Top 10 Denials in Medical Billing and How to Prevent Them

Claim denials are one of the biggest barriers to smooth reimbursement. Even when workflows seem well-organized, denials can still disrupt cash flow and increase administrative workload. They also slow down the revenue cycle and create unnecessary rework. Recent industry reports show denial rates have risen sharply in the past two years. Many of these are caused by something as basic as incorrect patient information or coding errors.

Understanding what is being denied and why is the first step in preventing revenue loss. When teams identify denial patterns early, they can build prevention checkpoints into scheduling, documentation, coding, and billing steps. This helps resolve issues before the claim reaches the payer.

In this article, we’ll review the top medical billing denials, explain the root causes, and share practical ways to prevent them. The goal is to support timely reimbursement and improve overall operational efficiency.

What Are Claim Denials? 

A claim denial occurs when an insurance payer reviews a submitted claim and decides not to issue payment for all or part of the billed services. The payer communicates this decision through a denial code along with an Explanation of Benefits (EOB) or an Electronic Remittance Advice (ERA), which explains why the claim was not paid.

It’s important to distinguish a denial from a rejection.

• A rejected claim never makes it through the processing stage due to basic issues such as missing data or formatting errors. These can usually be corrected and resubmitted quickly.
• A denied claim has already been processed by the payer, but payment is withheld because of issues like coverage limitations, documentation gaps, coding inconsistencies, or medical necessity concerns. Denied claims often require follow-up work and, in some cases, a formal appeal to resolve.

Understanding the difference helps teams respond more efficiently and reduce delays in reimbursement. 

Now that we’ve defined what a denial is, let’s look at the denial codes you’ll encounter most often.

Top 10 Denials in Medical Billing

1. Missing or Incomplete Patient Information (CO 16)

This denial occurs when essential patient or claim details are missing, incomplete, or entered incorrectly. Examples include wrong date of birth, incorrect insurance ID, missing subscriber information, or inaccurate demographic data. Because this information is used to verify eligibility and coverage, any mismatch can stop the claim from being processed.

Why it Happens:

• Inaccurate data entry during registration
• Patient information not updated at each visit

How to Prevent It:

• Verify demographics and insurance details at every appointment
• Use automated eligibility verification tools
• Train front-desk teams to check for accuracy before claim submission

2. Incorrect Patient Eligibility or Coverage (CO 109)

Denials for eligibility happen when the patient’s insurance is not active on the date of service or the plan does not cover the billed service. Common triggers include recent plan changes, policy terminations, or missing referral requirements. Since payers check enrollment records closely, any discrepancy can stop payment.

Why it Happens:

• Insurance not active for date of service
• Benefits not verified at each visit

How to Prevent It:

• Verify eligibility during scheduling and again at check-in
• Confirm coverage levels, copays, and network rules before service
• Use real-time eligibility checks integrated with your PM/EHR

3. Duplicate Claims (CO 18)

Duplicate claim denials occur when the same service is submitted more than once for the same patient, provider, and date of service. Sometimes this happens because teams are unsure whether a claim was processed, or the same encounter is billed under multiple systems or batches. Even small workflow gaps can cause a claim to be flagged as a duplicate.

Why it Happens:

• Same claim submitted multiple times
• Claim resubmitted before the payer finishes processing
• Lack of visibility between billing teams or systems

How to Prevent It:

• Track claim status before resubmitting
• Set clear internal rules for when and how claims can be refiled
• Use claim scrubber tools to detect duplicate submissions before sending to payer

4. Lack of Prior Authorization (CO 197)

Some services require prior authorization before they can be performed and billed. When authorization is missing, expired, or does not match the service provided, the claim is often denied. This can significantly delay reimbursement because these denials usually require rework and sometimes an appeal.

Why it Happens:

• Authorization was not requested before the service
• Authorization is approved for a different provider or procedure
• Authorization expired before the date of service

How to Prevent It:

• Verify authorization requirements during scheduling
• Track authorization numbers, valid dates, and matching CPT codes in the EHR
• Reconfirm authorization before services that are frequently updated, such as imaging or therapies

5. Invalid or Unsupported Diagnosis Code (CO 167)

A claim may be denied when the diagnosis code does not clearly support the services billed. If the diagnosis is outdated, too general, or not recognized as medically necessary for the procedure, the payer may refuse payment. Accurate clinical documentation is essential to ensure the diagnosis reflects the patient’s condition and aligns with payer rules.

Why it Happens:

• ICD-10 code does not justify the medical service
• Diagnosis does not match coverage criteria
• Code is outdated or no longer valid in current coding guidelines

How to Prevent It:

• Confirm diagnosis codes directly from recent clinical documentation
• Map diagnoses to services based on payer-specific medical policies
• Review ICD-10 code updates at least quarterly to avoid using discontinued codes

6. Invalid or Unsupported Procedure Code (CO 181)

These denials occur when the procedure code submitted does not align with payer guidelines or is not appropriate for the documented service. Sometimes the CPT or HCPCS code may be outdated, mismatched with the diagnosis, or not allowed under the patient’s plan. Even small coding inconsistencies can result in denied reimbursement.

Why it Happens:

• CPT/HCPCS code is outdated or not recognized
• Procedure does not match the supporting diagnosis
• Payer-specific billing rules were not followed

How to Prevent It:

• Confirm procedure codes against the latest CPT/HCPCS updates
• Check payer policies for coding rules, bundling, and frequency limitations
• Ensure clinical notes clearly support the service billed

7. Non-Covered Services (PR 96)

A non-covered service denial happens when the billed service is not included in the patient’s insurance plan. Even if the service was clinically appropriate, the payer may classify it as non-benefit, elective, or outside the scope of coverage. These denials often surface when coverage rules are not verified before treatment.

Why it Happens:

• The service is not included in the patient’s policy benefits
• The payer categorizes the service as elective or experimental
• Coverage limitations vary by plan, payer, or service setting

How to Prevent It:

• Verify coverage and benefit limitations during pre-visit checks
• Inform patients about out-of-pocket responsibility before scheduling
• Keep payer coverage policies updated for commonly performed procedures

8. Medical Necessity Denials (CO 50)

Medical necessity denials occur when the payer determines that the service provided was not justified based on the patient’s clinical condition. Even when the service is appropriate, insufficient documentation or missing details can lead to denial. Clear clinical notes are essential to demonstrate why the service was needed.

Why it Happens:

• Documentation does not clearly support the service
• Diagnosis codes do not align with the procedure performed
• Payer-specific medical policies are not met

How to Prevent It:

• Ensure clinical notes explicitly justify the treatment
• Use specific, accurate diagnosis codes to support the service
• Review payer medical necessity criteria before submitting claims

9. Late Claim Submission (CO 29)

Claims may be denied if they are submitted after the payer’s timely filing limit. Each insurance plan sets its own deadline for claim submission, and missing this window can result in automatic denial. Even valid claims can go unpaid if they are filed too late.

Why it Happens:

• Delays in documentation or internal claim preparation
• Lack of tracking for payer-specific filing deadlines
• Claims held for corrections or approvals before submission

How to Prevent It:

• Track payer filing deadlines within your billing system
• Submit claims promptly after services are rendered
• Establish internal timelines to ensure claims are prepared and filed on schedule

10. Coordination of Benefits Errors (CO 22)

Coordination of benefits (COB) denials happen when multiple insurance plans are involved and the payer cannot determine which plan is primary. If the primary and secondary coverage information is missing or incorrect, the claim may be denied until the details are clarified.

Why it Happens:

• Incorrect primary or secondary insurance information
• Missing COB details at the time of claim submission
• Changes in patient coverage not updated in the system

How to Prevent It:

• Collect and verify all active insurance plans during registration
• Confirm the order of responsibility for primary and secondary payers
• Update coverage changes promptly and communicate responsibilities to patients

Strengthening Denial Prevention Going Forward

Reducing denials isn’t just about fixing rejected claims. It requires strengthening processes across scheduling, registration, documentation, coding, and billing. Organizations that analyze denial trends regularly and adjust workflows see faster payments, fewer resubmissions, and better patient billing experiences.A knowledgeable revenue cycle partner like AnnexMed can provide end-to-end RCM support, helping streamline eligibility checks, automate authorization tracking, and implement standardized denial workflows without disrupting day-to-day operations. Their team’s expertise ensures that claims are processed efficiently, helping practices improve revenue collection while focusing on quality patient care.

FAQs in Denials of Medical Billing