usually fixes it |
|---|---|---|---|
| CO-16 | Claim lacks information or a submission/billing error that prevents correct adjudication | Coding or claim-data error — often a missing modifier, invalid diagnosis pointer, or incomplete field | Corrected claim once the specific missing element is identified, usually from an accompanying RARC |
| CO-18 | Duplicate claim | Submission process — a claim resubmitted before the original finished adjudicating, or a billing system re-sending automatically | Confirm against the original claim status before resubmitting; fix the process that caused the duplicate send |
| CO-27 | Expenses incurred after coverage terminated | Eligibility — coverage ended before the date of service, usually not caught at check-in | Rebill the correct current payer, or bill the patient where the coverage situation supports it |
| CO-29 | Timely filing expired | Timely filing — commonly the cited code for this, though it can also reflect a payer processing error on a claim that was actually filed on time | Appeal only with proof of timely submission; otherwise a deliberate write-off |
| CO-31 | Patient cannot be identified as our insured | Eligibility — wrong payer, wrong member ID, or a plan the patient was never enrolled in | Verify current coverage and rebill the correct payer with correct identifiers |
| CO-50 | Non-covered services because this is not deemed a "medical necessity" by the payer | Medical necessity — the diagnosis on the claim doesn't meet the payer's coverage policy for the billed service | Appeal with clinical documentation that meets the specific coverage policy criteria, or confirm the denial is correct |
| CO-97 | Payment adjusted because this service/procedure is not paid separately (bundled service) | Bundling — an NCCI edit pair or payer bundling rule folded this service into another billed the same day | Withdraw, or unbundle with the correct modifier and documentation supporting a distinct service |
Two things are worth noticing across that table. First, none of these codes tells you the fix by itself — each one narrows the investigation to a specific category, and the actual resolution still depends on the details of the individual claim. Second, several of these look similar on the surface but point to genuinely different problems: CO-27 and CO-31 are both eligibility codes, but CO-27 means coverage existed and ended, while CO-31 means the payer never recognized this patient as covered at all — a distinction that changes which payer to rebill and how the conversation with the patient goes if it comes to that.
Beyond this list, treat any CARC or RARC code you haven't personally confirmed as unknown rather than guessing from a similar-sounding code you do know. Codes get added, retired, and occasionally reused for a different purpose over time, and X12's published list is the only source that reflects the current, active definition.
Why "the payer is wrong" is the wrong default
The most expensive habit in denial management isn't missing a deadline — it's treating every denial as an adversarial event that needs a rebuttal, without first reading what the payer actually said. A meaningful share of denials are correctly applied: a genuine duplicate submission, a service the payer's coverage policy simply doesn't include, a claim sent to a payer the patient no longer has. Appealing those isn't persistence, it's wasted effort, because there's no path to reversal on a claim that was denied correctly on the facts — and every hour spent building that appeal is an hour not spent on a denial elsewhere in the queue that actually had a chance.
The alternative habit is cheap by comparison: read the CARC, and the RARC if one is present, before deciding anything. A CO-97 tells a biller this is a bundling question worth checking against the NCCI edit pairs, not a fight worth escalating on principle. A CO-16 tells them to look for the specific missing element, usually named in the accompanying remark code, rather than guessing at what might be wrong with the claim. A CO-50 tells them this is a medical-necessity question that needs the chart pulled and checked against the payer's actual coverage policy, not a form letter insisting the service was necessary without citing anything the payer can act on.
Reading the code first also protects the denials that are genuinely worth fighting. A team that reflexively appeals everything spreads its credibility thin with a payer's appeals reviewers over time, and a team that reflexively writes everything off leaves recoverable revenue on the table. A team that reads the code, sorts by category, and responds differently to each one builds a track record of appeals that are worth a reviewer's attention — which, over enough volume, tends to get better and faster outcomes than either extreme.
Not sure your team is reading denials correctly?
We categorize every denial by its actual CARC and RARC code before working it, so effort goes to claims with a real path to recovery.
Frequently asked questions
What's the difference between a CARC and a RARC code?
A CARC (Claim Adjustment Reason Code) states the category of a payment adjustment or denial — why the amount paid differs from the amount billed. A RARC (Remittance Advice Remark Code) often appears alongside it on the same remittance line to add detail the CARC alone doesn't capture, such as which specific element was missing. Read both together; the CARC tells you the category, the RARC frequently tells you the specific fix.
Where do I find the full, current list of CARC and RARC codes?
X12, the organization that maintains the electronic transaction standards behind the 835 remittance, publishes and maintains the authoritative code list at x12.org/codes/claim-adjustment-reason-codes. New codes are added and existing ones are periodically deactivated, so a code list saved from a few years ago can already be out of date on the less common codes.
Why shouldn't I just assume the payer got it wrong and appeal everything?
Because a meaningful share of denials are correctly applied — a genuine duplicate claim, a genuinely non-covered service, a claim sent after coverage actually ended. Appealing those wastes staff time on claims with no realistic path to reversal, time that a genuinely recoverable denial elsewhere in the queue needed instead. Reading the actual CARC code first tells you which category you're in before you commit the effort.
Confirm before you rely on this. Payer denial and appeal policies vary by plan and change over time. The process information on this page reflects standard industry practice as of August 2026 and is provided for general education — verify current appeal deadlines and requirements directly with the specific payer before relying on it.