10 CARC Codes Billing Companies See Most in 2026

Short answer

A CARC (Claim Adjustment Reason Code) travels on the 835 ERA and tells you why a payer adjusted, reduced, or denied a claim line. The ten codes below are the ones a billing-company analyst sees most, ranked roughly by encounter frequency. The top three across denial queues are CO-16 (lacks information or submission error), CO-97 (benefit bundled into another service), and CO-45 (charge exceeds fee schedule). The MGMA 2024 Cost and Revenue Data Report is reported to put the average initial denial rate near 11.8 percent (the underlying dataset is subscriber-gated), and the Experian Health 2025 State of Claims survey finds 41 percent of providers run denial rates of 10 percent or higher.

Each code's group code (CO, PR, OA, PI) decides the first routing action: appeal, patient bill, or contractual write-off. That routing matters more than the CARC number. Definitions are cross-referenced against the X12 CARC code list.

Sources: X12 CARC codes · FCSO Medicare CO-97 guidance · MGMA Cost and Revenue Data Report · Experian Health 2025 State of Claims

How this list is ranked

No public source ranks CARCs by raw volume; payers do not release that data. This ordering reflects the codes that recur across X12 definitions, CMS guidance, the MGMA and Experian reports above, and the denial queues Medi sees in billing-company workflows.

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

The most common denial code, and the most frustrating: it says something is wrong without naming it. The payer pairs CO-16 with a Remittance Advice Remark Code (RARC) that carries the specifics. Workflows that log the CARC and ignore the RARC make staff guess instead of fix.

• **Triggers:** missing or incorrect subscriber ID, invalid procedure or diagnosis code, NPI not on file with the payer, a required field left blank, or a duplicate-claim flag.
• **First step:** read the paired RARC before anything else. Common pairings are N382 (incomplete ordering provider name), MA27 (incomplete subscriber information), and M115 (requested information not provided).
• **Root cause:** upstream intake (eligibility, charge capture, claim build) skipped a 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.

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

The billed service is considered bundled into a service already paid. This is the primary code the CMS National Correct Coding Initiative (NCCI) uses to enforce Procedure-to-Procedure (PTP) edit pairs. Per FCSO Medicare guidance, a code with a "B" status indicator on the Medicare Physician Fee Schedule cannot be billed to Medicare separately.

• **Triggers:** a minor procedure billed alongside an E&M on the same day without modifier 25, two services NCCI treats as mutually exclusive, or an assistant-surgeon charge the procedure indicator does not allow.
• **First step:** look up the CPT pair in the NCCI edit tables to confirm the edit applies and whether a modifier overrides it. Common resolving modifiers are 25 (separate E&M), 59 (distinct procedural service), and XS/XE/XP/XU. Resubmit corrected lines only; resubmitting the whole claim triggers a duplicate denial.
• **Root cause:** scrubbing that does not load current NCCI edits, or both the comprehensive and component service billed without flagging the distinction.

See NCCI modifier context in the glossary.

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 is that the charge exceeds the fee schedule, maximum allowable, or contracted/legislated arrangement. For Medicare participating providers, CO-45 is the mandatory write-off between billed charge and Medicare-allowed amount. The patient cannot be billed the difference.

• **Triggers:** the billed charge is higher than the payer's contracted rate or published fee schedule. This is expected, not an error.
• **First step:** verify the expected allowed amount against the contracted rate. If paid matches contract, adjust it off. If paid is below the contracted rate, that is an underpayment to investigate, not a write-off.
• **Root cause:** billing charges above contracted rates is standard practice, so CO-45 appears on nearly every 835. Escalate only when the payer pays below the signed contract.

See ERA posting workflow.

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

The payer determined the service does not meet its medical-necessity criteria, usually tied to a National Coverage Determination (NCD) or Local Coverage Determination (LCD). One of the most heavily appealed codes, because the determination often conflicts with what the clinician documented.

• **Triggers:** a diagnosis that does not satisfy coverage criteria for the procedure, a missing modifier that would have qualified the service, or a service the LCD treats as investigational or non-covered for the indication.
• **First step:** pull the applicable LCD or NCD and compare claim diagnoses against covered indications. If documentation supports a covered diagnosis that was omitted, a corrected claim resolves it. If documentation genuinely supports necessity, appeal with progress notes, prior conservative care, and imaging or lab results.
• **Root cause:** the provider did not document the specific indicators the LCD requires, even though the service was appropriate. A documentation problem, not a clinical one.

See denial appeal workflow.

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

The coordination of benefits (COB) denial. The payer believes another insurer is primary, or it needs the primary's payment shown before the secondary will process.

• **Triggers:** a patient with multiple active plans where the secondary received the claim without primary payment data, a payer whose records show other coverage the claim did not carry, or an incorrect payer sequence on a workers' comp or auto-liability claim.
• **First step:** verify payer order by checking eligibility on the date of service; run a 270/271 against both payers. If the claim went to the wrong payer first, resubmit in correct sequence with primary ERA data. If COB changed (e.g., Medicare became primary by age), update the order and resubmit.
• **Root cause:** COB not updated at registration, or stale payer records from a prior enrollment. Rarely a coding issue.

See COB context in the glossary and eligibility verification guidance.

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

A hard-deadline denial: the claim arrived after the timely-filing window closed. Medicare Part B requires claims within 12 months of the date of service (42 CFR 424.44). Most commercial payers require 90 to 180 days, though contract terms vary.

• **Triggers:** submission that missed the deadline, whether from intake delay, an unnoticed clearinghouse rejection, or a returned claim never resubmitted.
• **First step:** pull submission history and confirm the original claim landed within the window. The 277CA acknowledgment date is generally what counts, not the date the billing system sent it. If you have proof of timely submission, appeal with the 277CA or clearinghouse confirmation attached.
• **Root cause:** most valid-claim CO-29s trace to a rejected claim that fell out of a queue without a resubmission. The claim was on time, the rejection was missed, and the window closed.

See the denial management workflow guide for the rejection-versus-denial distinction.

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

The service required prior authorization that was not in place when it was rendered. One of the most preventable denials; authorization belongs upstream, at scheduling or intake.

• **Triggers:** service rendered without the required authorization, an authorization for a different code or date than what was billed, or an authorization that lapsed before the service date.
• **First step:** check for a retro-authorization pathway. Some payers grant it for certain service types (especially clinical emergencies or payer-system failures); windows are short, often 72 hours to 30 days from denial. If retro-auth is unavailable, appeal documenting clinical necessity and why auth was not obtained in advance.
• **Root cause:** authorization lookup at scheduling that misses payer-specific, code-level requirements. A payer may authorize a CPT range but require separate auth for individual codes.

See prior auth context in the FAQ and the billing operations overview.

8. CO-11 — Diagnosis inconsistent with procedure

The diagnosis code does not support the procedure billed. A coding-linkage issue: the submitted ICD-10 codes do not satisfy the clinical rationale the payer expects for that CPT.

• **Triggers:** a too-nonspecific diagnosis, a laterality mismatch (diagnosis left, procedure right), a mental-health procedure linked to a general medical diagnosis, or a procedure that requires a specific chronic or acute diagnosis not on the claim.
• **First step:** pull documentation and claim together and confirm the most specific accurate diagnosis is present. Check whether the procedure requires a specific diagnosis range under the LCD or NCCI. If documentation supports a more specific code, submit a corrected claim. If it does not support the procedure, escalate to clinical before proceeding.
• **Root cause:** EHR auto-population pulling a problem-list diagnosis instead of the encounter-specific one, or documentation not updated before billing. High corrected-claim resolution rate when 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, CO-15 means a number was submitted but the payer cannot validate it: wrong, belongs to a different patient or service, or expired before the date of service.

• **Triggers:** a transposed or mistyped auth number, a number issued for a different CPT, a number obtained for one date but applied to a later one, or a payer-system mismatch.
• **First step:** pull the original authorization from the payer portal or phone log and compare it character by character against the claim. Auth numbers run 10 to 15 characters and are easy to mis-enter. If the number is correct and still rejects, call provider services; payer systems sometimes fail to link an auth to the claim.
• **Root cause:** manual auth-number entry at claim build with no validation. Clean fix: store the number against the encounter when obtained and pull it automatically, rather than retyping at submission.

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

The receiving payer does not cover the patient for the service billed: the patient was not enrolled on the date of service, the plan does not cover the location or service type, or the claim went to the wrong payer.

• **Triggers:** claim sent to the wrong payer (e.g., traditional Medicare when the patient had Medicare Advantage), a plan terminated or changed between scheduling and service, or a Medicare Part B jurisdictional mismatch (wrong MAC for the provider's location).
• **First step:** run a 270/271 for the exact date of service against the denying payer. If coverage was inactive, identify the correct payer from the insurance card or registration and resubmit. Medicare Advantage claims go to the MA plan, not traditional Medicare. If coverage genuinely lapsed, verify before moving the balance to the patient.
• **Root cause:** payer ID or plan selection captured at registration that does not match actual coverage on the date of service. Eligibility verification at scheduling and again at check-in is the control. CO-109 and CO-22 are the two codes most tied to front-desk registration accuracy.

See eligibility workflow guidance.

What these 10 codes have in common

• **Six are preventable front-end problems.** CO-16, CO-22, CO-29, CO-15, CO-109, and CO-197 are fixed upstream through eligibility verification, authorization capture, timely submission, and accurate registration. They are process problems, not coding problems.
• **Three need coding knowledge.** CO-97, CO-45, and CO-11 require a coder or the clinical team to investigate before a corrected claim goes out.
• **One is a clinical-determination call.** CO-50 turns on the payer's medical-necessity criteria. It resolves either by adding a covered diagnosis the LCD or NCD requires or by appealing with the clinical record, so it sits with the clinician, not the front desk or the coder.
• **All ten carry a CO group code.** The adjustment is contractual or payer-initiated, so none default to a patient bill. The PR series routes to the patient (PR-1 deductible, PR-2 coinsurance, PR-3 copay). Billing a patient for a CO denial, or appealing a PR code, is one of the most common operational errors.

Resolution is fastest when the CARC and its paired RARC are read together with the claim detail, eligibility history, and authorization record. Most of the codes above cannot be worked from the CARC number alone; the RARC, the contract, or the coverage policy supplies the actual reason.

Where each code routes in a billing workflow

Group code drives the first routing decision. CO stays in the billing company's work queue, PR moves to patient statements, OA requires payer-specific investigation first.

Frequently asked questions

What is a CARC code and where does it appear?

A CARC (Claim Adjustment Reason Code) is a standardized code on the 835 ERA, the electronic remittance advice a payer sends when it processes a claim. It explains why the payer adjusted, reduced, or denied a claim line. CARCs are maintained by X12, the body that governs HIPAA transaction formats. Every CARC pairs with a group code (CO, PR, OA, PI) that sets who is financially responsible. See the medical billing glossary for 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. CO-15 means an authorization number was submitted but the payer cannot validate it: wrong, mismatched, or expired. CO-197 requires a retro-authorization request or an appeal documenting why auth was unavailable. CO-15 requires verifying the 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. Investigation is warranted only when paid is less than the signed contract specifies for that code. That is an underpayment, not a CO-45 adjustment, and belongs in an underpayment-detection workflow. See ERA posting workflow.

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 one that appears present. Common culprits: an NPI not yet enrolled with that payer, a subscriber ID in a format the payer rejects, or a diagnosis valid at time of service but superseded before the claim processed. The paired RARC points at the specific gap. See denial investigation workflow.

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 varies by state, typically 90 to 365 days. Most commercial payers require 90 to 180 days, though contract terms govern. Medicare Advantage plans follow their own policies, often mirroring Medicare but sometimes shorter. The clock runs from the date of service, not the date a clean claim finally goes out, so submit within 30 days and work rejections immediately.

How does CO-109 differ from CO-22?

CO-22 means the payer thinks another insurer is primary: a COB sequencing problem fixed by resubmitting in the correct order. CO-109 means the payer does not cover the patient at all for that service, date, and location: a routing or eligibility problem. Both start with an eligibility check, but CO-22 usually resolves by resubmitting to the same payer with primary payment data, while CO-109 resolves by identifying the correct payer and submitting there.

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 filters by CARC, group code, payer, practice, age, and dollar amount. To see it for a billing-company workflow, request a demo or read the billing company operations overview.