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Key Takeaways
- There are three types of denial codes: group code, CARC, and RARC. All three are needed to fully understand why a claim was changed.
- CO means that the provider will pay for the change and won’t charge the patient. PR means that the patient needs to pay the rest.
- The five numbers that billing teams see the most are CO-16, CO-97, CO-45, CO-96, and CO-197.
- Fix the real problem before you send it again, not just the claim. Resubmitting without a fix will still get you turned down.
- Denials that don’t show up as code issues are caused by missing credentials, incorrect NPIs, and start-date errors.
- The official CARC and RARC lists are kept up to date by the X12 group. Several times a year, the X12 group adds, changes, and turns off codes.
- Appeals for Medicare redetermination must be made within 120 days of the first decision.
What are denial codes in medical billing?
There are codes on the Electronic Remittance Advice (ERA) or Explanation of Benefits (EOB) that tell you why a claim was changed or not paid. It has three parts: a group code (CO, PR, OA, or PI), a Claim Adjustment Reason Code (CARC) that says why the adjustment was made, and often a Remittance Advice Remark Code (RARC) that gives more information. The fastest way to fix a denied claim right the first time is to understand all three. See medical billing services for help from professionals.
What Are Denial Codes in Medical Billing?
With the 835 electronic remittance advice, payers send back a standard code whenever they change or refuse to pay a claim. In medical billing, these denial codes aren’t optional. They tell you what’s wrong, who needs to pay, and what needs to happen before the claim is paid.
The X12 standards committee, which works under the Accredited Standards Committee framework, keeps the codes up to date. Most commercial payers use the same system CMS uses to pay Medicare. The list changes all the time. A few times a year, X12 adds new codes and turns off old ones, so the version you’re talking about does matter.
An ERA’s denial line has three slots that work together. The group code is the first thing that tells you who is responsible for paying what. The CARC comes next and explains why the change was made. When there is a RARC, it fills in the blanks that the CARC doesn’t. If you don’t pay attention to any of those three, you’ll have to guess the fix.
Group Codes: CO, PR, OA, and PI
Before looking up a specific code number, the group code tells you the most important thing: who pays.
| Group Code | Full Name | Financial Impact | Can You Bill the Patient? |
| CO | Contractual Obligation | Provider adjusts the amount | No |
| PR | Patient Responsibility | Patient owes the balance | Yes |
| OA | Other Adjustment | Neither payer nor patient (e.g., secondary adjudication) | Situation-dependent |
| PI | Payer-Initiated Reduction | Payer reduces for administrative reasons | No |
CO is the group code that most people use. For example, when a claim gets a CO-45, the charge went over the agreed-upon fee schedule, and the provider writes off the difference. For most payers, billing the patient for a CO denial is against the terms of the contract. On the other hand, PR codes show how much the patient really pays for their deductible, coinsurance, or fee.
CARC vs RARC Codes: What’s the Difference?
You can find out why the adjustment took place in the CARC. The RARC tells you what to do or gives you more information than the CARC.
The numbers on CARCs are CO-16, PR-1, and OA-23. The first letter of a RARC is either M, MA, or N. A payer might send back CO-16 along with the note code MA27, which says that the patient’s birth date is missing. You know there’s a problem with the information, but not which one, without that RARC.
The CARC is like the header, and the RARC is like the editor’s note below it. Read both all the time. If you fix the CARC problem but don’t pay attention to the RARC detail in your resubmission, you will often get the same denial.
Most Common Denial Codes: Top 25 List
| Code | Brief Meaning | Group | Common Cause |
| CO-4 | Modifier missing or inconsistent | CO | Procedure billed without required modifier |
| CO-11 | Diagnosis inconsistent with procedure | CO | Wrong ICD-10 paired with CPT |
| CO-16 | Claim lacks required information | CO | Missing data element; check RARC |
| CO-18 | Exact duplicate claim | CO | Claim submitted twice |
| CO-22 | May be covered by another payer | CO | COB not completed before filing |
| OA-23 | Prior payer adjudication impact | OA | Secondary claim filed without primary EOB |
| CO-24 | Covered under capitation/managed care | CO | Sent to wrong payer type |
| CO-27 | Coverage expired before date of service | CO | Patient was not active on that date |
| CO-29 | Timely filing expired | CO | Claim filed past payer’s deadline |
| CO-45 | Charge exceeds fee schedule | CO | Billed above contracted rate |
| CO-50 | Not medically necessary | CO | Documentation doesn’t support the service |
| CO-96 | Non-covered charge | CO | Service excluded from plan benefits |
| CO-97 | Included in allowance for another service | CO | Bundling; service not separately payable |
| CO-109 | Not covered by this payer | CO | Sent to wrong carrier |
| CO-119 | Benefit maximum reached | CO | Patient hit annual or lifetime limit |
| CO-151 | Does not support these many services | CO | Frequency exceeds plan allowance |
| CO-167 | Diagnosis not covered | CO | Specific ICD-10 excluded from plan |
| CO-197 | Prior authorization absent | CO | Auth not obtained or not attached |
| PR-1 | Deductible | PR | Patient’s deductible applies |
| PR-2 | Coinsurance | PR | Patient’s coinsurance applies |
| PR-3 | Copayment | PR | Patient’s copay applies |
| PR-31 denial code | Patient not identified as insured | PR | Wrong member ID or spelling error |
| PR-204 | Not covered under current benefit plan | PR | Service excluded from patient’s specific plan |
| CO-236 | Procedure combination incompatible (NCCI) | CO | NCCI edit triggered; modifier may resolve |
| CO-252 | Attachment or documentation required | CO | Supporting records not submitted with claim |
Top Denial Codes Explained
CO-97 Denial Code: Bundled Service
CO-97 means the billed procedure is already covered by payment for another service on the same claim or a claim that was just decided. Most of the time, this happens when you bill a small procedure along with a larger one that already covers it. To start fixing it, you need to see if an appropriate unbundling modifier, like modifier 59 or the X modifiers, is present. For help with individual modifiers, see modifiers XU and modifiers XP.
CO-16 Denial Code: Missing Information
CO-16 is the denial that depends on what you do the most because it’s never clear by itself. Always pair it with the RARC. It could be a date of birth, a referring provider NPI, place of service codes, or a taxonomy number. The comment code tells you exactly what is missing. Fix only the part that the RARC names, not everything that comes to mind at once.
CO-96 Denial Code: Non-Covered Charge
This isn’t a rejection that requires an appeal. CO-45 is generated when the billed amount is more than the payer’s contract rate or authorized amount. The provider writes off the difference under the provisions of the contract. This matters during fee schedule inspections. If CO-45 adjustments are extremely large, they may need to renegotiate the agreed-upon rate.
CO-45 Denial Code: Exceeds Fee Schedule
Not really a denial that needs to be appealed in this case. When the billed charge is more than the payer’s agreed-upon rate or allowed amount, CO-45 goes off. Under the terms of the contract, the provider writes off the difference. When fee schedule checks are done, it matters: if CO-45 changes are particularly large, the agreed-upon rate may need to be renegotiated.
OA-23 Denial Code: Secondary Payer Adjustment
OA-23 is the claim adjustment used on secondary-payer claims to show what the primary payer has already paid. It’s not a mistake; it’s an update of the information.” The primary payment plus what OA-23 shows should equal the amount invoiced and any patient obligation. If the math doesn’t work, the primary EOB may have been utilized incorrectly.
CO-252 Denial Code: Documentation Required
CO-252 says the payer wants to see supporting medical documents before they will process the claim. This is typically true for inpatient treatment, certain pricey procedures, and some DME goods. Complete the paperwork as quickly as you can, and keep an eye on the new judgment date. If CO-252 is allowed to go by, you may have a timely submission issue.
CO-197 Denial Code: No Prior Authorization
It throws CO-197 when a service required a permit and either didn’t get one, or it wasn’t linked to the claim. Depends on when it occurs. If the service has not yet been provided, get permission first. If this has already been done, before you file an appeal, be sure the payer will accept retroactive permission for that sort of service.
CO-22 Denial Code: Coordination of Benefits
CO-22 Payer believes another carrier is primary. It triggers when there is no COB information or the payer’s data indicate the patient has several insurance plans. Before the appointment, identify patient eligibility, review main and secondary payer orders, and when charging the secondary payer, resubmit with the primary payer’s EOB attached.
CO-109 Denial Code: Wrong Payer
CO-109 is a routing error. Someone has the claim, but they pay nothing for the patient’s care. Verify the patient’s current insurance information is accurate, discover the proper payer, and refile. Don’t submit an appeal for CO-109 with the incorrect carrier.
CO-29 Denial Code: Timely Filing
There are fewer avenues for appeal when CO-29 kicks in. Most payers will accept a timely appeal if the delay was caused by them or a known system issue on the provider’s side. There’s a lesson upstream: make sure your billing process includes warnings for payers about filing deadlines so this doesn’t pile up.
CO-50 Denial Code: Medical Necessity
The CO-50 form is a paperwork refusal that resembles a denial of coverage. The provider has a claim. The professional proof that it has (or can receive) doesn’t prove that the quantity of service is required. It’s not a generic message; it’s a request based on clinical considerations in the payer’s LCD or coverage policy.
CO-4 Denial Code: Modifier Issue
CO-4 is initiated when a claim lacks a modifier or the modifier on the claim is not valid for the paid procedure. Check if the CPT code requires a modifier, whether the modifier you applied is valid for that CPT code, and if the modifier value and place of service code value match. Add or fix the modifier and resend.
PR-204 Denial Code: Not Covered Under Benefit Plan
The patient is liable for PR-204, and providers don’t receive a tax benefit. This service is not covered under the patient’s current plan. Explain this to the patient at the time of treatment or before the service, since you can typically determine it from the benefits check.
How to Read Denial Codes on an ERA or EOB
Start with the code for the group. That lets you know right away if the change is a write-off (CO) or a patient responsibility (PR). After that, check out the CARC. That puts you in the reason group. You should then look for a RARC, which is usually in the same line or in the comments section. That’s where the actionable detail is.
You’ll see something like this on a normal ERA: group code CO, cause code 16, and comment code MA27. Go from left to right. Do not skip the RARC. When CO-16 shows up, most billers learn to read the RARC first, since CO-16 by itself doesn’t tell you much useful.
How to Fix a Denied Claim: Correct, Resubmit, or Appeal
Which way to go depends on what made them say no. If the original claim had a mistake, like the wrong modifier, wrong date of birth, or no diagnosis code, a corrected claim could work. When the claim is right, and the denial is wrong, you can file an appeal. In between these two is a “reconsideration request,” which asks the payer to review the same information again.
A redetermination request is the first level of appeal for Medicare. It must be sent in within 120 days of receiving the first determination. Commercial payer dates are different for each contract, so look at the payer’s agreement before you write an appeal. Keep the original claim, the ERA, the physician notes, and, if available, the clearance number.
For claims that need an entirely new process, claims reprocessing handles the resubmission workflow.
Denials Caused by Credentialing and Enrollment Problems
Many denials have nothing to do with coding. They have something to do with credentials. The provider wasn’t signed up with the buyer before the service. There is a date gap after a contract change. The claim has the wrong NPI or category code. A provider switched groups but didn’t move enrollments properly.
It’s really annoying that these denials keep happening even though we fixed the code. We need to fix the main enrollment problem first. With proper insurance credentialing, these gaps are found before any claims are submitted, not after a stack of denials builds up.
How to Prevent Denials
Most denials can be avoided. Check your benefits and eligibility before every appointment. Check to see if the procedure and payer have any prior authorization requirements. Keep your CAQH profiles up to date. Keep track of the timely filing windows for each payer by carrier. Every month, check your denial spread by code to find trends early.
The most common medical billing denials post covers denial reasons in more detail, and the guides on how to reduce claim denials and what to do when claims keep getting denied walk through specific prevention strategies by denial type. For behavioral health practices, behavioral health claim denials have specialty-specific coverage.
Conclusion
In medical billing, denial codes aren’t just extra nonsense. They are a structured way to diagnose problems, and claims are paid faster when you read them correctly. Know your group codes so you don’t charge a patient for a CO denial. Review your CARC and RARC together to fix the right issue. Make the change part of the process so that next month’s denial doesn’t happen again.
If you use any of these codes often, a medical billing audit can tell you whether the problem is a one-off or widespread. The denial management services team at Credex Healthcare will look over your denial records by code and build the fix into your process for free.
Frequently Asked Questions
What are denial codes in medical billing?
When a payer changes or refuses to pay a claim, a denial code shows up on the ERA or EOB. They include a group code (CO, PR, OA, or PI), a CARC that explains why, and often a RARC with more detail.
What is the difference between CARC and RARC codes?
The reason a claim was changed is shown on a CARC. A RARC adds extra information and usually says which field is missing or what paperwork is needed. A RARC starts with M, MA, or N.
What does CO mean on a denial?
CO stands for Contractual Obligation. The provider can’t charge the patient the difference because it violates their contract with the payer.
What is the difference between CO and PR denial codes?
As per the contract, CO denials are the provider’s financial responsibility and must be written off. PR denials, which show deductible, coinsurance, or copay amounts, are the patient’s responsibility.
What is the most common denial code?
In most billing situations, CO-16 and CO-97 show up most often. CO-45, CO-50, and CO-197 follow closely behind. The exact distribution depends on the specialty and the type of payer.
How do I fix a CO-16 denial?
Read the RARC that is inside the CO-16. It identifies the missing or incorrect data element. Fix that one part of the claim and send it again. Do not try to guess what is missing.
How long do I have to appeal a denied claim?
For Medicare, you have 120 days from the first notice of determination to ask for a redetermination. In the provider contract, commercial payers set their own dates, which are usually 90 to 180 days. Check each payer’s deal directly to see what they say.
Can credentialing problems cause claim denials?
Yes. If the provider isn’t registered with the payer, has an effective-date gap, or has the wrong NPI or category on the claim, it will be denied. This will look like a coverage issue, but it’s actually a licensing problem. They can’t do that because they have the right insurance credentialing before the bills go out.
