10 CARC Codes Billing Companies See Most in 2026

Short answer

The three CARC codes that dominate denial queues across billing companies are CO-16 (claim lacks information or has a submission error), CO-97 (benefit bundled into another service), and CO-45 (charge exceeds fee schedule). Together with CO-50, CO-22, and CO-197, these six codes account for the large majority of actionable denial volume that billing staff work every day. The MGMA 2024 Cost and Revenue Report puts the average initial claim denial rate at 11.8 percent, and the Experian Health 2025 State of Claims survey finds that 41 percent of providers experience denial rates of 10 percent or higher. The ten codes below are the ones a billing company analyst will see most on the 835 ERA, ranked roughly by encounter frequency and cross-referenced against X12 CARC code definitions.

How this list was built

This ranking synthesizes four sources: the official X12 CARC code list, FCSO Medicare CO-97 bundling guidance, the MGMA 2024 Cost and Revenue Report, and the Experian Health 2025 State of Claims survey. No single public source ranks all CARC codes by raw volume — payers do not publish that data. The ordering here reflects the codes that show up most consistently across industry summaries, CMS guidance, and the denial queues Medi sees in billing-company workflows. Each code carries a group code (CO, PR, OA, PI) that determines whether the right next action is appeal, patient bill, or contractual write-off. That group-code routing matters more than the CARC number itself.

1. CO-16 — Claim or service lacks information or has submission errors

CO-16 is the most common denial code billing companies encounter, and it is also the most frustrating because it tells you something is wrong without naming what. The payer pairs CO-16 with a Remittance Advice Remark Code (RARC) that carries the actual specifics, but many billing workflows log the CARC and ignore the RARC, which means staff spend hours guessing instead of fixing.

What triggers it: missing or incorrect subscriber ID, invalid procedure or diagnosis code, NPI not on file with the payer, required field left blank on the claim, or a duplicate claim flag where the payer's system treated the submission as already received.

First investigation step: read the paired RARC before doing anything else. The RARC is not optional context — it is the actual answer. Common RARCs with CO-16 include N382 (missing or incomplete ordering provider name), MA27 (missing or incomplete subscriber information), and M115 (the requested information was not provided). Once you know the specific gap, the correction is usually straightforward.

Common root cause: the upstream intake process — eligibility check, charge capture, or claim building — skipped a required field, or the payer's NPI file is stale. CO-16 is often a front-end problem presenting as a denial. See denial workflow investigation guidance for the full investigation stack.

2. CO-97 — Benefit included in payment for another service

CO-97 tells you the billed service is considered bundled into another service or procedure that was already paid. It is the primary code the CMS National Correct Coding Initiative (NCCI) uses to enforce its Procedure-to-Procedure (PTP) edit pairs. Per FCSO Medicare CO-97 guidance, if the procedure code has a "B" status indicator on the Medicare Physician Fee Schedule database, it should not be billed to Medicare separately.

What triggers it: billing a minor procedure alongside an E&M on the same day without modifier 25, billing two services that NCCI considers mutually exclusive, or billing an assistant-surgeon charge when the procedure's indicator does not allow it.

First investigation step: look up the CPT pair in the CMS NCCI edit tables to confirm whether the edit applies, and whether a modifier can override it. The most common modifiers that resolve CO-97 are 25 (separate E&M), 59 (distinct procedural service), and XS, XE, XP, XU for more specific separation. FCSO also notes that resubmitting the entire claim causes a duplicate denial — submit corrected lines only.

Common root cause: claim-scrubbing that does not load current NCCI edits, or clinical staff billing both the comprehensive and component service without flagging the distinction. See NCCI modifier context in the billing glossary for modifier 59 and the X-modifier family.

3. CO-45 — Charge exceeds fee schedule or maximum allowable

CO-45 is almost always a contractual write-off, not an appealable denial. The X12 definition states the charge exceeds the fee schedule or maximum allowable or contracted/legislated fee arrangement. For Medicare participating providers, CO-45 represents the mandatory write-off between the billed charge and the Medicare-allowed amount. The patient cannot be billed the difference, and appealing it accomplishes nothing if the contracted rate is correctly applied.

What triggers it: the billed charge is higher than the payer's contracted rate or published fee schedule for that code. This is expected behavior, not an error, in most cases.

First investigation step: verify the expected allowed amount against the contracted rate in the fee schedule or in the payer's online provider portal. If the amount paid matches the contracted rate, CO-45 is informational — adjust it off and move on. If the paid amount is less than what the contract specifies, that is an underpayment requiring investigation, not a contractual write-off.

Common root cause: billing charges that significantly exceed contracted rates is standard practice (it protects against renegotiation leverage), so CO-45 appears on nearly every 835. The exception worth escalating is when the payer applies a lower rate than the signed contract specifies. See ERA posting workflow for how contracted adjustments flow through the posting process.

4. CO-50 — Non-covered service not deemed medically necessary

CO-50 signals the payer determined the service does not meet their coverage criteria for medical necessity, typically tied to a National Coverage Determination (NCD) or Local Coverage Determination (LCD). It is one of the most heavily appealed codes in billing because the payer's medical necessity determination often conflicts with what the treating clinician documented.

What triggers it: a diagnosis code that does not satisfy the payer's coverage criteria for the procedure, a missing modifier that would have qualified the service, or a service type the payer's LCD categorizes as investigational or non-covered for the given indication.

First investigation step: pull the applicable LCD or NCD for the procedure code and compare the diagnosis codes on the claim against the covered indications. If the documentation supports a covered diagnosis that was not included on the claim, a corrected claim resolves it. If the clinical documentation genuinely supports medical necessity and the diagnosis was correct, the path is a medical necessity appeal with progress notes, prior conservative care documentation, and imaging or lab results.

Common root cause: the ordering or treating provider did not document the specific clinical indicators the payer's LCD requires, even though the service was clinically appropriate. This is a documentation problem, not a clinical one. See denial appeal workflow for packet-building guidance.

5. CO-22 — This care may be covered by another payer

CO-22 is the coordination of benefits (COB) denial. The payer is saying it believes another insurer is primary and the claim should have been billed there first, or that the claim needs to show what the primary insurer paid before the secondary will process it.

What triggers it: a patient with multiple active insurance plans where the secondary insurer received the claim without primary insurance payment information, a payer whose records show the patient has other coverage but the claim did not carry COB data, or an incorrect payer sequence on a workers' compensation or auto-liability claim.

First investigation step: verify the correct payer order by checking eligibility on the date of service — run a 270/271 transaction against both payers to confirm who is primary. If the claim went to the wrong payer first, resubmit in the correct sequence with primary ERA data attached. If the patient's COB has changed (e.g., Medicare became primary due to age, not employer plan), update the payer order and resubmit.

Common root cause: COB information not updated at registration, or the payer's internal records are stale from a prior enrollment period. CO-22 is rarely a coding issue — it is almost always a registration or eligibility workflow issue. See COB and eligibility context in the glossary and eligibility verification guidance.

6. CO-29 — The time limit for filing has expired

CO-29 is a hard deadline denial. The payer is saying the claim arrived after their timely filing window closed. Medicare Part B requires claims within 12 months of the date of service under 42 CFR 424.44. Most commercial payers require 90 to 180 days from date of service, though contract terms vary.

What triggers it: claim submission that missed the payer's timely filing deadline, whether due to intake delay, clearinghouse rejection that went unnoticed, or a returned claim that was not resubmitted.

First investigation step: pull submission history and confirm whether the original claim was submitted within the filing window. The date the clearinghouse accepted the claim — the 277CA acknowledgment date — is generally the date that counts for timely filing purposes, not the date the billing system sent it. If the original submission was within the window and you have proof, appeal with the 277CA acknowledgment or clearinghouse confirmation report attached.

Common root cause: most CO-29 denials that billing companies receive on valid claims trace back to a rejected claim that fell out of a queue without triggering a re-submission workflow. The claim was submitted on time, but the rejection was not caught, and by the time the denial came back, the window had closed. See the denial management workflow guide for rejection versus denial distinction.

7. CO-197 — Precertification or authorization was not obtained

CO-197 means the service required prior authorization that was not in place when the service was rendered. It is one of the most preventable denials in a billing company's queue — authorization responsibility belongs upstream of the claim, at scheduling or intake.

What triggers it: service rendered without obtaining the authorization the payer required for that CPT code, an authorization obtained for a different procedure code or service date than what was ultimately billed, or an authorization that lapsed before the service date.

First investigation step: check whether the payer offers a retro-authorization pathway. Some payers will grant retroactive authorization for certain service types (especially when there was a clinical emergency or a payer-system failure). If retro-auth is available, request it immediately because these windows are short — often 72 hours to 30 days from the denial. If retro-auth is not available, the appeal must document why the service was clinically necessary and why authorization was not obtained in advance.

Common root cause: authorization lookup at scheduling that does not account for payer-specific code-level requirements. A payer may authorize a CPT range but require separate authorization for individual codes within it. See prior auth context in the FAQ and the billing operations overview.

8. CO-11 — Diagnosis inconsistent with procedure

CO-11 signals the payer's system found the diagnosis code on the claim does not support the procedure code billed. This is a coding linkage issue — the ICD-10 code(s) submitted do not satisfy the coverage or clinical rationale the payer expects for that CPT.

What triggers it: a diagnosis code that is too nonspecific, a laterality mismatch (e.g., the diagnosis specifies left but the procedure specifies right), a mental health procedure linked to a general medical diagnosis, or a procedure that requires a specific chronic or acute diagnosis not present on the claim.

First investigation step: pull the patient's clinical documentation and the claim together and verify that the most specific, accurate diagnosis code supporting the procedure is on the claim. Check whether the procedure code requires a specific diagnosis code range under the payer's LCD or NCCI guidance. If the documentation supports a more specific diagnosis, submit a corrected claim. If the documentation does not support the procedure, escalate to the clinical team before proceeding.

Common root cause: EHR auto-population that pulls a problem-list diagnosis rather than the encounter-specific diagnosis, or encounter documentation that was not updated before billing. CO-11 has a high corrected-claim resolution rate when the underlying documentation is solid.

9. CO-15 — Authorization number is missing, invalid, or does not apply

CO-15 is distinct from CO-197. Where CO-197 means no authorization existed at all, CO-15 means an authorization number was submitted but the payer cannot validate it — the number is wrong, it belongs to a different patient or service, or it expired before the date of service.

What triggers it: a transposed or mistyped authorization number entered during claim build, an authorization issued for a different CPT code than what was billed, an authorization that was obtained for one date of service but applied to a later service date, or a payer system mismatch where the authorization reference does not match the payer's internal record.

First investigation step: pull the original authorization confirmation from the payer's portal or the phone log, and compare it character-by-character against what appears on the submitted claim. Authorization numbers are often 10 to 15 characters and errors are easy to introduce during manual entry. If the number is correct but the payer still rejects it, call the payer's provider services line — their systems occasionally fail to link an authorization to the correct claim.

Common root cause: manual authorization number entry during claim build with no validation step. This is a workflow problem with a clean fix: authorization numbers should be stored against the patient encounter at the time they are obtained and pulled automatically into the claim, not retyped at submission.

10. CO-109 — Claim not covered by this payer or contractor

CO-109 means the payer receiving the claim does not cover the patient for the service billed. This can mean the patient was not enrolled with that payer on the date of service, the plan type does not cover the service location or service type, or the claim was routed to the wrong payer entirely.

What triggers it: claim submitted to the wrong payer (e.g., Medicare when the patient had Medicare Advantage that day), patient plan terminated or changed between scheduling and service, or a payer jurisdictional mismatch for Medicare Part B (e.g., wrong MAC for the provider's location).

First investigation step: run a 270/271 eligibility check for the patient on the exact date of service against the payer that issued the denial. If coverage was not active, identify the correct payer from the patient's insurance card or registration records and resubmit. If the patient was in a Medicare Advantage plan, the claim goes to the MA plan, not traditional Medicare. If coverage genuinely lapsed, the patient may be responsible — but verify before moving the balance.

Common root cause: payer ID or plan selection captured at registration that does not reflect the patient's actual coverage on the date of service. Eligibility verification at scheduling and again at check-in is the preventive control. CO-109 and CO-22 are the two codes most directly tied to front-desk registration accuracy. See eligibility workflow guidance for verification best practices.

What these 10 codes have in common

Looking across all ten, three patterns emerge. First, six of them (CO-16, CO-22, CO-29, CO-15, CO-109, CO-197) are directly preventable through front-end workflow controls — eligibility verification, authorization capture, timely submission, and accurate registration. They are not coding problems; they are process problems. Second, three of them (CO-97, CO-45, CO-11) require coding knowledge to investigate correctly, and they are the ones most likely to involve the clinical team or a coder before a corrected claim goes out. Third, all ten codes are resolved faster when the billing system surfaces the CARC and the paired RARC together in the denial queue, with the claim detail, eligibility history, and authorization record on one screen. When staff have to move between systems to gather that context, each denial takes four to six times longer than it should.

The other common thread: the CO group code on all ten means the adjustment is contractual or payer-initiated. None of these go to the patient as a bill by default. The PR series (PR-1 deductible, PR-2 coinsurance, PR-3 copay) is what routes to the patient statement workflow. Mixing them up — billing the patient for a CO denial, or appealing a PR code — is one of the most common operational errors in billing-company workflows.

Where each code routes in a billing workflow

Group code determines the first routing decision. CO codes stay in the billing company's work queue. PR codes move to patient statements. OA codes require payer-specific investigation before routing.

Frequently asked questions

What is a CARC code and where does it appear?

A CARC (Claim Adjustment Reason Code) is a standardized code that travels on the 835 ERA — the electronic remittance advice a payer sends when it processes a claim. The CARC explains why the payer adjusted, reduced, or denied a claim line. CARCs are maintained by X12, the standards body that governs HIPAA-mandated transaction formats. Every CARC pairs with a group code (CO, PR, OA, PI) that determines who is financially responsible for the adjustment. See the medical billing glossary for a full breakdown of CARC, RARC, and group code definitions.

What is the difference between CO-197 and CO-15?

CO-197 means no prior authorization was obtained before the service was rendered. CO-15 means an authorization number was submitted on the claim but the payer cannot validate it — the number is wrong, mismatched, or expired. Both end up in the authorization failure bucket, but the resolution paths differ. CO-197 requires a retro-authorization request or an appeal documenting why prior auth was unavailable. CO-15 requires verifying the authorization number itself and resubmitting a corrected claim with the valid number.

Can CO-45 ever be appealed?

CO-45 as a contractual write-off to the fee schedule is not appealable — the payer applied the contract correctly. The scenario where investigation is warranted is when the amount paid is less than what the signed contract specifies for that code. That is an underpayment, not a CO-45 contractual adjustment, and it belongs in an underpayment detection workflow. See ERA posting workflow for how the two are distinguished in the posting process.

Why does CO-16 appear on claims that look complete?

CO-16 fires when the payer's adjudication system cannot find or validate a required field, even if that field appears to be present on the claim. Common culprits include an NPI that is not yet enrolled with that payer, a subscriber ID with a format the payer's system does not accept, or a diagnosis code that was valid in ICD-10 at time of service but was superseded before the claim processed. The RARC paired with CO-16 is the most reliable signal — it points at the specific gap. See denial investigation workflow for how to build a CO-16 correction queue.

What is the timely filing window for the most common payers?

Medicare Part B requires claims within 12 months of the date of service (42 CFR 424.44). Medicaid timely filing varies by state — typically 90 to 365 days. Most commercial payers require 90 to 180 days from date of service, though contract terms govern. Medicare Advantage plans follow their own policies, which often mirror Medicare but can be shorter. The safest practice is submitting claims within 30 days of service and working rejected claims immediately, because the timely filing clock runs from the date of service, not the date a clean claim finally went out.

How does CO-109 differ from CO-22?

CO-22 means the payer thinks another insurer is primary — it is a COB sequencing problem, and the fix is resubmitting in the correct payer order. CO-109 means the payer receiving the claim does not cover the patient at all for that service, on that date, at that location. CO-109 is a routing or eligibility problem, not a sequencing problem. Both require an eligibility check as the first step, but CO-22 usually resolves by resubmitting to the same payer with primary insurance payment data attached, while CO-109 usually resolves by identifying the correct payer and submitting there instead.

Where can billing companies see these codes in a live workflow?

Medi presents CARC and RARC codes in the ERA review queue alongside the claim detail, eligibility snapshot, and authorization status. The denial queue is filterable by CARC, group code, payer, practice, age, and dollar amount. If you want to see how that looks for a billing-company workflow, request a demo or read the billing company operations overview.