Common denial codes in medical billing: what each CARC means and who pays
Denial codes are the reason codes on a remittance advice that explain why a payer reduced or denied part of a claim. Group codes show who pays: CO means the practice absorbs it, PR means the patient owes it. Numetix posts each code correctly so write-offs are not mistaken for bad debt.
Quick Answer: What do denial codes mean?
- A claim adjustment reason code, or CARC, states why the payer paid less than billed, and the group code before it says who bears the difference.
- CO means the contract makes the practice absorb it, PR means the patient owes it, and OA and PI mark adjustments that need a closer read.
- Many codes are adjustments, not true denials, so each needs a posting decision: write off, bill the patient, correct, or appeal.
Numetix is the expert-led, AI-powered, human-in-the-loop bookkeeping layer that reads every remittance line and posts it correctly, so contractual write-offs, patient balances, and rebills never end up in the same bucket. This guide lists the 28 codes practices meet most often, in plain language, with the first fix for each.
What are denial codes?
Each remittance advice line can carry a group code, a claim adjustment reason code, and one or more remark codes. The group code says who is responsible, the CARC gives the reason, and the remark code adds detail. X12, the standards body that maintains the lists, publishes the official claim adjustment reason code list, which holds more than 400 reason codes and is updated regularly. This page was reviewed against that list in October 2026.
A denial and an adjustment are not the same thing. A true denial means the payer refused to pay for a service. An adjustment can simply be the contract at work, such as the difference between a billed charge and an allowed amount. Treating every code as a denial sends staff to appeal items that should just be written off.
CO, PR, OA, and PI: Who pays
Group code | Official name | What it means for the books |
|---|---|---|
CO | Contractual obligation | The contract assigns the amount to the practice; do not bill the patient |
PR | Patient responsibility | The patient owes the amount; post it to the patient account |
OA | Other adjustment | Informational or other; read the reason before posting |
PI | Payor initiated reduction | The payer reduced the payment on its own; read the remark code and contract first |
For most reason codes the standard does not fix the group code, so the payer picks it line by line. Always read the prefix printed on the remittance rather than assuming the usual pairing in the table below.
The 28 most common claim adjustment reason codes
Meanings below are written in plain language and are not the official wording. The group code shown is the one most often seen with the reason. Codes with a guide of their own link to it.
Code | Usual group | Plain meaning | First fix |
|---|---|---|---|
PR | Amount applied to the patient deductible | Bill the patient; do not write off | |
PR | Amount applied to the patient coinsurance | Bill the patient; confirm the allowed amount | |
PR-3 | PR | Amount applied to the patient copayment | Collect or bill the copay |
CO-4 | CO | The procedure code does not agree with the modifier on the claim | Correct the modifier and resubmit |
CO | The claim is missing information or has a billing error | Fix the item named in the remark code and resubmit | |
OA-18 | OA | The claim or service is an exact duplicate | Confirm the first claim paid before resubmitting |
CO-22 | CO | Another payer may owe this care under coordination of benefits | Verify other coverage and bill the right payer first |
OA-23 | OA | Shows the effect of what a prior payer paid or adjusted | Post it as information; do not write it off |
CO-24 | CO | The charges fall under a capitation or managed care arrangement | Check the capitation contract before billing |
CO-27 | CO | The expense was incurred after coverage ended | Verify coverage dates; bill the patient or correct payer |
CO | The time limit for filing has expired | Look for proof of timely filing and appeal | |
CO-31 | CO | The patient cannot be identified as the payer's insured | Recheck member ID and eligibility |
CO | The charge is above the fee schedule or contracted amount | Post a contractual write-off | |
CO | The payer decided the service was not medically necessary | Review the coverage policy; appeal with documentation | |
CO-55 | CO | The service is treated as experimental or investigational | Check the policy; appeal only with support |
CO-59 | CO | The service was paid under multiple or concurrent procedure rules | Compare the reduction with the payer multiple-procedure policy |
CO-96 | CO | The charge is not covered, and a remark code explains why | Read the remark code before deciding |
CO | The payment is already included in another service paid | Check bundling edits and modifiers | |
CO-109 | CO | This payer does not cover the claim; send it to the right payer | Bill the correct payer |
CO-119 | CO | The benefit maximum for the period has been reached | Verify the limit; bill the patient if allowed |
CO | The documentation does not support this many services | Check frequency limits before scheduling | |
CO-167 | CO | The diagnosis is not covered | Verify the diagnosis coding; appeal if correct |
CO-170 | CO | Payment is denied when billed by this type of provider | Check provider type and credentials |
CO | Precertification or authorization was absent | Check authorization status; request retroactive review if available | |
CO-204 | CO | The service is not covered under the current benefit plan | Verify the plan; bill the patient if allowed |
CO-236 | CO | The procedure combination is not compatible under coding edits | Review the edit and the modifier |
CO-252 | CO | An attachment or documentation is required | Send the documentation and resubmit |
CO-B7 | CO | The provider was not certified or eligible for this service on this date | Check enrollment and credentialing dates |
What is the CO-27 denial code?
CO-27 means the expense was incurred after the patient's coverage ended. Check the coverage termination date against the date of service. If coverage had truly ended, the claim belongs with the patient or another payer. If the dates are wrong, correct the eligibility record and resubmit.
How to post a denial in the books
Every remittance line ends in one of three outcomes: write off, bill the patient, or fix and resubmit. Take an illustrative $240.00 charge. The payer allows $150.00, so a $90.00 contractual adjustment posts under CO-45. The payer pays $120.00, and the remaining $30.00 is PR-2 coinsurance. The three pieces add up: $90.00 plus $120.00 plus $30.00 equals $240.00.
Reason codes that point to a correctable error, such as CO-16 or CO-252, should return to the billing queue, not be written off. Misclassifying them inflates bad debt, understates accounts receivable, and hides fixable revenue, a pattern covered in common medical billing myths and in the healthcare revenue cycle guide.
How remark codes pair with reason codes
Some reason codes, including CO-16, CO-96, and CO-252, must be accompanied by at least one remark code according to the official list. The remark code names the missing item or the policy behind the adjustment. Our guide to RARC remark codes explains how to read the pair together. The remittance format is described in the CMS Medicare Claims Processing Manual, chapter 22.
Frequently asked questions
Is a denial code the same as a rejection?
No. A rejection means the claim failed front-end edits and never entered adjudication, so it usually needs correction and resubmission. A denial code appears on a remittance after the payer processed the claim. Many reason codes describe adjustments rather than refusals, which is why each code needs a posting decision before anyone starts an appeal.
What is the difference between CO and PR?
CO means contractual obligation: the contract assigns the amount to the practice, so it is written off and not billed to the patient. PR means patient responsibility: the patient owes the amount, such as a deductible, coinsurance, or copayment. The same reason code can appear with either prefix, so read the group code on each line.
Where can I find the official list of denial codes?
X12 publishes the official claim adjustment reason code list and the remark code list on its website. The lists change regularly as codes are added or retired, so a printed copy goes stale. Practices should check the current list before relying on a code meaning, and should keep their own quick reference dated.
Numetix is the expert-led, AI-powered, human-in-the-loop bookkeeping layer that posts every remittance line to the right place. See our bookkeeping services for healthcare practices.
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