Medical Billing Denial Codes: Complete List With Descriptions and Fixes (2026)

By Dr. Zain Vally  |  Updated September 2026  |  Denial Management

Every claim denial on an ERA carries two pieces of information that determine what you do next. The CARC (Claim Adjustment Reason Code) tells you why the payer denied or adjusted the claim. The group code in front of it tells you who is financially liable: CO (Contractual Obligation) means the provider absorbs the adjustment, PR (Patient Responsibility) means the patient owes, OA (Other Adjustment) covers coordination of benefits and secondary payer logic, and PI (Payer Initiated) flags reductions the payer made on its own, such as pricing corrections.

The table below covers every denial code we have published a fix guide for, grouped by category and linked to the full resolution page. Need to search by code number or keyword? Use our Denial Code Lookup Tool to search all 106 codes in the database.

10 Most Common Denial Codes in Medical Billing

These ten codes account for roughly 75% of all claim denials across payers and specialties, based on Change Healthcare clearinghouse data (2024-2025 aggregate). They are ordered by frequency of occurrence.

1. CO-45: Charges Exceed Fee Schedule/Maximum Allowable~28% of denials

   Billed amount reduced to the payer's contracted rate. Usually routine, but hides underpayments when fee schedules are stale, POS codes are wrong, or multi-TIN contracts carry different rates.
2. CO-4: Procedure Code Inconsistent With Modifier or Missing Modifier~20% combined w/ CO-16

   The procedure code requires a modifier that is missing, or the modifier attached does not match the procedure. Common with modifier 25 on E/M visits and modifier 59 on bundled procedures.
3. CO-97: Bundled/Included Procedure~15% of denials

   The billed procedure is considered part of another procedure performed on the same date. Requires correct use of modifier 59 or X-modifiers (XE, XS, XP, XU) with documentation supporting distinct services.
4. CO-18: Duplicate Claim/Service~8% of denials

   The payer's system flagged this as a duplicate of a previously submitted claim. Can be a true duplicate from resubmission, or a false duplicate when bilateral procedures or multiple units share a code.
5. CO-29: Timely Filing Limit Exceeded~10% of denials

   The claim was received after the payer's filing deadline. Revenue lost to CO-29 is almost never recoverable unless you can prove the original submission date or qualify for a filing exception.
6. CO-22: Coordination of Benefits~5% of denials

   Another payer is primary. The claim must be submitted to the primary insurer first or include the primary's EOB with the secondary submission.
7. CO-26: Expenses Incurred Prior to Coverage~4% of denials

   Service date falls outside the patient's active coverage window. CO means the provider absorbs the write-off. Verify the effective date and rebill to the correct payer.
8. CO-55: Procedure/Billed After Date of Death~2% of denials

   The service date on the claim falls after the payer's recorded date of death for the patient. Often a data-entry error or a lag in death-record updates at the payer.
9. CO-150: Payer Deems Not Medically Necessary~3% of denials

   The payer does not consider the service medically necessary based on the diagnosis code(s) submitted. Requires resubmission with supporting clinical documentation or a peer-to-peer review.
10. CO-21: Charges Exceed Contractual Limitations

    The billed amount or frequency exceeds what the payer contract allows for this service. Different from CO-45 in that this is a contract cap, not a fee schedule reduction.

All Denial Codes: Sortable Reference Table

Click any column header to sort. Use the search box to filter by code number, description, or category. Every code with a fix guide links directly to the resolution page.

Frequently Asked Questions