🏥 Medical Billing Denial Resource Guide


This quick-reference guide is designed for billing staff who need a practical way to understand, prevent, and appeal claim denials. Use it to identify common denial patterns, take the right next step, and reduce rework through consistent front-end processes. Keep it handy for day-to-day triage and training.








🔢 Common Denial Codes


Code Reason What It Means
CO-16 Claim/service lacks information Something required is missing or incomplete (e.g., **modifier**, **NPI**, **diagnosis**, dates, units, or attachments).
CO-18 Duplicate claim/service The payer believes this was already billed or processed for the same patient/date/service.
CO-22 Coordination of benefits The payer needs **other insurance** information or expects another payer to pay first.
CO-27 Coverage terminated Coverage was not active on the date of service, or the plan ended before the visit.
CO-29 Timely filing The claim was received after the payer’s filing deadline.
CO-50 Not medically necessary The payer says the service does not meet **medical necessity** rules for the diagnosis, setting, or frequency.
CO-96 Non-covered charge(s) The service is not covered under the patient’s benefits for that plan or situation.
CO-97 Included in another service The payer considers it bundled into another billed service and won’t pay separately.
CO-109 Claim not covered by this payer The payer says they are not responsible (wrong payer, wrong plan, or coverage not in effect).
CO-119 Benefit maximum reached The patient has used up plan limits (visits, dollars, or frequency) for the benefit period.
CO-197 Precertification/authorization required The payer expected **prior authorization** (or a referral) and did not find it on file.
PR-1 Deductible amount The amount is applied to the patient’s **deductible** and is typically patient responsibility.
PR-2 Coinsurance amount The amount is the patient’s **coinsurance** share based on plan rules.
PR-3 Copayment amount The amount is the patient’s **copay** for the visit/service.
OA-23 Impact of prior payer(s) The payment/adjustment reflects what another payer already paid or allowed under **COB**.

📋 Major Denial Types & How to Work Them


Eligibility Issues


These denials occur when **coverage** is inactive, the patient is not eligible for the billed benefit, or the payer has different member details on file. They are often preventable with consistent front-end verification and accurate registration.

  1. Verify eligibility for the **date of service** (plan, effective dates, and benefit type).
  2. Confirm patient demographics match the payer record (name, DOB, member ID).
  3. Check **COB** and update other insurance details; rebill the correct primary payer if needed.
  4. If coverage is active, submit a corrected claim with proof (eligibility response, portal screenshot, or payer reference #).


Authorization Failures


These denials happen when **prior authorization** or a referral was required but missing, expired, or did not match the billed service/location/provider. They commonly affect imaging, procedures, DME, and some specialty visits.

  1. Confirm whether authorization was required for the **CPT**, place of service, and rendering provider.
  2. Locate the authorization and validate dates, units/visits, and approved codes match the claim.
  3. If authorization was not obtained, request a retro-authorization when allowed and document payer guidance.
  4. Submit an appeal with the authorization number and supporting clinical documentation when appropriate.


Coding Errors


Coding denials often involve **bundling**, incorrect or missing **modifiers**, and **diagnosis-to-procedure mismatches**. Small coding differences can change whether a service is payable, bundled, or requires additional documentation.

  1. Review the EOB/EOP for bundling edits and confirm whether a modifier is appropriate (e.g., distinct service, laterality, repeat).
  2. Validate the **ICD-10** supports the service and matches documentation (medical decision-making, findings, and indication).
  3. Check units, dates, and NCCI edits; correct coding and rebill as a corrected claim when allowed.
  4. If coding is correct, appeal with a short rationale and attach key documentation (op note, report, progress note).


Timely Filing Violations


These denials occur when the claim is received after the payer’s **timely filing** deadline. They may be recoverable if you can prove timely submission or if the delay was caused by payer processing, COB, or eligibility issues.

  1. Confirm the payer’s filing limit and whether the claim was submitted as original, corrected, or reconsideration.
  2. Pull proof of timely submission (clearinghouse acceptance, payer acceptance, trace/ICN, or submission reports).
  3. If COB delayed billing, include primary EOB and the date it was received to support an exception.
  4. Submit a timely filing appeal with documentation and a clear timeline of events.


Duplicate Claims


Duplicate denials happen when the payer believes the same service was already billed or paid. This can be caused by resubmissions, corrected claims filed incorrectly, or overlapping dates/units.

  1. Compare the denied claim to the paid/processed claim (dates, CPTs, units, modifiers, and provider).
  2. If it truly duplicated a prior submission, stop further resubmissions and adjust internal work queues.
  3. If it was a corrected claim, ensure it was sent with the correct **frequency/type of bill** and original claim reference number.
  4. If the payer processed the wrong claim, request a reconsideration with claim references and a brief explanation.


Missing Information


These denials occur when required claim fields or attachments are missing, invalid, or inconsistent. Common issues include **NPI**, taxonomy, modifiers, diagnosis pointers, accident details, and documentation requests.

  1. Identify the exact missing element from the denial message and payer edits (don’t guess).
  2. Correct the claim data (provider identifiers, patient info, codes, units, dates) and add required attachments if applicable.
  3. Resubmit as a corrected claim and retain proof of acceptance by the payer/clearinghouse.
  4. If the payer’s request was unclear, contact payer support and document the reference number for follow-up.


Medical Necessity Denials


Medical necessity denials occur when the payer says the service does not meet **coverage criteria** based on diagnosis, frequency, setting, or documentation. These are best addressed by aligning documentation to payer policy and clearly stating why the service was needed.

  1. Locate the payer policy or LCD/NCD (when applicable) tied to the denied service and diagnosis.
  2. Confirm the billed **ICD-10** reflects the documented indication and any required severity/failed conservative treatment details.
  3. Submit an appeal with a concise clinical summary and attach only the most relevant records (order, note, results, prior treatment).
  4. If appropriate, request a peer-to-peer review and document the outcome and next steps.


💡 Prevention Tip


The single most effective way to reduce denials is consistent **front-end verification** paired with **clean claim submission**. Verify eligibility, COB, and authorization requirements before the visit, then run claim edits to catch missing data and coding issues before the claim leaves your system.


⚖️ Appeal Best Practices


  • Reference the payer’s denial reason and cite the exact claim/ICN number in the first line.
  • State the requested outcome (reprocess, overturn, or pay) and the specific line(s) in dispute.
  • Attach only relevant documentation (e.g., order, progress note, op note, report) and label it clearly.
  • Include proof of timely filing or submission acceptance when deadlines are involved.
  • Use payer-specific forms/portals and confirm the correct appeal level (reconsideration vs. formal appeal).
  • Track appeal dates, confirmation numbers, and follow-up intervals in your work queue.
  • Escalate to peer-to-peer or supervisor review when clinical judgment or policy interpretation is central.
  • Standardize templates and maintain an internal library of successful appeal language by denial type.


Payer Type Filing Deadline
Medicare 12 months (1 calendar year) from date of service
Medicaid Varies by state (commonly 90–365 days)
Commercial/Private Varies by contract (commonly 90–180 days)
Medicare Advantage Varies by plan (often aligns to Medicare or plan policy)
Workers’ Comp Varies by state and carrier (often 90 days–1 year)
TRICARE Typically 1 year for professional claims; longer windows may apply in some situations


⚠️ Important Reminder

Timely filing rules vary by payer, plan, and claim type—always confirm the exact deadline in the specific payer contract or provider manual before appealing a filing denial.




✅ Quick Reference Checklist


  • Work denials daily by **payer** and **denial category** to prevent aging and missed appeal windows.
  • Validate eligibility, COB, and authorization status for every denied claim before rebilling.
  • Correct claim errors (demographics, NPIs, modifiers, diagnosis pointers, units) and resubmit as the proper claim type.
  • Attach required documentation and keep submission proof (acceptance reports, reference numbers).
  • Track appeal deadlines and follow up on open appeals at set intervals (e.g., every 14–21 days).
  • Trend top denial reasons weekly and assign owners for root-cause fixes (registration, coding, auth, charge capture).