The Most Common Medical Billing Denial Codes (CARC/RARC) and How to Fix Them

By Clayton Severson, Principal Consultant · Published July 6, 2026

The denial codes you will see most are CO-16 (missing or invalid information), CO-197 (no prior authorization), CO-11 (diagnosis inconsistent with the procedure), CO-50 (not medically necessary), and CO-29 (timely filing expired). Most are preventable before the claim ever goes out — the durable fix is upstream at registration, eligibility, and coding, not in the appeal.

What are CARC and RARC codes?

When a payer pays a claim for less than the billed amount — or nothing at all — it explains the adjustment with standardized codes on the electronic remittance (the 835) and the explanation of benefits. Two code sets do the work, and they travel together.

The codes are maintained nationally and updated a few times a year, so the exact list shifts at the edges — but the denials that drive the bulk of lost revenue are remarkably stable from payer to payer.

What are the most common denial codes?

The table below lists the denial codes that recur across specialties and payers, the root cause behind each, and where in the revenue cycle it actually gets fixed. All are CO (contractual obligation) group codes — the denials a practice can act on.

Code	What it means	Typical root cause	Prevent / fix at
CO-16	Claim lacks information or has a submission error	Missing or invalid field — member ID, NPI, diagnosis (a RARC names which)	Registration & claim scrubber
CO-197	Precertification / authorization absent	Service performed without the required prior auth	Pre-service authorization
CO-11	Diagnosis is inconsistent with the procedure	Dx-to-CPT linkage error or wrong code	Coding
CO-50	Not deemed a medical necessity by the payer	Diagnosis does not support the service under payer policy	Coding & documentation
CO-29	The time limit for filing has expired	Claim submitted past the payer's filing deadline	Billing workflow & A/R follow-up
CO-18	Exact duplicate claim or service	Resubmission or a system double-send	Billing system / clearinghouse
CO-97	Benefit is included in another, already-adjudicated service	Bundling / NCCI edit / missing modifier	Coding
CO-22	May be covered by another payer (coordination of benefits)	Primary vs. secondary not verified	Eligibility & COB check
CO-109	Not covered by this payer — send to the correct one	Claim routed to the wrong payer or plan	Eligibility & registration
CO-27	Expenses incurred after coverage terminated	Eligibility not re-checked at the date of service	Eligibility verification
CO-4	Procedure inconsistent with the modifier, or a required modifier is missing	Modifier error or omission	Coding
CO-B7	Provider not eligible / certified to be paid for this service on this date	Enrollment or credentialing gap with the payer	Provider enrollment & credentialing

Two codes people often file under "denials" do not belong here. CO-45 (charge exceeds the fee schedule or contracted amount) is the single most frequent adjustment on most remittances, but it is a contractual write-off, not a denial — the difference between your billed charge and the negotiated rate. And PR-1, PR-2, and PR-3 (deductible, coinsurance, copay) are patient-responsibility balances, not payer denials. Chasing those as if they were denials is one of the quiet ways a denial team loses hours it does not have.

Official CARC definitions, verbatim

The table above is our operational read. This one is the source text: the official X12 definition of each Claim Adjustment Reason Code, quoted verbatim from the national list at x12.org. Aggregator sites routinely paraphrase these and drift; where a definition here differs from what your vendor's software displays, the text below is the standard. Several official definitions close with a "Usage: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present" instruction, trimmed here for readability — it points you to the payer policy reference carried on the remittance, and says nothing about the denial itself.

CARC	Official X12 definition
CO-4	The procedure code is inconsistent with the modifier used.
CO-11	The diagnosis is inconsistent with the procedure.
CO-16	Claim/service lacks information or has submission/billing error(s). Usage: Do not use this code for claims attachment(s)/other documentation. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT.)
CO-18	Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO)
CO-22	This care may be covered by another payer per coordination of benefits.
CO-27	Expenses incurred after coverage terminated.
CO-29	The time limit for filing has expired.
CO-45	Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement. Usage: This adjustment amount cannot equal the total service or claim charge amount; and must not duplicate provider adjustment amounts (payments and contractual reductions) that have resulted from prior payer(s) adjudication. (Use only with Group Codes PR or CO depending upon liability)
47	This (these) diagnosis(es) is (are) not covered, missing, or are invalid. Deactivated by X12 effective 02/01/2006 — if a payer still returns it, that is legacy output, not a current code.
CO-50	These are non-covered services because this is not deemed a 'medical necessity' by the payer.
CO-55	Procedure/treatment/drug is deemed experimental/investigational by the payer.
CO-96	Non-covered charge(s). At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT.)
CO-97	The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated.
CO-107	The related or qualifying claim/service was not identified on this claim.
CO-109	Claim/service not covered by this payer/contractor. You must send the claim/service to the correct payer/contractor.
CO-119	Benefit maximum for this time period or occurrence has been reached.
CO-193	Original payment decision is being maintained. Upon review, it was determined that this claim was processed properly.
CO-197	Precertification/authorization/notification/pre-treatment absent.
CO-204	This service/equipment/drug is not covered under the patient's current benefit plan
CO-251	The attachment/other documentation that was received was incomplete or deficient. The necessary information is still needed to process the claim. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT).
CO-252	An attachment/other documentation is required to adjudicate this claim/service. At least one Remark Code must be provided (may be comprised of either the NCPDP Reject Reason Code, or Remittance Advice Remark Code that is not an ALERT).
CO-253	Sequestration - reduction in federal payment
CO-B7	This provider was not certified/eligible to be paid for this procedure/service on this date of service.
CO-B15	This service/procedure requires that a qualifying service/procedure be received and covered. The qualifying other service/procedure has not been received/adjudicated.

A note on the group code. The prefix is not part of the CARC — it is a separate group code that assigns financial liability: CO (contractual obligation: write it off, do not bill the patient), PR (patient responsibility), OA (other adjustment), and PI (payer-initiated reduction). The same reason code can arrive under different group codes and mean very different things for your A/R, which is why "denial code 197" and "CO-197" are the same reason code while a PR-197 would move the balance to the patient. X12 constrains the valid pairings for some codes — CO-18 above is defined for use with OA except where state workers' compensation rules require CO.

How do you read a denial code on an 835 or EOB?

Read it in three parts: group code, CARC, then RARC. A line that reads CO-16 with remark M76 means a contractual denial for missing information (CO-16), and the RARC (M76) specifies the culprit — a missing, incomplete, or invalid diagnosis. The CARC tells you the category; the RARC tells you exactly what to fix. A denial without a useful RARC is a signal to call the payer rather than guess, because resubmitting blind usually earns the same denial a second time.

Common RARC codes and what they mean

RARCs are the supplemental codes that tell you which field a payer objected to. They are maintained by X12, the standards body that publishes the national list, and Medicare uses that same list. The definitions below are the official X12 text, verbatim; the two right-hand columns are our read on what each one usually means in a real billing operation and where it gets fixed.

RARC	Official X12 definition	What it usually means in practice	Fix at
M76	Missing/incomplete/invalid diagnosis or condition.	The diagnosis field is blank, truncated, or not a valid ICD-10 code for the date of service	Coding & claim scrubber
M51	Missing/incomplete/invalid procedure code(s).	The CPT/HCPCS code is absent, deleted for that year, or invalid for the place of service	Coding
M79	Missing/incomplete/invalid charge.	A line-level charge amount is missing or zero	Billing system / charge entry
MA27	Missing/incomplete/invalid entitlement number or name shown on the claim.	The member ID or the patient name does not match the payer's record — a very common CO-16 companion	Registration & eligibility
N382	Missing/incomplete/invalid patient identifier.	Same family as MA27 — the identifier itself is unusable	Registration & eligibility
MA112	Missing/incomplete/invalid group practice information.	Group NPI or tax ID missing or not matched to the rendering provider	Provider enrollment & billing setup
MA130	Your claim contains incomplete and/or invalid information, and no appeal rights are afforded because the claim is unprocessable. Please submit a new claim with the complete/correct information.	The important one: this claim was never adjudicated, so there is nothing to appeal — correct and resubmit	Claim scrubber, before submission
N26	Missing itemized bill/statement.	Payer wants the itemized detail before it will process	Billing & medical records
N130	Consult plan benefit documents/guidelines for information about restrictions for this service.	A benefit or policy limitation applies — check the plan before appealing	Eligibility & benefits verification
N19	Procedure code incidental to primary procedure.	Bundling — the line was folded into the primary procedure; usually arrives with CO-97	Coding (modifier / NCCI review)
N56	Procedure code billed is not correct/valid for the services billed or the date of service billed.	The CPT/HCPCS is not valid for that date of service — often a code deleted or replaced in the annual update	Coding & annual code-set maintenance
N95	This provider type/provider specialty may not bill this service.	A scope-of-billing restriction, not a documentation problem — the service is not payable to this specialty	Provider enrollment & charge routing
N115	This decision was based on a Local Coverage Determination (LCD). An LCD provides a guide to assist in determining whether a particular item or service is covered.	Read the LCD before appealing — if the documentation does not meet it, the appeal fails on the same grounds	Coding & documentation
N152	Missing/incomplete/invalid replacement claim information.	A corrected claim went out without the original claim reference (frequency code / payer claim control number)	Billing system — corrected-claim setup
N408	This payer does not cover deductibles assessed by a previous payer.	Secondary-payer COB issue — the primary's deductible is not this payer's liability	Coordination of benefits
N702	Decision based on review of previously adjudicated claims or for claims in process for the same/similar type of services.	A history-based decision — the payer looked at other claims, so the answer usually lies elsewhere in the claim history, not on this line	A/R follow-up & claim-history review
N803	Submission of the claim for the service rendered is the responsibility of the Contracted Medical Group or Hospital.	Delegated-risk arrangement — bill the medical group or IPA, not the plan	Eligibility & payer routing
N880	Original claim closed due to changes in submitted data. Adjustment claim will be processed under a new claim number.	Informational — track the new claim number or the payment will look like a missing remit	A/R follow-up
N830	Alert: The charge[s] for this service was processed in accordance with Federal/ State, Balance Billing/ No Surprise Billing regulations. As such, any amount identified with OA, CO, or PI cannot be collected from the member and may be considered provider liability or be billable to a subsequent payer. Any amount the provider collected over the identified PR amount must be refunded to the patient within applicable Federal/State timeframes. Payment amounts are eligible for dispute pursuant to any Federal/State documented appeal/grievance process(es).	No Surprises Act / balance-billing protection — do not bill the patient beyond the PR amount, and refund any overcollection within the applicable timeframe; disputes run through the federal or state IDR process	Patient billing & compliance
MA67	Alert: Correction to a prior claim.	Informational only — an adjustment to something already processed, not a new denial	No action; reconcile the adjustment
M15	Separately billed services/tests have been bundled as they are considered components of the same procedure. Separate payment is not allowed.	Bundling — the line was folded into a procedure already paid; check whether a modifier legitimately applies before appealing	Coding (bundling & modifier review)
M47	Missing/incomplete/invalid Payer Claim Control Number. Other terms exist for this element including, but not limited to, Internal Control Number (ICN), Claim Control Number (CCN), Document Control Number (DCN).	A corrected or voided claim went out without the original payer claim number — the same failure as N152, named from the payer's side	Billing system — corrected-claim setup
M67	Missing/incomplete/invalid other procedure code(s).	A secondary or additional procedure code is absent or invalid; the primary line often still pays	Coding
MA01	Alert: If you do not agree with what we approved for these services, you may appeal our decision. To make sure that we are fair to you, we require another individual that did not process your initial claim to conduct the appeal. However, in order to be eligible for an appeal, you must write to us within 120 days of the date you received this notice, unless you have a good reason for being late.	An appeal-rights notice, not a denial reason — but it starts a 120-day clock, so date-stamp the remit when it lands	A/R follow-up — calendar the appeal deadline
N48	Claim information does not agree with information received from other insurance carrier.	Coordination-of-benefits mismatch — what you submitted does not reconcile against the primary payer's remit	Coordination of benefits
N59	Alert: Please refer to your provider manual for additional program and provider information.	A pointer, not a reason — it never travels alone, so work the CARC and the other RARCs on the line	No action on its own
N122	Add-on code cannot be billed by itself.	The add-on CPT was submitted without its required primary procedure — either the primary was omitted or it was itself denied	Coding & charge entry
N197	The subscriber must update insurance information directly with payer.	The payer's own member record is stale — the patient, not the billing office, has to correct it	Patient outreach & eligibility
N307	Missing/incomplete/invalid adjudication or payment date.	Usually a secondary claim submitted without the primary payer's adjudication date	Coordination of benefits / secondary billing
N362	The number of Days or Units of Service exceeds our acceptable maximum.	A units cap — the quantity billed exceeds the payer's maximum for that code; confirm it is not a keying error before appealing	Charge entry & coding
N448	This drug/service/supply is not included in the fee schedule or contracted/legislated fee arrangement.	Not a documentation problem — the item is simply off the contracted fee schedule, which makes it a contracting question rather than an appeal	Payer contracting
N517	Resubmit a new claim with the requested information.	An explicit instruction to resubmit rather than appeal — a frequent MA130 companion	Claim scrubber, before resubmission
N822	Missing procedure modifier(s).	A required modifier was omitted from the line	Coding
N823	Incomplete/Invalid procedure modifier(s).	A modifier is present but is wrong for the code, the payer, or the date of service	Coding

MA130 deserves special attention because it is routinely worked incorrectly. It states outright that no appeal rights attach — the claim was rejected as unprocessable rather than denied on the merits. Filing an appeal against an MA130 accomplishes nothing except burning days off the timely-filing clock (CO-29). The correct action is always to fix the flagged field and submit a new claim.

Which denials are preventable?

Most of them. Look back at the "prevent / fix at" column: registration, eligibility, authorization, and coding account for nearly every high-volume denial in the table. Those are all front-end steps that happen before the claim is submitted. Genuinely clinical denials — a true medical-necessity dispute that needs a physician's documentation — are the minority. That is the single most useful thing to internalize about denials: the codes look like a back-office billing problem, but the causes are overwhelmingly upstream, at the front desk and in the coding queue.

How do you fix and prevent the most common denials?

Prevention and rework are two different disciplines, and a healthy revenue cycle runs both:

• Verify eligibility and benefits at scheduling — real-time eligibility catches terminated coverage (CO-27), wrong-payer routing (CO-109), and COB issues (CO-22) before the visit, not after the denial.
• Automate prior authorization — flag services that require auth at booking so CO-197 never happens; it is close to 100% preventable and rarely appealable after the fact.
• Scrub claims before submission — an edit engine that checks required fields, NPIs, modifiers, and NCCI conflicts kills most CO-16, CO-4, and CO-97 denials at the door.
• Track timely-filing clocks — a worklist sorted by days-to-deadline stops CO-29, which is the most infuriating denial because the money was earned and simply left on the table.
• Work denials by reason code, not by claim — a queue grouped by CARC lets you fix the pattern (and the upstream cause) instead of re-solving the same denial one claim at a time.
• Close the loop to the root cause — every worked denial should feed a trend so a recurring CO-197 from one payer becomes an authorization rule, not a permanent tax on your A/R.

That last two points are where denial management with AI earns its keep: classifying raw payer codes into workable buckets, routing each to the right owner, and surfacing the reason-code trends a human staring at one claim at a time will never see.

Where denial management actually breaks down

In the billing operations I have worked in, the denials themselves are rarely the hard part — the workflow around them is. Two failures recur. The first is that denials get worked reactively, one claim at a time, so the same CO-16 or CO-197 shows up every week and nobody ever fixes the upstream gap that produces it; the queue never shrinks because the faucet is still running. The second is classification: payers return the same underlying problem under a thicket of slightly different codes and free-text remarks, and without a crosswalk that collapses them into a handful of workable categories, the team cannot even see which denials are worth chasing. Fixing those two things — pattern over one-off, and a clean reason-code taxonomy over raw payer output — usually recovers more revenue than any individual appeal ever will.

Frequently asked questions