How to Appeal a Denied Medical Claim Step by Step: Payer Appeal Process, Timelines, and CARC Code Responses

How to Appeal a Denied Medical Claim Step by Step: Payer Appeal Process, Timelines, and CARC Code Responses is a question billing teams face daily, because claim denials cost U.S. healthcare practices an estimated $262 billion in rework costs annually, per the American Medical Association. A denied claim is not a final answer. Every commercial payer, Medicare Administrative Contractor (MAC), and state Medicaid program is required by law or contract to provide a structured appeals process. The outcome depends almost entirely on whether your billing team understands which appeal level to use, what documentation to attach, and how to read the CARC (Claim Adjustment Reason Code) on the Explanation of Benefits (EOB) or Electronic Remittance Advice (ERA). MMBS resolves 85% of appealable denials on the first submission, compared to the industry average of 35-45% for in-house billing teams without a dedicated denial workflow.

Reviewed by David Park, CPC — Senior Medical Billing Specialist at MMBS. David has 14+ years of experience in specialty billing and case study research and holds active AAPC (American Academy of Professional Coders) certification.

Reading the CARC Code Before Filing Any Appeal: What CO-16, CO-18, CO-29, and CO-97 Mean

The first step in appealing a denied claim is reading the CARC code on the ERA (Electronic Remittance Advice, the 835 transaction file your EHR or practice management system receives from the payer) or the paper EOB (Explanation of Benefits) mailed to the provider. CMS (Centers for Medicare & Medicaid Services) publishes CARC codes through the Health Care Code Committee, and every payer operating under Medicare Part B or commercial contracts is required to use this standardized set.

• Process name: Medicare Part B Claim Appeal
• Governing agency: CMS (Centers for Medicare & Medicaid Services, 42 CFR 405.940-405.986)
• Formal levels: 5 (Redetermination, QIC Reconsideration, ALJ Hearing, Medicare Appeals Council, Federal District Court)
• Level 1 deadline: 120 days from denial notice date
• Level 1 decision window: 60 days from receipt of appeal
• Level 3 amount threshold (2026): $180 (ALJ hearing minimum)
• CARC codes governing most appeals: CO-16, CO-18, CO-29, CO-97
• Record closes: At QIC (Level 2) , new clinical evidence not submittable after Level 2

Four CARC codes generate the highest appeal volume at MMBS:

• CO-16 (Claim lacks information or has submission or billing errors): This is a technical denial, not a clinical one. The fix is adding the missing element and resubmitting. Common triggers include a missing NPI (National Provider Identifier, the 10-digit CMS-issued identifier required on every claim under 45 CFR Part 162.408), an absent prior authorization number, or a missing modifier. CO-16 does not require a formal appeal in most cases because the corrected claim can be resubmitted within the original timely filing window.
• CO-18 (Exact duplicate claim or service): The payer received two identical claims. Confirm in your EHR that only one claim was sent, then submit a written reconsideration with proof of a single submission. If the payer processed the same claim twice internally, request a check reissue rather than resubmitting a corrected claim.
• CO-29 (The time limit for filing has expired): Timely filing denials are the hardest to overturn, but clearinghouse transmission records, 999 acknowledgment files, and ERA audit trails can prove the claim was submitted on time and force the payer to reverse the denial.
• CO-97 (The benefit for this service is included in the payment for another service): CMS's NCCI (National Correct Coding Initiative) edits drive most CO-97 denials. Overturning them requires operative or procedure notes showing distinct anatomical sites or separate encounters, paired with the appropriate modifier (Modifier 59 or XE, XS, XP, XU).

Additional denial codes that frequently reach a denial prevention workflow include CO-4 (procedure code inconsistent with the modifier), CO-22 (care may be covered by another payer), and CO-50 (non-covered service). Each CARC code carries a specific appeal pathway; submitting a generic appeal letter that does not address the stated CARC reason is the single biggest reason appeals fail.

Timely Filing Windows by Payer: Medicare, Medicaid, and Major Commercial Carriers

Timely filing deadlines are contractual and regulatory hard stops. Missing the appeal window forfeits your right to payment. Key windows to know:

Medicare Part B: Initial claims must be filed within 12 months of the date of service, per 42 CFR 424.44. Redetermination appeals must be submitted within 120 days of the denial notice. QIC Reconsideration must be filed within 180 days of the Redetermination decision. ALJ hearings must be requested within 60 days of the QIC decision, and the 2026 threshold is $180.

Medicaid: Each state Medicaid program sets its own rules under CMS guidance. Most require initial claim submission within 90 to 365 days and appeals within 30 to 90 days of the denial. Always check the state-specific Medicaid provider manual before filing, because MAC contractors differ by region.

Major commercial payers: UnitedHealthcare, Anthem, Aetna, and Cigna each set their own appeal windows by contract, but 180 days from the denial date on the EOB or ERA is the most common standard. One detail billing teams miss: the timely filing clock on an appeal starts from the denial date on the remittance, not the date of service. Calendar your deadlines the same day you receive a denial notice.

The Four Formal Medicare Appeal Levels: Redetermination Through Council Review

Medicare structures its appeal ladder across five levels, with billing teams typically resolving denied claims at Levels 1 or 2. Understanding each level helps manage AR days (Accounts Receivable days, the average time from service date to payment receipt) and set realistic expectations with practice administrators.

• Level 1 , Redetermination by the MAC: The MAC (Medicare Administrative Contractor, the regional CMS contractor for Part B claims in your state) reviews the claim plus any new documentation you submit. The MAC must issue a decision within 60 days. Submit via the provider portal for your region: Novitas, CGS, Palmetto, WPS, or Noridian. Most technical and documentation denials resolve here.
• Level 2 , Reconsideration by a QIC: If the MAC upholds the denial, escalate to a QIC (Qualified Independent Contractor), a CMS-contracted reviewer independent from the MAC. The QIC must decide within 60 days. The record closes at this level: new clinical evidence that existed at the time of service cannot be introduced after Level 2. Attach everything at Level 1.
• Level 3 , ALJ Hearing at OMHA: If the QIC upholds the denial and the disputed amount meets the 2026 threshold ($180), request a hearing at OMHA (Office of Medicare Hearings and Appeals). Hearings run in person, by video, or by phone. OMHA wait times have historically run 18 to 36 months, so Level 3 suits high-value claims where the recovery justifies the timeline.
• Level 4 , Medicare Appeals Council: If the ALJ rules against you, request review by the Medicare Appeals Council within the HHS Departmental Appeals Board within 60 days of the ALJ decision. The Council reviews the full record for legal and factual error. Level 5 (federal district court) is available after Council review but is rare in standard billing practice.

Supporting Documentation Requirements by Denial Type

The documents attached to an appeal determine whether the payer reverses the denial or upholds it. A generic cover letter without supporting evidence fails at a high rate. Required items by denial category, relevant to any revenue cycle management process:

• Medical necessity denials: Complete progress note, signed physician order, the ICD-10 diagnosis code with full description from the ICD-10-CM set published annually by CMS, and the applicable LCD or NCD from the MAC.
• Timely filing denials (CO-29): Clearinghouse transmission confirmation, 999 acknowledgment file, ERA for the original submission, and any payer correspondence documenting system downtime if the delay originated on their end.
• Bundling denials (CO-97): Operative or procedure note documenting separate anatomical sites or distinct encounter times, a written explanation citing the specific NCCI edit pair and the clinical reason it does not apply, and the appropriate modifier (Modifier 59, or Modifiers XE, XS, XP, XU).
• Authorization denials: Authorization number, payer approval letter, fax confirmation of prior auth submission, and the physician's clinical summary. If the auth was verbal, include the rep's name, date, and call reference number.
• Coordination of Benefits denials (CO-22): Current insurance cards for both payers, a copy of the primary payer's EOB showing adjudication, and the patient's signed COB statement.

How MMBS Handles Denial Appeals: Performance Benchmarks and Workflow

MMBS (MyMedicalBillSolution.com) manages denial appeals as a structured workflow, not an ad hoc task. Every denied claim is triaged within 24 hours of ERA posting. CARC codes are categorized by appeal type (technical vs. clinical vs. contractual), and each category triggers a specific documentation checklist before the appeal letter is drafted.

MMBS's AAPC-certified billing team, all holding active CPC or COC credentials, brings the denial rate down at the source: a 98.2% clean claim rate across all specialties means fewer claims reach the appeals queue in the first place. For claims that do receive a denial, the team's first-pass appeal resolution rate holds at 85% on appealable denials, because every letter is matched to the CARC code before it is submitted. Average AR days across MMBS-managed practices run 28-32, compared to the industry standard of 45-55 AR days.

For practices managing their own billing, the realistic first-pass appeal resolution rate without a dedicated denial team is closer to 35-45%, per MGMA benchmarking surveys. The gap between 45% and 85% is largely attributable to documentation completeness, deadline management, and understanding of CARC code-specific appeal pathways. These patterns differ significantly by specialty and payer mix, including mental health billing, cardiology claims workflows, and orthopedic prior authorization processes.

All MMBS appeal workflows comply with HIPAA (Health Insurance Portability and Accountability Act, governed by 45 CFR Parts 160 and 164). Patient health information transmitted in appeal letters travels via secure, encrypted channels and only to authorized payer representatives.

Commercial Payer Appeals and External Review: Knowing When to Escalate

Commercial payers (UHC, Anthem, Aetna, Cigna, Humana) typically offer two levels of internal appeal before independent external review becomes available. The ACA (Affordable Care Act, enacted in 2010 under Public Law 111-148) requires all non-grandfathered group health plans to offer external review once internal options are exhausted. An accredited IRO (Independent Review Organization) conducts the review, and its decision binds the payer. For clinical necessity denials where a physician's judgment conflicts with the payer's medical policy, external review resolves in the provider's favor at a meaningfully higher rate than internal appeals, because the reviewer is an independent clinician rather than a payer medical director.

Always submit commercial appeals in writing, even when a phone option is offered. A written record with a date-stamped confirmation protects your right to external review and creates an audit trail under HIPAA compliance standards. Practices that want to offload the entire denial workflow, including CARC triage, appeal letter drafting, deadline calendaring, and escalation decisions, can transfer that workload through MMBS's outsourced billing and denial management services.

Frequently Asked Questions

How long does a Medicare claim appeal (Redetermination) take to resolve?

The MAC (Medicare Administrative Contractor) must issue a Redetermination decision within 60 days of receiving the appeal request. In practice, many straightforward Redeterminations close within 30 to 45 days. If the MAC does not issue a decision within 60 days, you may escalate to the QIC (Qualified Independent Contractor) level without waiting for the Redetermination to close, a right established under 42 CFR 405.960.

What is the difference between a CO-16 denial and a CO-18 denial?

CO-16 (Claim lacks information or has submission or billing errors) flags a missing data element such as an NPI, prior authorization number, or required modifier. CO-18 (Exact duplicate claim or service) means the payer logged two identical claims for the same date of service, procedure, and patient. CO-16 resolves through a corrected resubmission; CO-18 requires a written reconsideration with proof of single submission. Both CARC codes are governed by the Health Care Code Committee under CMS standards and apply uniformly across Medicare Part B and commercial payers.

Can a timely filing denial (CO-29) be overturned on appeal?

Yes. A clearinghouse transmission confirmation report, a 999 acceptance acknowledgment file, an ERA showing receipt, or certified mail documentation each constitute acceptable proof that the claim was submitted within the filing window. If a payer portal outage caused the delay, the payer is generally required to accept the late filing under the doctrine of payer error. MMBS retains all clearinghouse acknowledgment files for a minimum of 7 years, consistent with HIPAA (45 CFR Part 164) record retention requirements.

What documentation is required to appeal a CO-97 bundling denial?

A CO-97 denial reflects a NCCI (National Correct Coding Initiative) bundling edit that CMS applies to prevent double-billing for component services. Overturning it requires the operative or procedure note proving the services were performed at distinct anatomical sites or during separate encounters, a written explanation citing the specific NCCI edit pair and the clinical reason it does not apply, and the appropriate modifier (Modifier 59 for distinct procedural service or Modifiers XE, XS, XP, XU for more specific distinction). Attach all documentation at Level 1, because the evidentiary record closes at the QIC level for Medicare claims.

How many appeal levels does Medicare provide for denied claims?

Medicare provides five formal appeal levels: Redetermination by the MAC, Reconsideration by a QIC, an ALJ hearing at OMHA (Office of Medicare Hearings and Appeals), review by the Medicare Appeals Council within the HHS Departmental Appeals Board, and federal district court review. Most billing teams resolve denied claims at Levels 1 or 2. ALJ hearings are pursued for high-value claims where the disputed amount meets the annual threshold, currently $180 for 2026. The QIC is the last level at which new clinical evidence can be introduced into the record.

What is the average first-pass appeal resolution rate for medical billing practices?

Per MGMA benchmarking data, practices managing appeals in-house without a dedicated denial management team resolve approximately 35-45% of appealable claims on the first submission. MMBS's denial management workflow achieves 85% first-pass resolution on appealable denials, a gap attributable to CARC-matched documentation, AAPC-certified reviewers (CPC, COC credential holders), and disciplined deadline calendaring across all 50 states. Practices that want to close that gap can explore MMBS's end-to-end billing and AR recovery services.

If your practice is losing revenue to unresolved denials or missed appeal deadlines, MMBS's claims management and appeal workflow covers CARC triage, appeal letter drafting, payer follow-up, and AR recovery across all specialties and all 50 states. Contact MMBS at mymedicalbillsolution.com/contact-us to request a free billing assessment.