Did you know that according to AHA, nearly 12% of all medical claims in the U.S. are denied on first submission? But how’d you know why your claim was denied? What was the reason? These rejections are communicated through denial codes, which provide providers with specific reasons why a claim was not paid or reduced. By knowing denial codes, providers and their staff can quickly identify the issue, correct it, and resubmit the claim before deadlines pass.
This blog covers the most common denial codes, why they occur, and how to prevent them. The tips will help make billing smoother and ensure your practice gets paid on time.
What Are Denial Codes?
Denial codes are standardized codes that explain why an insurance payer has rejected or reduced payment for a medical claim. Instead of simply stating that a claim is “not paid,” the payer provides a denial code to indicate the specific reason.
Who Issues Denial Codes?
Denial codes are issued by insurance payers, not by providers. This includes:
- Government payers – Medicare, Medicaid, and Medicare Advantage plans
- Commercial payers – Private insurers such as Aetna, Blue Cross, or UnitedHealthcare
Providers receive denial codes on the remittance advice or explanation of benefits (ERA/EOB) after a claim has been processed. It is then the provider’s responsibility, often through their billing staff or revenue cycle management partner, to review the code, resolve the issue, and resubmit the claim if appropriate.
Common Denial Codes in 2026
The following are the most common denial codes providers faced in recent years.
CO-4
Denial code CO-4 usually shows up when a required modifier is missing or when the wrong one is attached. The rules are complex and change often, so even a small error can cause a denial. To avoid this, use the correct CPT modifier so the payer knows what service was provided.
Example: A surgeon performs a bilateral knee procedure but forgets to add modifier -50. The claim is denied because the payer cannot confirm that both sides were treated.
CO-11
Denial code CO-11 occurs when the diagnosis listed doesn’t justify the procedure performed. Ideally, the ICD code should fully support medical necessity for the CPT code billed. The difficulty for providers is that documentation and coding must line up perfectly, and payers are quick to reject anything that looks mismatched.
Example: A colonoscopy is billed with only a screening diagnosis. Without a supporting risk factor or abnormal finding, the payer denies it for medical necessity.
CO-15
Denial code CO-15 is triggered when a prior authorization number is missing or not valid. The right process is to secure the authorization before care and then include it on the claim. Providers struggle here because every payer has its own rules, and keeping track of approvals across multiple insurers can be a real administrative burden.
Example: An MRI is performed without prior approval. The payer denies the claim because authorization rules were not followed.
CO-16
CO 16 denial code happens when something important is left out, such as demographics, NPI, or required modifiers. A clean claim means every field is correct. But even a small typo or missing detail, which is easy to overlook in a busy practice, can lead to a denial.
Example: A claim is submitted without the provider’s NPI. The payer rejects it until the missing information is corrected.
CO-18
Denial code CO-18 is a duplicate claim denial. This means the payer thinks the same service was billed more than once. A correct submission should be one claim for one service on the correct date. Providers get stuck here because resubmissions or corrected claims often look like duplicates to the payer’s system, creating unnecessary denials.
Example: A provider resubmits a claim with corrected details, but the system flags it as a duplicate of the first submission.
CO-22
Denial code CO-22 shows up when the payer says another insurer should have been billed first. The primary plan must be billed first, then the secondary. Providers struggle because patients may not share full coverage, and insurers often change coordination rules without warning.
Example: A patient has both Medicare and a commercial plan. The claim is sent to Medicare first, but the commercial plan is actually primary, so the claim is denied.
CO-27
Denial code CO-27 happens when the patient’s insurance coverage has expired. Eligibility should be checked before every visit. But lapses or retroactive terminations often slip through, leaving providers with claims unlikely to be paid.
Example: A visit occurs in March, but the patient’s policy ended in February. The claim is denied for lack of coverage.
CO-29
Denial code CO-29 means the claim was filed too late. Each payer sets strict deadlines, sometimes only 30 days. Providers struggle because rules vary, and delays in paperwork, coding, or review can easily push a claim past the limit.
Example: Medicaid requires claims within 90 days. The provider files at 120 days, and the payer denies the claim for late submission.
CO-45
CO 45 denial code indicates the billed charge is higher than what the payer allows. The right approach is to align charges with the contracted fee schedule. Providers struggle because fee schedules often change, and without regular reconciliation, it’s easy to either overbill or accept underpayments.
Example: A provider bills $300 for a service, but the contract allows $200. The payer reduces payment to the allowed amount.
CO-50
Denial code CO-50 is used when the payer decides the service was not medically necessary. Ideally, documentation should clearly prove the need for the service. The challenge for providers is that “medical necessity” is defined differently by each payer, and meeting those varied standards is not always straightforward.
Example: A chiropractor bills for 20 spinal adjustments. After 12 visits, the payer denies the rest as maintenance care.
CO-97
Denial code CO-97 means the service has already been paid for or was bundled into another service. Proper billing should avoid separate charges for bundled items. Providers face problems because bundling rules are complicated, and sometimes it isn’t obvious what counts as included in the package.
Example: A post-operative visit is billed in addition to the surgical package. The payer denies the visit as bundled.
CO-167
Denial code 167 or CO-167 appears when a service simply isn’t covered under the patient’s plan. The correct approach is to bill only covered services to insurance and shift non-covered ones to the patient. The pain point is that benefit exclusions aren’t always clear upfront, and patients often don’t know their own coverage limits.Example: A patient undergoes a cosmetic Botox injection. The payer denies the claim because the service is not covered.
Types of Denial Codes in Medical Billing
In medical billing, payers deny claims for different reasons, and each type needs a different response. Knowing these denial types helps providers determine whether a claim should be corrected, appealed, or written off. Here are the main types explained in simple terms:
Soft Denial Codes
These are temporary denials that can often be corrected and resubmitted. For example, missing patient information or modifier errors will cause a soft denial. The good part is that once the error is fixed, the claim can still be paid.
Hard Denial Codes
These are irreversible denials where payment cannot be recovered under any circumstances. A common example is a claim filed after the payer’s timely filing deadline. Hard denials may also occur when a service is not covered under the patient’s plan or the provider is not credentialed with the payer. Once issued, these denials cannot be corrected or appealed, and the revenue is permanently lost.
Tips To Prevent Denial Claims
Here are practical tips providers can use to prevent common claim denials.
Verify patient eligibility before every visit
Check insurance coverage, active dates, and benefits before the appointment. Many denials (like CO-27 for expired coverage or CO-22 for coordination of benefits) can be avoided with this step. Use automated eligibility tools if possible.
Use accurate and up-to-date codes
Make sure CPT, ICD-10, and HCPCS codes are current. Cross-check that the diagnosis supports the procedure (avoiding CO-11 or CO-50). Coding software and payer-specific guidelines are very useful here.
Apply correct modifiers
Modifiers like 25, 59, 26, and TC must be attached properly. Train staff regularly on modifier use to prevent CO-4 and CO-97 denials.
Complete claim data with no blanks
Confirm that the patient’s demographics, provider NPI, place of service, and required attachments are filled in. Even one missing field can trigger a CO-16 denial.
Submit claims on time
Track payer filing limits and use billing software to flag approaching deadlines. This prevents hard denials like CO-29 that cannot be appealed.
Check claims for duplicates
Use claim scrubbers or workflow checks before resubmitting corrections. This cuts down on CO-18 denials where payers see duplicates.
Hire a trusted medical billing company
All these tips may sound exhausting, but the truth is, denials cost time and revenue. Outsourcing medical billing can cut errors and lower costs. A team like MedCare MSO offers complete denial management, accurate coding, and payer follow-ups to keep your cash flow steady.
Conclusion:
Coding errors cause up to 81% of claim denials. Most billing denials are avoidable. With proper data checks, accurate patient info, and reliable software, your practice can reduce denials and improve claims handling.
Fewer denials mean less stress and a steady income for your practice. With support from a team like MedCare MSO, billing becomes easier, and you can focus more on patient care.