If you're looking for the fastest fixes to the most frequent denials, our top 10 denial codes guide covers those in detail. This guide is the other half: a complete, categorized reference to the top 20 denial codes in medical billing, built for coders and billing specialists who need the full picture, not just the shortcuts.
Already Know the Essentials? Here's the Quick Recap
These 10 are covered in depth, with fixes, in our top 10 guide:
CO-4 (modifier issue), CO-11 (diagnosis-procedure mismatch), CO-16 (missing information), CO-18 (duplicate claim), CO-22 (coordination of benefits), CO-29 (filing deadline expired), CO-50 (not medically necessary), CO-97 (bundled service), CO-119 (benefit maximum reached), and CO-167 (diagnosis not covered).
If any of those is the one you're dealing with right now, that guide has the fix. What follows are the other 10 codes worth knowing, organized by category.
Eligibility and Coverage Denials
CO-204: Service Not Covered Under the Patient's Benefit Plan
The service, equipment, or drug billed simply isn't part of what the patient's plan covers, regardless of medical necessity.
How to prevent it: Confirm covered services during eligibility verification, not after the claim is filed. Plan benefit summaries change yearly, so don't rely on last year's coverage file.
CO-27: Expenses Incurred After Coverage Terminated
The date of service falls after the patient's coverage ended.
How to prevent it: Re-verify eligibility close to the date of service, especially for patients scheduled weeks in advance. Coverage lapses happen more often around plan renewal periods.
CO-96: Non-Covered Charges
The specific charge isn't covered under the patient's plan, separate from the broader benefit plan denial above.
How to prevent it: Check the payer's non-covered services list for the specific CPT or HCPCS code before billing, particularly for newer procedures or add-on services.
Authorization Denials
CO-197: Missing Precertification, Authorization, or Notification
The service required prior approval, and either it wasn't obtained or wasn't documented on the claim.
How to prevent it: Build authorization checks into scheduling, not billing. By the time the claim reaches your billing team, it's often too late to get retroactive approval.
Coding Accuracy Denials
CO-6: Procedure/Revenue Code Inconsistent With Patient's Age
The code billed doesn't align with the patient's age, whether that's an age-banded preventive code, a pediatric versus adult distinction, or a simple date-of-birth error at registration.
How to prevent it: Confirm date of birth at every visit, not just at initial intake, and flag age-restricted codes in your coding software before submission.
CO-199: Revenue Code Inconsistent With Procedure Code
The revenue code and the procedure code on the claim don't align with each other, which triggers an automatic rejection regardless of medical accuracy.
How to prevent it: Cross-check revenue and procedure code pairings against your clearinghouse's edit rules before submission, especially on facility claims where this pairing is checked closely.
Coordination of Benefits Denials
OA-23: Prior Payer's Adjudication Information
This one usually isn't a denial in the traditional sense. It reflects how much a prior payer already paid or adjusted on a secondary claim, so the current payer can calculate what's left.
How to prevent it: Make sure the primary payer's EOB is attached and accurate before submitting to the secondary payer. Most issues here come from incomplete or mismatched prior payment data.
Provider and Patient Identification Denials
CO-31: Patient Cannot Be Identified as Insured
The payer's system can't match the patient on the claim to a covered member on file.
How to prevent it: Verify the patient's name, date of birth, and member ID exactly as they appear on the insurance card, not as they appear in your practice management system.
CO-38: Services Not Provided by Network or Authorized Providers
The rendering provider isn't in the payer's network for this patient's plan, or isn't authorized to bill for the service performed.
How to prevent it: Confirm network status per plan, not just per payer. The same insurer often has multiple networks, and a provider can be in one and out of another.
Contractual Adjustments
CO-45: Charge Exceeds Fee Schedule
This one isn't really a denial. It's the contractual difference between what you billed and what the payer's fee schedule allows, and it's already built into your agreement with that payer.
How to prevent it: There's nothing to fix here; it's expected. The only action worth taking is confirming the adjustment matches your actual contracted rate, in case the fee schedule was applied incorrectly.
Turning This Reference Into Fewer Denials
Knowing the top 20 denial codes in medical billing is a starting point, not the goal. A reference guide tells you what a code means. Preventing the denial in the first place takes it a step further: eligibility verification before the visit, accurate coding at submission, and someone tracking claim status so issues get caught before they age out. That's the core of what our medical coding and billing outsourcing services are built to handle for practices that don't have the bandwidth to chase every denial manually.
If denials are already piling up in your accounts receivable cycle, talk to our team about where the recurring issues are coming from.
