Claim Adjustment Reason Codes (CARCs) and Remittance Advice Remark Codes (RARCs) appear on every Electronic Remittance Advice (ERA) your practice receives, and the Group Code attached to each CARC determines financial responsibility before any appeal decision is valid. Reading a CO-97 denial differently from a PR-96 is the difference between a mandatory write-off and an improper balance bill to a Medicare beneficiary. This guide covers the Group Code logic, CARC corrective workflows, NCCI interaction, and Medicare appeal mechanics for the denial codes billing staff encounter most frequently in Medicare fee-for-service.
The 835 Health Care Claim Payment/Advice transaction (version 005010X322A1) is the HIPAA-mandated format for all ERA data. CARCs appear in the CAS segment at the service line level and explain the primary reason a payer adjusted a claim from the billed amount. RARCs supplement a CARC with more specific detail or carry informational processing notes; they appear in the LQ segment (service line) or the MIA/MOA segment (claim level) of the 835.
Every CARC is paired with a Group Code that assigns financial liability. Medicare contractors use only three Group Codes: CO (Contractual Obligation, provider write-off), PR (Patient Responsibility, billable to the beneficiary), and OA (Other Adjustment). The Group Code PI (Payer Initiated) is not used by Medicare per CMS Claims Processing Manual, Chapter 22.
Alert RARCs, prefixed with "Alert:", carry informational notices only and are not tied to any payment adjustment. Both CARC and RARC code sets are maintained by ASC X12 and updated three times per year (approximately March 1, July 1, and November 1). CMS issues Change Requests after each cycle to update MREP and PC Print software at the MAC level. RARC requests are reviewed monthly; CARC requests, three times per year.
| CARC | Short Description | Group Code | First Action |
|---|---|---|---|
| 4 | Modifier inconsistency or missing modifier | CO | Review NCCI PTP tables; correct modifier placement |
| 5 | Procedure/bill type inconsistent with place of service | CO | Correct POS code; resubmit with documentation |
| 11 | Diagnosis inconsistent with procedure | CO | Verify ICD-10-CM linkage; add supporting diagnosis |
| 16 | Missing information or submission error | CO | Read accompanying RARC before any corrective action |
| 18 | Exact duplicate claim | CO | Appeal with original submission evidence |
| 22 | Coordination of benefits; another payer primary | CO or OA | Submit with primary EOB attached |
| 29 | Timely filing limit expired | CO | Assess statutory exception eligibility |
| 45 | Charge exceeds fee schedule allowable | CO | Write off contractual difference; do not bill patient |
| 50 | Non-covered service; medical necessity not established | CO | Review LCD/NCD criteria; appeal with complete record |
| 96 | Non-covered charge; Remark Code required | CO | Read RARC; assess ABN status before action |
| 97 | Service bundled into separately paid procedure | CO | Verify NCCI modifier availability; accept write-off if none |
| 167 | Diagnosis not covered | CO | Verify LCD criteria; confirm most specific ICD-10-CM code |
| 204 | Service not covered under current benefit plan | CO | Verify plan benefits; issue ABN prospectively |
| 236 | Procedure/modifier combination NCCI-incompatible | CO | Apply NCCI-associated modifier if clinically appropriate |
The Group Code check is the mandatory first step on any ERA. CO adjustments are never billable to the Medicare patient under the participation agreement. PR adjustments are billable only when the ABN requirement is satisfied. The flowchart below maps from Group Code to corrective action for the most common denial scenarios.
graph TD
A[ERA Adjustment Received] --> B{Group Code?}
B -->|PR| C[Bill patient; verify ABN on file if required]
B -->|OA| D[Internal adjustment; no patient billing]
B -->|CO| E{CARC Number?}
E -->|18 Duplicate| F[Appeal with original claim submission proof]
E -->|29 Timely Filing| G{42 CFR 424.44b exception applies?}
G -->|Yes| H[File redetermination with exception documentation]
G -->|No| I[Write-off; revise submission workflow]
E -->|16 Missing Info| J[Read RARC for specific missing element]
J --> K[Correct data element and resubmit]
E -->|97 or 236 NCCI| L{NCCI-associated modifier clinically appropriate?}
L -->|Yes| M[Resubmit with modifier and procedure documentation]
L -->|No| N[Accept CO write-off]
E -->|50 or 167 Medical Necessity| O[Review applicable LCD and NCD criteria]
O --> P{Record supports coverage?}
P -->|Yes| Q[File Level 1 Redetermination with complete medical record]
P -->|No| R[Write-off; issue ABN for future services]
E -->|45 Fee Schedule| S[Write-off contractual difference; never bill patient]
The compliance distinction here is absolute: no CO-adjusted amount may be shifted to the patient under Medicare, regardless of whether the provider believes the service was medically necessary.
X12 descriptor: "The procedure code is inconsistent with the modifier used or a required modifier is missing."
CARC 4 fires when a modifier is applied to the wrong code in an NCCI PTP pair (typically the Column 1 code instead of the Column 2 code), when an NCCI-associated modifier is absent, or when a Medically Unlikely Edit (MUE) reduces units of service. Auditors look for high-volume CARC 4 patterns as indicators of systematic modifier placement errors across a code family.
Documentation: The operative or procedure note must specify distinct clinical circumstances supporting the modifier. For MUE unit overrides, document each unit of service with date, time, and individual clinical rationale.
Modifier rule: NCCI-associated modifiers (25, 59, 91, XE, XS, XP, XU) may override a CARC 4 denial when services are clinically distinct and documentation supports it. Modifiers 22, 76, and 77 are not NCCI PTP-associated modifiers and cannot override NCCI bundling.
X12 descriptor: "The procedure code is inconsistent with the place of service."
A POS code mismatch between the documented site of service and what was billed triggers CARC 5. Common examples include billing POS 11 (office) for a service performed in an ambulatory surgery center (POS 24) or hospital outpatient department (POS 22). Verify the POS code against the medical record, correct the claim, and resubmit. The medical record must clearly reflect the actual site of service.
X12 descriptor: "The diagnosis is inconsistent with the procedure."
CARC 11 fires when ICD-10-CM diagnosis code(s) on the claim do not support the medical necessity of the billed procedure per LCD or NCD criteria, or when the diagnosis is clinically incompatible with the procedure. Verify diagnosis linkage on the claim face and add supporting diagnosis codes that reflect the full clinical picture documented in the medical record.
X12 descriptor: "Claim/service lacks information or has submission/billing error(s). At least one Remark Code must be provided."
CARC 16 is among the most frequently issued denial codes in Medicare fee-for-service. CMS MLN MM6229 identifies CARC 16 as a generic code; the accompanying RARC carries the actionable detail identifying the specific missing data element (NPI, diagnosis code, authorization number, rendering provider information, date of service format, etc.). Billing staff who correct and resubmit without reading the RARC will receive the same denial. Always extract and act on the RARC before resubmission.
Common RARC pairings: N56, N130, N265, N290, N522. Verify current definitions at x12.org, as each RARC specifies a distinct missing or erroneous data element.
X12 descriptor: "Exact duplicate claim/service."
A claim matching a previously processed claim on all key data elements (patient, provider, date of service, procedure code, service line details) generates CO-18. If the denial is erroneous because a resubmission was processed as a new claim due to a system error, appeal via Level 1 Redetermination within 120 days of the RA date and include original submission evidence: clearinghouse timestamp, 997/999 EDI acknowledgment, or MAC portal confirmation. Submitting a new corrected claim creates another duplicate entry; the correct path is always the appeal track.
X12 descriptor: "This care may be covered by another payer in accordance with the coordination of benefits."
CO-22 or OA-22 appears when Medicare processes a claim that should have been submitted to a primary payer first under Medicare Secondary Payer (MSP) rules per CMS MSP Manual Chapter 1. Submit the claim to the primary payer, obtain the primary payer's EOB, and resubmit to Medicare as a secondary payer claim with COB data in the 837P transaction. The MSP questionnaire must be on file before billing.
X12 descriptor: "The time limit for filing has expired."
Medicare Part B claims must be filed within 12 months of the date of service per 42 CFR 424.44. CO-29 cannot be appealed on the merits of the underlying claim. Three statutory exceptions under 42 CFR 424.44(b) permit late filing: (1) administrative error or misrepresentation by an HHS employee, Medicare contractor, or authorized agent; (2) retroactive Medicare entitlement on or before the date of service; (3) retroactive Medicare entitlement where a State Medicaid Agency recoups money from a provider six or more months after the date of service.
Proof of timely filing: Clearinghouse batch submission report with timestamp; EDI 997/999 functional acknowledgment; MAC portal submission confirmation. These records must be retained for the full timely filing window.
X12 descriptor: "Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement."
CO-45 represents the contractual difference between the billed charge and the Medicare allowed amount. This is a mandatory write-off under Medicare participation agreements. X12 guidance specifies CARC 45 must be used with Group Code CO. Billing a Medicare patient for the CO-45 difference violates Medicare assignment rules. The RARC M77 may accompany CO-45 to provide fee schedule limitation detail (verify current M77 definition at x12.org).
X12 descriptor: "These are non-covered services because this is not deemed a 'medical necessity' by the payer."
CO-50 is issued when a service does not meet the coverage criteria of an applicable NCD or LCD, searchable at the Medicare Coverage Database. If clinical documentation supports coverage but was not submitted with the claim, file a Level 1 Redetermination with the complete medical record, diagnosis linkage, and LCD criteria citation. If documentation does not support coverage, issue an ABN prospectively for future services so the patient assumes financial responsibility and the Group Code shifts to PR. Routine acceptance of CO-50 denials without review is a documentation gap that RAC auditors identify in targeted reviews.
X12 descriptor: "Non-covered charge(s). At least one Remark Code must be provided."
Like CARC 16, CARC 96 is a generic non-coverage code requiring a RARC to carry the specific reason. If the service is non-covered and no ABN was issued, the loss is a provider write-off (CO). If a valid ABN (Form CMS-R-131) was issued before the service and the patient signed it acknowledging potential non-coverage, the Group Code shifts to PR and the patient may be billed.
X12 descriptor: "The benefit for this service is included in the payment/allowance for another service/procedure that has already been adjudicated."
CO-97 is the primary NCCI PTP bundling CARC. The Column 1 code in the PTP edit pair is paid; the Column 2 code is denied as included in the Column 1 allowance. An NCCI-associated modifier may override the edit when the modifier indicator for the edit pair is 1 and the services are clinically distinct. Modifier indicator 0 means no modifier can override the edit under any circumstance.
CARC 97 also appears in non-NCCI bundling scenarios (for example, an add-on code billed without its primary code). Confirm whether the denial is NCCI-driven before selecting the corrective path.
X12 descriptor: "Procedure/modifier combination incompatible with the National Correct Coding Initiative."
CO-236 fires when the combination of procedure code and modifier violates NCCI PTP edit requirements. The distinction from CO-97 is that CO-236 specifically identifies the modifier combination as the violation. An NCCI-associated modifier applied correctly to the Column 2 code in the pair, supported by clinical documentation, may resolve a CO-236 denial on resubmission or Level 1 Redetermination.
CARC 97 and CARC 236 are the two NCCI-specific denial codes. PTP edits are updated quarterly (January 1, April 1, July 1, October 1) and published at the CMS NCCI for Medicare page. Each edit pair carries a modifier indicator: indicator 1 allows an NCCI-associated modifier to override the edit when clinically appropriate; indicator 0 means no modifier overrides, period.
MUEs are a separate NCCI mechanism limiting units of service per code per date of service. MUE denials typically generate CARC 4 or appear as partial payment with a unit reduction. Appeals for both PTP and MUE denials go to the MAC, not the NCCI contractor.
The NCCI-associated modifiers that may override PTP edits (when indicator permits): 25, 59, 91, XE, XS, XP, XU. Modifiers 22, 76, and 77 are explicitly not NCCI PTP-associated modifiers and cannot unbundle NCCI-paired codes.
Unbundling audit triggers auditors look for: high-volume CO-97 denials resubmitted with modifier 59 across the same code pair without supporting documentation; Column 2 codes billed separately in a pattern suggesting systemic upcoding rather than genuinely distinct services.
Medicare contractors are restricted to Group Codes CO, OA, and PR per CMS Claims Processing Manual, Chapter 22. Any CO-adjusted amount is a mandatory write-off; billing a Medicare beneficiary for a CO balance is a violation of the participation agreement. The only valid path to patient billing is Group Code PR, and ABN-required services require a signed ABN before the service date.
For Medicare Advantage, OIG Report OEI-09-18-00260 (2022) found MAOs reversed many denials on appeal, indicating systematic denial errors. MAO denials remain subject to ongoing OIG monitoring.
The 12-month timely filing limit under 42 CFR 424.44 is jurisdictional. CO-29 cannot be appealed on the underlying claim's merits; only a statutory exception applies. Proof of timely submission (clearinghouse timestamp, EDI 997/999 acknowledgment, MAC portal receipt) is the foundation of any CO-29 appeal. Practices should automate submission confirmation archiving at 30, 60, and 90 days post-service as a standard workflow.
CARCs 15 and 96 with authorization-related RARCs are the ERA signals for prior authorization failures. The CMS Interoperability and Prior Authorization Final Rule (CMS-0057-F) requires impacted payers (Medicare Advantage, Medicaid, CHIP) to implement operational PA transparency provisions by January 1, 2026 and the FHIR Prior Authorization API by January 1, 2027. Under the WISeR Prior Authorization Model (effective July 1, 2025), CMS finalized PA requirements for select services under the Wasteful and Inappropriate Services Reduction model.
For CY 2026 (requests filed on or after January 1, 2026): ALJ hearing threshold is $200 (up from $190 in CY 2025); Federal District Court threshold is $1,960 (up from $1,900 in CY 2025), per the Federal Register CY 2026 AIC Adjustment. Claims may be aggregated to meet the threshold at each appeal level.
1. Acting on CARC 16 without reading the RARC. Billing staff correct and resubmit a generic CARC 16 denial without identifying the specific missing element from the RARC. The claim returns with an identical denial and another timely filing day is consumed. Always extract the RARC, identify the exact data element, and correct it before resubmission.
2. Billing the patient for CO-adjusted amounts. Any balance remaining after a CO adjustment is a contractual write-off under the Medicare participation agreement. Issuing a patient statement for a CO-45 or CO-50 balance is an improper billing violation, not a billing error.
3. Using modifier 59 to override a modifier indicator 0 NCCI edit. PTP edits with modifier indicator 0 cannot be overridden by any modifier. Appending modifier 59 to the Column 2 code does not change the payer response and may flag the claim for audit scrutiny.
4. Resubmitting a CO-18 duplicate denial as a new corrected claim. A resubmission billed as a new claim generates another duplicate entry. If the original CO-18 denial was erroneous, the corrective action is a Level 1 Redetermination with original submission proof, not a new claim on a fresh invoice.
5. Appealing CO-29 on medical necessity grounds. The underlying clinical necessity of the service is irrelevant in a timely filing denial. The only valid argument is proof of timely submission or a statutory exception under 42 CFR 424.44(b). Appeals that argue medical necessity for a CO-29 denial will be denied at every level of the five-level appeals process.
6. Accepting CO-50 denials as routine write-offs without review. Medical necessity denials systematically accepted without review signal a documentation gap. RAC auditors and MAC probe reviews identify patterns of CO-50 acceptance as indicators of insufficient clinical documentation, which can lead to extrapolated overpayment demands across a provider's claim history.
7. Applying the NCCI-associated modifier to the Column 1 code instead of Column 2. CARC 4 fires when the modifier is on the wrong code in the PTP pair. The NCCI-associated modifier belongs on the Column 2 (denied) code to indicate the distinct service.
8. Missing the ABN window for CARC 96 or CARC 50 denials. For services with predictable non-coverage under an LCD or statutory exclusion, a prospective ABN signed before the service shifts financial responsibility to the patient (Group Code PR). After the service is rendered without an ABN, the CO adjustment is final and the patient cannot be billed.
Scenario 1: CO-29 Timely Filing with Administrative Delay A physician practice submitted a Medicare Part B claim for a January 5, 2025 office visit. The clearinghouse batch confirmation shows submission on December 30, 2025 (within 12 months), but the MAC system recorded receipt on January 22, 2026 due to a year-end processing delay; the ERA shows CO-29. Action: File a Level 1 Redetermination within 120 days of the RA date. Submit the clearinghouse timestamp and the 997/999 EDI acknowledgment as proof of timely submission. Cite the administrative delay exception under 42 CFR 424.44(b)(1); the MAC's own processing delay constitutes an administrative error by an agent of CMS. The appeal should not address the clinical merits of the visit.
Scenario 2: CO-97 NCCI Mutually Exclusive Bundling A general surgeon bills CPT 44950 and CPT 44960 for the same patient on the same date. The ERA returns CO-97 on CPT 44950. The NCCI PTP edit identifies CPT 44950 as the Column 2 code, bundled into CPT 44960 as Column 1. Action: Accept the CO-97 write-off for CPT 44950. These two codes are mutually exclusive per NCCI; no modifier overrides a mutually exclusive edit (modifier indicator 0). Confirm the operative note supports CPT 44960 as the definitive comprehensive service.
Scenario 3: CO-16 with RARC Specifying Missing COB Information An outpatient therapy group submits a Medicare claim. The ERA shows CO-16 with RARC N56. RARC N56 specifies the adjustment is tied to a missing primary payer EOB (verify current N56 definition at x12.org before acting). Action: Obtain the primary insurer's EOB, confirm the MSP questionnaire is on file, and resubmit as a Medicare secondary payer claim with COB data populated in the 837P transaction. Do not submit as a new primary Medicare claim; that creates a coordination error and restarts the duplicate claim risk.
Scenario 4: CO-50 Power Wheelchair Medical Necessity A provider bills HCPCS K0856 for a Medicare patient. The ERA returns CO-50. The applicable MAC LCD requires a face-to-face evaluation by the treating practitioner within 45 days of the order and a written order from a treating physician; the order was signed by a PA only and no face-to-face note was submitted with the claim. Action: Pull the applicable MAC LCD from the Medicare Coverage Database. If the face-to-face evaluation was performed and documented and a treating physician co-signed the order, file a Level 1 Redetermination within 120 days with the complete documentation. If the PA-signed order alone does not satisfy the LCD, the denial stands; issue a prospective ABN for any future power mobility device orders to shift financial responsibility appropriately.
Scenario 5: CO-236 NCCI Incompatibility, Shave Removals A dermatologist bills CPT 11300 and CPT 11305 for two shave removals on the trunk on the same date. The ERA returns CO-236 on CPT 11305. Modifier 59 was not appended at the time of submission. Action: If the procedure note documents distinct anatomical locations for each lesion (separate sites with individual measurements), resubmit or appeal with modifier 59 (or XS for separate structure) appended to CPT 11305 and attach the procedure note specifying both anatomical sites. If the note does not differentiate the locations, accept the write-off and revise documentation practices for future multi-lesion encounters.
| Code | Type | Description |
|---|---|---|
| CARC 15 | CARC | Authorization number missing, invalid, or does not apply; primary denial code for prior authorization failures |
| CARC 24 | CARC | Charges covered under capitation agreement; Medicare Advantage and managed care plan denials |
| M77 | RARC | Fee schedule or limitation detail; typically accompanies CARC 45 (verify current definition at x12.org) |
| M86 | RARC | Supplemental payment adjustment detail; pairs with CO and OA adjustments (verify at x12.org) |
| Alert RARCs | RARC | Informational ERA notices prefixed with "Alert:"; no financial adjustment attached |
| Form CMS-R-131 | ABN | Advance Beneficiary Notice of Noncoverage; required for ABN-required services to shift Group Code from CO to PR |
| 997/999 | EDI | Functional Acknowledgment transaction; primary documentary evidence for CO-29 timely filing appeals |
| ASC X12 835 | Transaction | Health Care Claim Payment/Advice; the HIPAA-mandated ERA format containing all CARC, RARC, and Group Code data |
| Code | Description |
|---|---|
| 11300 | Shaving of epidermal or dermal lesion, single lesion, trunk, arms or legs; lesion diameter 0.5 cm or less |
| 11305 | Shaving of epidermal or dermal lesion, single lesion, scalp, neck, hands, feet, genitalia; lesion diameter 0.5 cm or less |
| 44950 | Appendectomy; |
| 44960 | Appendectomy; for ruptured appendix with abscess or generalized peritonitis |
| K0856 | Power wheelchair, group 3 standard, single power option, sling/solid seat/back, patient weight capacity up to and including 300 pounds |
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