Denial Code Reference
Plain-English explanations for the 17 CARC denial codes that drive most A/R work. Each page covers what the code actually means, who absorbs the balance, the specific steps to work it, and whether it is worth appealing.
Denial codes tell you what went wrong; the payer's own policy tells you how to fix it. For deadline-driven codes like CO-29 and authorization denials like CO-197, check the payer billing reference for that payer's verified filing limits and appeal windows.
Group CO
Contractual Obligation - the provider absorbs the balance and cannot bill the patient
CO-16
Claim or service lacks required information
CO-18
Exact duplicate claim or service
CO-22
Care may be covered by another payer per coordination of benefits
CO-29
The time limit for filing has expired
CO-45
Charge exceeds the contracted fee schedule amount
CO-50
Not deemed medically necessary by the payer
CO-97
Service is bundled into another service already adjudicated
CO-109
Claim not covered by this payer or contractor
CO-151
Payer deems the information does not support this many services
CO-197
Precertification, authorization, or notification is absent
CO-236
Procedure combination is not compatible with another procedure
Group PR
Patient Responsibility - the balance transfers to the patient
Group OA
Other Adjustment - typically a coordination-of-benefits or crossover adjustment
Denial code frequently asked questions
- What is a CARC denial code?
- A Claim Adjustment Reason Code (CARC) is the code on a remittance advice that explains why a payer adjusted or denied a line. It is paired with a group code - CO, PR, OA, or PI - that determines who absorbs the balance. CARCs are often accompanied by RARC remark codes that add specifics.
- What is the difference between CO and PR denial codes?
- CO means contractual obligation: the provider absorbs the balance and cannot bill the patient. PR means patient responsibility: the balance transfers to the patient. Mistaking a CO code for a PR code and billing the patient is a contract violation.
- Which denial codes are the most common?
- CO-16 (missing or invalid information) is the highest-volume denial in most practices, followed by CO-197 (missing prior authorization), CO-97 (bundled service), CO-18 (duplicate claim), and CO-29 (timely filing expired). CO-29 is typically the most costly because it is rarely overturned.
- Can every denial be appealed?
- No. Some denials are correct adjustments rather than errors - CO-45 records a normal contractual write-off, and PR-1 through PR-3 record ordinary patient cost sharing. Others, such as a bundling edit with modifier indicator 0, cannot be bypassed regardless of documentation. Identifying which denials are genuinely appealable is what makes A/R work efficient.
These are original plain-English summaries written for reference, not the official X12 code descriptions. Payer handling of any code varies by contract - always verify against the remittance advice and your payer agreement.