Top 10 Denial Codes in Medical Billing and How to Fix Them in 2026

Every denial code tells the practice why a claim failed to pay correctly. Behind that code, there is usually a specific billing issue: inactive coverage, missing authorization, incorrect coding, weak documentation, duplicate submission, or wrong payer selection.

For practices, these denials directly affect cash flow because they delay reimbursement and add more follow-up work for the billing team. This guide breaks down the top 10 denial codes in medical billing, what each code means, why it happens, and how to fix it before the same issue repeats.

TL;DR

Most denial codes are not random payer responses. They usually point to repeat problems in eligibility checks, claim data, coding, documentation, authorization, timely filing, or payer selection.

This guide explains the top 10 denial codes in medical billing, what each code means, why it happens, and the first action a billing team should take to fix it. The main goal is not just to correct one denied claim, but to find the workflow issue behind the denial and prevent the same problem from affecting future

What Are Denial Codes in Medical Billing?

Denial codes are codes used by insurance payers to explain why a medical claim was not paid, was paid less than expected, or was denied completely. When a claim is processed, the payer reviews the submitted details and then uses denial codes to show the reason payment was not approved as billed.

These codes usually appear on the EOB or ERA. They help the billing team understand what went wrong with the claim and what needs to be checked next. Without denial codes, it would be difficult to know whether the issue came from patient information, insurance coverage, coding, documentation, authorization, or payer rules.

A denial code may point to issues like missing claim details, expired insurance, lack of medical necessity, missing prior authorization, duplicate billing, or billing the wrong payer. For example, one denial code may show that the patient’s insurance was not active on the date of service, while another may show that the service was not covered under the patient’s plan.

Top 10 Denial Codes in Medical Billing: Quick Reference Table

The table below gives a quick overview of the top 10 denial codes in medical billing, what each code means, why it usually happens, and what action the billing team should take first. This helps practices identify the issue faster before moving into detailed claim review or appeal steps.

Denial Code	Meaning	Common Cause	Quick Fix
CO-16	Claim lacks information or has invalid information	Missing or incorrect claim data	Correct claim details and resubmit
CO-18	Duplicate claim or service	Same claim submitted twice	Check claim status before resubmission
CO-27	Coverage expired	Policy inactive on date of service	Verify eligibility and update payer info
CO-29	Timely filing expired	Claim submitted late	Appeal only with valid proof
CO-50	Non-medically necessary service	Documentation does not support service	Submit clinical records
CO-96	Non-covered charges	Service not covered by plan	Verify benefits before service
CO-109	Wrong payer billed	Incorrect payer responsibility	Confirm primary/secondary insurance
CO-167	Diagnosis not covered	Diagnosis does not support service	Review payer coverage rules
CO-197	Authorization absent	Prior auth not obtained	Submit auth proof or appeal
CO-11	Diagnosis inconsistent with procedure	ICD-CPT mismatch	Correct diagnosis/procedure pairing

 

1. CO-16: Claim Lacks Information or Has Invalid Information

CO-16 means the claim is missing required information or has incorrect details. The payer cannot process the claim until the missing or invalid information is corrected.

This is one of the most common denial codes because it can happen for many small claim errors.

Why CO-16 Denials Happen

• Missing patient details

• Invalid subscriber ID
• Missing provider NPI
• Incorrect modifier
• Missing authorization or referral number
• Incorrect place of service
• Invalid payer or provider information

Example of CO-16 Denial

A claim is submitted without the rendering provider’s NPI, or the insurance ID entered on the claim does not match the payer’s record. Because the required information is missing or invalid, the payer denies the claim with CO-16.

How to Fix CO-16

Review the EOB or ERA to identify what information is missing or incorrect. Then check the claim against the patient record, insurance details, provider information, and documentation. Correct the error and resubmit the claim as soon as possible.

How to Prevent CO-16

Use a pre-billing checklist before claim submission. Run claims through a claim scrubber to catch missing fields, invalid modifiers, incorrect IDs, or payer-specific errors. Also, regularly validate provider NPI, payer ID, subscriber details, and authorization information before submitting claims.

2. CO-18: Duplicate Claim or Service

CO-18 means the payer believes the same claim or service has already been submitted or processed. The payer denies the second claim because it looks like a duplicate.

This often happens when a claim is resubmitted before checking the status of the original claim.

Why CO-18 Denials Happen

• Same claim submitted more than once

• Claim resubmitted before payer processing is complete
• Corrected claim sent incorrectly
• Same service billed twice for the same date of service
• Clearinghouse or system transmission error
• Billing team did not check claim status before resubmission

Example of CO-18 Denial

A billing team submits an office visit claim. A few days later, before checking the payer status, the same claim is submitted again. The payer identifies it as a duplicate and denies the second claim with CO-18.

How to Fix CO-18

First, check the original claim status. If the first claim is still processing or already paid, do not resubmit. If the service was actually repeated, use the correct modifier or submit supporting documentation to explain why the service was billed again.

How to Prevent CO-18

Always check payer status before resubmitting a claim. Keep a clear workflow for corrected claims, resubmissions, and appeals. Use clearinghouse reports to confirm whether the claim was accepted, rejected, pending, paid, or denied.

3. CO-27: Insurance or Coverage Expired

CO-27 means the patient’s insurance coverage was not active on the date of service. The payer is saying the policy had expired or was inactive when the service was provided.

This denial usually points to an eligibility verification issue.

Why CO-27 Denials Happen

• Insurance was not verified on the date of service

• Patient gave outdated insurance information
• Coverage ended before the appointment date
• Policy was retroactively terminated
• Incorrect date of service entered on the claim
• Insurance changes were not updated in the billing system

Example of CO-27 Denial

A patient visits the practice on May 5, but their insurance coverage ended on April 30. The claim is submitted to the expired plan, and the payer denies it with CO-27.

How to Fix CO-27

Verify the patient’s coverage directly with the payer. If the coverage was active on the date of service, submit proof of eligibility and appeal the denial. If coverage was truly inactive, update the patient account and follow the practice’s self-pay or patient billing policy.

How to Prevent CO-27

Verify eligibility before every visit, not only when the appointment is scheduled. Ask patients about insurance changes at check-in, especially at the start of a new month or year. Use real-time eligibility tools whenever possible.

4. CO-29: Timely Filing Limit Expired

CO-29 means the claim was submitted after the payer’s allowed filing deadline. Even if the claim details are correct, the payer may deny it because it was sent too late.

This is one of the harder denials to overturn unless there is valid proof.

Why CO-29 Denials Happen

• Claim was held too long before submission
• Staff missed payer-specific filing deadlines
• Corrected claim was submitted late
• Claim was first sent to the wrong payer
• Missing information delayed submission
• Timely filing limits were not tracked properly

Example of CO-29 Denial

A payer allows 90 days from the date of service to submit a claim. If the practice submits the claim after 120 days, the payer may deny it with CO-29.

How to Fix CO-29

Check the payer’s timely filing policy first. If the deadline was missed because of payer error, retroactive eligibility, COB issues, or proof of earlier submission, submit an appeal with documentation. If there is no valid proof, the denial may not be recoverable.

How to Prevent CO-29

Submit claims as soon as possible after the date of service. Track payer-specific timely filing limits and monitor claims that are on hold. High-dollar or aging claims should be reviewed before the filing deadline is reached.

5. CO-50: Non-Medically Necessary Service

CO-50 means the payer does not believe the billed service was medically necessary. The payer may deny the claim if the diagnosis, documentation, or clinical notes do not support the service.

This denial is often linked to documentation and medical necessity rules.

Why CO-50 Denials Happen

• Diagnosis does not support the procedure
• Clinical documentation is incomplete
• Payer medical necessity policy was not met
• LCD or NCD guidelines were not followed
• Service appears routine, preventive, or cosmetic
• Prior authorization was missing when required

Example of CO-50 Denial

A provider orders an advanced diagnostic test, but the medical record does not explain why the test was necessary. The payer reviews the claim and denies it with CO-50.

How to Fix CO-50

Review the payer’s medical necessity policy and compare it with the medical record. If the service was medically necessary, submit an appeal with clinical notes, test results, physician documentation, and any supporting records.

How to Prevent CO-50

Make sure the diagnosis supports the service before claim submission. Providers should document the reason for the service clearly in the medical record. Billing and coding teams should check payer medical necessity rules for high-risk services.

6. CO-96: Non-Covered Charges

CO-96 means the service billed is not covered under the patient’s insurance plan. The payer is saying the patient’s benefits do not include payment for that service.

This is usually a benefit coverage issue.

Why CO-96 Denials Happen

• Service is excluded from the patient’s plan
• Wrong insurance plan was billed
• Service requires authorization but none was obtained
• Diagnosis does not support coverage
• Patient’s plan does not cover that service type
• Benefits were not checked before service

Example of CO-96 Denial

A cosmetic skin procedure is billed to a medical insurance plan. Since the plan does not cover cosmetic services, the payer denies the claim with CO-96.

How to Fix CO-96

Review the patient’s benefits and confirm whether the service is covered. If the wrong payer or plan was billed, correct the claim and resubmit it. If the service is truly non-covered, follow the practice’s patient billing policy.

How to Prevent CO-96

Verify benefits before the service is provided. Check coverage limits, exclusions, authorization rules, and payer guidelines. If a service may not be covered, inform the patient in advance and collect the required acknowledgment.

7. CO-109: Claim Not Covered by This Payer or Contractor

CO-109 means the claim was sent to the wrong payer. The payer is saying they are not responsible for payment because another insurance company or contractor should have been billed.

This often happens when payer responsibility is not confirmed correctly.

Why CO-109 Denials Happen

• Wrong payer selected during claim submission
• Patient changed insurance plans
• Primary and secondary insurance were not confirmed
• Medicare Advantage plan was missed
• Incorrect payer ID was used
• COB information was not updated

Example of CO-109 Denial

A claim is submitted to Medicare, but the patient is actually enrolled in a Medicare Advantage plan. Medicare denies the claim with CO-109 because the Medicare Advantage payer should have been billed.

How to Fix CO-109

Verify the patient’s current insurance coverage and payer responsibility. Confirm primary and secondary insurance details. Then update the payer information and resubmit the claim to the correct payer.

How to Prevent CO-109

Confirm active insurance and payer responsibility at every visit. Pay close attention to Medicare Advantage plans, secondary insurance, and coordination of benefits. Keep payer IDs and insurance records updated in the billing system.

8. CO-167: Diagnosis Not Covered

CO-167 means the diagnosis code used on the claim is not covered for the billed service. The payer does not consider that diagnosis payable under its coverage rules.

This denial is closely related to diagnosis and payer policy matching.

Why CO-167 Denials Happen

• Diagnosis does not support the service
• Diagnosis code is too general or unspecified
• Payer coverage policy was not checked
• LCD or NCD requirements were not met
• Screening diagnosis used when a clinical diagnosis was required
• Wrong ICD-10 code selected during coding

Example of CO-167 Denial

A wound care service is billed with a diagnosis that does not support wound treatment. The payer denies the claim with CO-167 because the diagnosis is not covered for that service.

How to Fix CO-167

Review the medical record and payer coverage policy. If the documentation supports a more accurate diagnosis, correct the diagnosis code and resubmit the claim. If needed, include supporting documentation.

How to Prevent CO-167

Check diagnosis-to-procedure compatibility before claim submission. Avoid using unspecified diagnosis codes when a more specific code is available. Use payer coverage guidelines to confirm which diagnoses support the billed service.

9. CO-197: Precertification or Authorization Absent

CO-197 means prior authorization or precertification was required but was not obtained before the service. The payer denies the claim because approval was missing.

This denial is common for services that require payer review before treatment.

Why CO-197 Denials Happen

• Prior authorization was not requested
• Authorization was approved for the wrong date
• Authorization was linked to the wrong provider or facility
• Approved visit limits were exceeded
• Authorization number was missing from the claim
• Staff did not confirm payer authorization rules before service

Example of CO-197 Denial

A patient receives an MRI, but the payer required prior authorization before the test. Since authorization was not obtained, the payer denies the claim with CO-197.

How to Fix CO-197

Check whether authorization was required for the service. If authorization was already approved, submit the authorization number and proof. If the service was urgent or medically necessary, submit an appeal with clinical documentation.

How to Prevent CO-197

Verify authorization requirements before scheduling services. Track authorization numbers, approval dates, units, visits, and expiration dates. Make sure authorization details are entered correctly in the billing system before claim submission.

10. CO-11: Diagnosis Inconsistent with Procedure

CO-11 means the diagnosis code does not match or support the procedure billed. The payer sees a mismatch between the patient’s condition and the service provided.

This usually happens when the ICD-10 and CPT codes do not support each other.

Why CO-11 Denials Happen

• ICD-10 code does not support the CPT code
• Wrong diagnosis selected in the EHR
• Provider documentation does not support the procedure
• Diagnosis and procedure were not linked correctly
• Payer-specific coding rules were not followed
• Medical necessity support is missing

Example of CO-11 Denial

A procedure is billed for a foot-related service, but the diagnosis code submitted is unrelated to the foot condition. The payer denies the claim with CO-11 because the diagnosis does not support the procedure.

How to Fix CO-11

Review the medical record and confirm the correct diagnosis and procedure. If the wrong diagnosis was used, update the claim with the correct ICD-10 code. Resubmit the claim with proper documentation if needed.

How to Prevent CO-11

Validate CPT and ICD-10 code combinations before submission. Use coding tools, payer edits, and documentation review to make sure the diagnosis supports the procedure. Train coding teams to check diagnosis-procedure pairing carefully.

Other Common Denial Codes Practices Should Track

The top 10 denial codes cover the most common claim issues, but some additional codes can still affect reimbursement. These codes may not appear as often for every practice, but they are important to track when reviewing denial trends.

CO-204: Service Not Covered Under the Patient’s Current Benefit Plan

CO-204 means the billed service is not covered under the patient’s current insurance benefits. This can happen when the plan excludes the service, the wrong benefit option was selected, or the service does not meet the payer’s coverage rules.

To reduce CO-204 denials, practices should verify benefits before the service, check coverage limitations, and confirm whether the service is covered under the patient’s active plan.

CO-B7: Provider Not Credentialed with the Payer

CO-B7 means the provider is not recognized as credentialed or enrolled with the payer for the billed service. This often happens when a new provider starts seeing patients before credentialing is complete, claims are submitted before the credentialing effective date, or the wrong NPI or taxonomy is used.

To prevent CO-B7 denials, practices should track payer enrollment status, confirm credentialing effective dates, and verify billing/rendering NPI and taxonomy details before submitting claims.

Denial Codes vs Rejection Codes

Denial codes and rejection codes are often confused, but they happen at different stages of the claim process.

A rejection usually happens before the claim reaches full payer review. This means the claim failed a basic validation check, such as a missing subscriber ID, invalid payer ID, wrong format, or missing required field. Rejected claims usually need to be corrected and resubmitted.

A denial happens after the payer reviews the claim. The payer has processed the claim but decided not to pay it as submitted. This may happen because of expired coverage, missing authorization, duplicate billing, medical necessity issues, or non-covered services.

Point	Denial Code	Rejection Code
When it happens	After payer review	Before payer adjudication
Where it appears	ERA or EOB	Clearinghouse or payer front-end report
Main issue	Payer processed the claim but did not pay as expected	Claim failed validation
Example	CO-16, CO-18, CO-197	Invalid payer ID, missing subscriber ID
Next step	Correct, appeal, or adjust	Fix and resubmit

 

How to Reduce Common Denial Codes in Medical Billing?

Most denial codes can be reduced when the billing workflow is checked before the claim reaches the payer. The goal is not only to fix denied claims after they happen, but to prevent the same errors from repeating across future claims.

To reduce common denial codes in medical billing, practices should:

• Verify eligibility before every visit to confirm active coverage, plan details, copay, deductible, and payer responsibility.
• Confirm authorization before scheduled services so procedures, imaging, therapy visits, and other approval-based services are not denied later.
• Review CPT and ICD-10 compatibility before submission to make sure the diagnosis supports the billed procedure.
• Use claim scrubbing before sending claims to catch missing fields, invalid modifiers, incorrect payer IDs, and formatting errors.
• Track denials by payer, provider, location, and denial reason to identify where the same issues are happening repeatedly.
• Review ERA and EOB trends weekly so recurring denial patterns are found before they turn into larger A/R problems.
• Train front desk, coding, and billing teams on recurring denial patterns because many denials start before the claim is created.
• Escalate high-dollar denials before timely filing deadlines so appealable claims are not lost because of late action.

Reducing denial codes requires a consistent process across eligibility, authorization, coding, claim submission, and denial follow-up. When each step is reviewed properly, practices can reduce rework, speed up reimbursement, and protect revenue from avoidable claim delays.