Every year, healthcare organizations grapple with an inconvenient truth: a significant percentage of medical claims are denied on the first submission. The American Hospital Association reports that denial rates remain high nationwide, slowing down cash flow, increasing administrative burden, and creating operational stress for practices of all sizes.
The encouraging reality? Most denials are avoidable.
Denial codes serve as a form of feedback. They explain exactly why a claim wasn’t paid and illuminate the path to resolution. When teams understand the “why” behind denials, they recover revenue faster and prevent issues before they reach a payer’s system.
This guide walks through the most common denial codes practices faced in 2025, explains why they occur, and illustrates them with relevant examples from specialties including ophthalmology, endocrinology, rural health, dermatology, family medicine, behavioral health, oncology, and more.
What Are Denial Codes?
Denial codes are standardized messages that explain why a claim was rejected or reduced by insurance. Instead of leaving your team guessing, they point to the exact issue, missing data, incorrect coding, authorization problems, or medical necessity concerns.
These codes appear on the explanation of benefits(EOB) after the payer reviews a claim and are essential to effective claims management.
Who Issues Denial Codes?
Denial codes come from insurance payers, including:
- Government payers: Medicare, Medicaid, Medicare Advantage
- Commercial payers: UnitedHealthcare, Aetna, Cigna, Blue Cross Blue Shield, and others
Once the codes appear on the remittance, the responsibility to interpret, correct, and resubmit the claim falls to your practice, or your revenue cycle management partner, such as the AMS team.
Most Common Denial Codes in 2025
CO-4: Missing or Incorrect Modifier
A required modifier is missing or incorrect.
Example (Ophthalmology):
A surgeon performs cataract surgery on the patient’s right eye but forgets to add modifier -RT to indicate laterality. The payer denies the claim due to unclear documentation of which eye was treated.
Prevention: Maintain updated modifier guidelines and specialty-specific cheat sheets; double-check laterality modifiers for all eye procedures.
CO-11: Diagnosis Does Not Support Procedure
The diagnosis does not justify the service billed.
Example (Endocrinology):
A CGM sensor placement is billed using a nonspecific fatigue diagnosis rather than a diabetes-related ICD-10, failing medical necessity.
CO-15: Missing or Invalid Prior Authorization
The payer requires authorization, but none is listed or the authorization has expired.
Example (Hematology/Oncology):
A chemotherapy infusion is completed, but the previously approved authorization expired days earlier under new 2026 authorization standards.
CO-16: Missing Claim Information
Some element of required claim data is incomplete or missing.
Example (Rural Health Clinic):
A telehealth encounter is billed with the wrong POS code under updated 2026 Medicare telehealth site rules, requiring correction.
CO-18: Duplicate Claim
The payer believes the service has already been billed.
Example (Dermatology):
A corrected claim for a shave biopsy lacks appropriate frequency coding, making the payer’s system mark it as a duplicate.
CO-22: Coordination of Benefits Issue
Another insurer should have been billed first.
Example (Family Medicine):
A chronic care visit is billed to Medicare, but the patient’s employer-based insurance is primary, leading to denial.
CO-27: Coverage Terminated
The patient’s plan was not active on the date of service.
Example (Behavioral Health):
A therapy appointment occurs after the patient’s insurance terminated mid-month, leading to a denial on submission.
CO-29: Timely Filing Limit Expired
The claim was filed too late to be considered.
Example (Emergency Medicine):
Documentation delays cause a high-acuity visit to be submitted after the payer’s shortened 2026 filing window.
CO-45: Charge Exceeds Fee Schedule
The billed charge is higher than the payer’s allowed amount.
Example (Optometry):
A retinal OCT scan is billed using outdated 2024 fees instead of updated 2026 contracted rates.
CO-50: Service Not Medically Necessary
The payer deems the service unnecessary based on policy.
Example (Telehealth):
A virtual follow-up is billed for a stable chronic condition that the payer now requires in-person evaluation for under 2026 policy updates.
CO-97: Service Bundled Into Another Service
The billed service is considered part of another code.
Example (Oncology):
Hydration after a chemotherapy infusion is billed separately, though it is bundled per 2026 NCCI edits.
CO-167: Non-Covered Service
The patient’s plan does not include coverage for the service.
Example (Dermatology):
A benign mole removal done strictly for cosmetic reasons is denied as a non-covered service.
Types of Denial Codes
Soft Denials
Correctable and reversible.
Examples: missing modifiers, incomplete demographics, COB issues.
Hard Denials
Permanent.
Examples: expired filing limits, non-covered services, provider not in network.
Hard denials represent revenue that cannot be recovered.
How Practices Can Reduce Denials in 2026
✔ Verify eligibility at every visit
Especially critical for specialties with high visit frequency (family medicine, behavioral health).
✔ Use up-to-date 2026 code sets
Review annual updates to ICD-10, CPT, and payer policies.
✔ Apply accurate modifiers
Eye-specific modifiers (RT/LT), oncology therapy modifiers, or dermatology lesion coding are common problem areas.
✔ Submit clean claims
Double-check demographics, NPI, POS, PA numbers, and attachments.
✔ Monitor timely filing deadlines
Keep calendars updated for payers that adjusted deadlines for 2026.
✔ Avoid duplicate submissions
Use appropriate frequency codes and clearly label corrected claims.
✔ Work with an experienced denial management team
If your staff is overwhelmed or struggling to keep up with policy changes, outsourcing can significantly reduce costly errors.
Stop Losing Revenue to Avoidable Denials. Let AMS Strengthen Your Entire Claims Process
If you’re spending too much time troubleshooting denials, you’re not alone. Most practices don’t have the internal bandwidth to keep up with shifting payer rules, new 2026 requirements, and increasing documentation demands.
Applied Medical Systems helps practices:
- prevent denials before they happen
- correct issues quickly
- navigate payer policies
- improve cash flow and revenue stability
Explore how our team can support your practice with full denial management and clean-claim optimization.
Learn more on our Medical Claim Denial Management page.
Or reach out to schedule a consultation.
Fewer denials. More revenue. And more time for patient care.