Common Denial Codes in Medical Billing: What Each One Means and Who Owes the Balance
Open any remittance and you’ll see a row of codes like CO-45, PR-2, CO-97 and OA-18. They look alike, but they mean very different things. Some are real denials you need to fix. Some are normal contract adjustments. Some are amounts you’re allowed to bill the patient.
This guide covers the common denial codes in medical billing that small and mid-size practices see most. For each one you get the official meaning, the usual cause, who owes the balance, and the first thing to try. Every code links to our full guide if you need the step-by-step fix.
How to read a denial code in 10 seconds
Every adjustment on an electronic remittance (the 835 file, or the EOB your payer posts) has three parts:
- Group code. Two letters that say who is responsible for the amount.
- Claim Adjustment Reason Code (CARC). A number (or letter-number) that says why the amount wasn’t paid.
- Remittance Advice Remark Code (RARC). An optional extra note, such as M51 or M127, that gives more detail.
So “CO-197” means group code CO (contractual obligation) plus CARC 197 (authorization absent). Always read the group code first, because it tells you whether the patient can be billed.
Group codes: who owes the money
| Group code | Official name | What it means for you | Can you bill the patient? |
|---|---|---|---|
| CO | Contractual Obligation | The amount is your responsibility under your contract or payer rules | No. Fix it, appeal it, or write it off |
| PR | Patient Responsibility | The amount is the patient’s share (deductible, coinsurance, copay, non-covered service they agreed to) | Yes, under your contract and any signed waiver |
| OA | Other Adjustment | Doesn’t fit CO or PR. Often duplicates or prior-payer amounts | Usually no. Read the reason code |
| PI | Payor Initiated Reduction | The payer reduced payment for a reason outside your contract | No. Review it and appeal if it’s wrong |
If you remember one thing from this page, make it this: never bill a patient for a CO adjustment. That’s the fastest way to turn a billing error into a compliance problem.
Example 1: a real denial
| CPT | Billed | Allowed | Paid | Adjustment | Remark codes |
|---|---|---|---|---|---|
| 99214 | $180 | $0 | $0 | CO-16, $180 | M51, MA130 |
Read it left to right. CO means you can’t bill the patient. 16 means the claim lacks information or has a billing error. M51 narrows it down: “Missing/incomplete/invalid procedure code(s).” MA130 adds that the claim is unprocessable and has no appeal rights. So don’t appeal. Fix the procedure code and send the claim again.
Example 2: not a denial at all
| CPT | Billed | Allowed | Paid | Adjustments |
|---|---|---|---|---|
| 99214 | $180 | $120 | $96 | CO-45, $60 · PR-2, $24 |
Here nothing is wrong. You billed $180, the payer’s allowed amount is $120, and it paid 80% ($96). CO-45 is the $60 contract write-off, which you can’t bill to the patient. PR-2 is the patient’s 20% coinsurance ($24), which you can bill. Many practices waste time “working” lines like this one.
Top 20 denial codes in medical billing: quick reference
| ode | Official meaning (X12) | Usual cause | First fix | Full guide |
|---|---|---|---|---|
| CO-16 | Claim/service lacks information or has submission/billing error(s) | Missing or invalid data: NPI, modifier, date, referring provider | Read the RARC, correct the field, resubmit | CO-16 guide |
| OA-18 / CO-18 | Exact duplicate claim/service | Claim resent before the first one finished processing | Check claim status, don’t resubmit duplicates. Use a corrected claim if a change is needed | |
| CO-22 | This care may be covered by another payer per coordination of benefits | Payer thinks another plan is primary | Verify primary coverage, bill the right payer first | CO-22 guide |
| PR-27 | Expenses incurred after coverage terminated | Patient’s coverage had ended on the date of service | Verify coverage, find new insurance or bill the patient | PR-27 guide |
| CO-29 | The time limit for filing has expired | Claim sent after the payer’s filing deadline | Appeal only with proof of timely filing, otherwise write off | CO-29 guide |
| CO-45 | Charge exceeds fee schedule/maximum allowable or contracted/legislated fee arrangement | Normal contract write-down, usually not a denial | Post the adjustment. Check underpayments against your contract | CO-45 guide |
| CO-50 | Non-covered because not deemed a “medical necessity” by the payer | Diagnosis doesn’t support the service under payer policy | Check documentation and diagnosis codes, then correct or appeal | |
| CO-96 / PR-96 | Non-covered charge(s) | Service isn’t covered under the plan | Check plan benefits and whether the patient signed a waiver | CO-96 & PR-96 guide |
| CO-97 | Benefit included in payment for another service/procedure | Bundling (NCCI edits) or a missing modifier | Check NCCI edits, add a modifier only if it’s truly separate | CO-97 guide |
| CO-109 | Claim/service not covered by this payer/contractor | Sent to the wrong payer or wrong Medicare contractor | Find the correct payer or contractor and resubmit there | |
| CO-151 | Payer deems the information submitted does not support this many/frequency of services | Units or visits exceed the payer’s frequency limits | Check units and policy limits, appeal with records if justified | |
| CO-167 | This (these) diagnosis(es) is (are) not covered | Diagnosis isn’t covered for this service | Confirm the most specific accurate diagnosis code | |
| CO-197 | Precertification/authorization/notification/pre-treatment absent | No prior authorization on file | Request a retro-authorization if the payer allows, otherwise appeal | CO-197 guide |
| PR-204 | This service/equipment/drug is not covered under the patient’s current benefit plan | Plan excludes the service | Bill the patient only if they were told and agreed beforehand | |
| CO-252 | An attachment/other documentation is required to adjudicate this claim/service | Payer needs records to decide | Send the requested documents before the deadline | CO-252 guide |
| CO-4 | The procedure code is inconsistent with the modifier used | Wrong or missing modifier | Correct the modifier and resubmit | |
| CO-11 | The diagnosis is inconsistent with the procedure | Diagnosis and CPT code don’t match | Fix the code pairing from the documentation | |
| CO-B7 | Provider not certified/eligible to be paid for this service on this date | Provider wasn’t enrolled or credentialed on that date | Check enrollment and effective date with the payer | |
| PR-1 | Deductible amount | Patient hasn’t met the deductible | Bill the patient | |
| PR-2 / PR-3 | Coinsurance amount / Co-payment amount | Patient’s share | Bill the patient (collect copays at check-in) |
The top 10 denial codes most practices run into
- CO-16: missing or wrong claim information
- CO-97: service bundled into another one
- CO-197: no prior authorization
- CO-22: another payer may be primary
- CO-29: filing deadline missed
- CO-50: not medically necessary under the payer’s policy
- PR-27: coverage had ended
- CO-252: payer needs records
- OA-18 / CO-18: duplicate claim
- CO-109: sent to the wrong payer
Common denial codes grouped by where they start
Most denials trace back to one of three stages. Fixing the stage stops the denial from coming back.
Front end: before the visit
Eligibility and coverage (PR-27, CO-109, CO-22). These start at check-in. Coverage ended, the claim went to the wrong payer, or another plan should have paid first. Verifying eligibility before every visit, not just the first one, prevents most of them. Our guide to insurance eligibility verification walks through what to check, and the COB guide explains which payer goes first.
Authorization (CO-197). If a service needs prior authorization and there’s none on file, the payer can deny the entire claim. Track which payers require authorization for which services, and attach the authorization number to the claim.
Provider enrollment (CO-B7). The provider wasn’t enrolled with the payer on the date of service. This often happens with new providers who start seeing patients before their effective date. It’s a credentialing problem dressed up as a billing problem.
Middle: coding and claim creation
Bundling (CO-97). The payer considers the service part of another one billed the same day. Sometimes that’s correct. Sometimes the service was truly separate and needed a modifier. The answer is in the documentation, not in adding modifiers by habit.
Medical necessity and diagnosis (CO-50, CO-167, CO-11). The diagnosis on the claim doesn’t support the service under the payer’s policy. Often the documentation supports a more specific diagnosis than the one coded.
Modifier errors (CO-4). The modifier doesn’t fit the procedure, or a required one is missing.
Frequency limits (CO-151). More units or visits than the payer’s policy allows. Check the policy before the visit, especially for therapy, injections and repeat tests.
Back end: submission and follow-up
Missing information (CO-16). The most common “easy” denial. The RARC usually tells you exactly which field is wrong.
Duplicates (OA-18 / CO-18). Usually caused by resubmitting a claim that was still processing. Check claim status first. If something needs to change, send a corrected claim (frequency code 7), not a new one.
Timely filing (CO-29). The claim reached the payer after its deadline. Medicare’s limit is 12 months from the date of service, and commercial limits are often much shorter. These are hard to win unless you have proof the claim was filed on time.
Documentation requests (CO-252). The payer wants records before it decides. Miss the response deadline and a fixable claim becomes a lost one.
Not really denials: payment adjustments
CO-45 is your contract working as expected. You billed your full fee, and the payer paid its allowed amount. You post the difference as a write-off. It only becomes a problem when the payer pays less than your contracted rate, and that’s an underpayment you can appeal.
PR-1, PR-2 and PR-3 are the patient’s deductible, coinsurance and copay. These are amounts to collect from the patient, not errors.
Denials vs. rejections: not the same thing
A rejection happens before the payer processes the claim. The clearinghouse or payer’s front-end check bounces it for a format problem, such as an invalid member ID or a missing NPI. A rejected claim was never accepted, so there’s no remittance and no denial code. Your filing deadline keeps running.
A denial happens after the payer processes the claim. It comes back on the remittance with a group code and a CARC.
Treat rejections as urgent. Fix them the same day, because the payer’s clock never stopped.
Commonly confused denial codes
| These two | The difference |
|---|---|
| CO-22 vs. CO-109 | CO-22: another payer may be primary. Bill them first, then come back. CO-109: this payer doesn’t cover the claim at all. Send it to the right one. |
| CO-96 vs. PR-96 vs. PR-204 | Same idea (not covered), different responsibility. With CO, you can’t bill the patient. With PR, you usually can, if the patient was told and agreed in advance. |
| CO-50 vs. CO-167 | CO-50: the service isn’t medically necessary for this diagnosis. CO-167: the diagnosis itself isn’t covered. |
| CO-16 vs. CO-252 | CO-16: something on the claim is missing or wrong. Fix the claim. CO-252: the claim is fine, but the payer wants records. Send documents. |
| OA-18 vs. CO-18 | Same reason (exact duplicate). X12 says CARC 18 belongs with group code OA, except where state workers’ comp rules require CO. Some payers still send CO-18. |
| CO-45 vs. a real denial | CO-45 is a normal contract write-down with payment. A real denial pays $0 on the line. |
What to do with each denial: decision table
| If the line shows… | Do this |
|---|---|
| CO + a fixable data error (CO-16, CO-4, CO-11) | Correct and resubmit as a corrected claim |
| CO + a judgment call (CO-50, CO-97, CO-151, CO-197) | Appeal with documentation if the record supports the service. Otherwise write off |
| CO + coordination or wrong payer (CO-22, CO-109) | Send to the correct or primary payer first |
| CO-29 with no proof of timely filing | Write off and fix the process that caused the delay |
| CO-252 | Send the requested records before the deadline |
| PR-1, PR-2, PR-3 | Bill the patient |
| PR-27, PR-96, PR-204 | Bill the patient only if they were informed in advance, or find their current coverage |
| CO-45 | Post the adjustment, then compare the paid amount to your contract |
| OA-18 / CO-18 | Don’t resubmit. Check the original claim’s status |
How to prevent the most common denials
Fixing denials one by one keeps your team busy. Fixing the reason they happen makes them stop.
- Check eligibility before every visit, including returning patients, and confirm which plan is primary.
- Track prior authorization rules by payer and service, and don’t schedule until approval is on file.
- Code from the documentation, and check NCCI edits before adding modifiers.
- Scrub every claim for missing fields before it goes out.
- Know each payer’s filing deadline, and send claims within days, not weeks.
- Confirm provider effective dates before new providers see patients.
- Keep a denial log by code, payer and provider, and review it monthly. The same few codes usually cause most of the lost money.
That last step is where most practices fall short. Without a log, every denial looks like a one-off. With one, patterns show up fast: one payer, one provider, one missing modifier.
A simple denial log you can start today
| Date | Patient account | Payer | Provider | CPT | Group + CARC | RARC | Root cause | Action taken | Result |
|---|---|---|---|---|---|---|---|---|---|
| 09/12 | 10482 | Payer A | Dr. B | 97110 | CO-151 | N/A | Units over visit limit | Appealed with notes | Paid 10/02 |
Sample row for illustration.
Sort it by CARC once a month. The top three codes on that list are your biggest fixes. If the same code keeps showing up for the same payer, the fix is a process change, like a new eligibility step or a scrubber rule, not more appeals.
This is also why billing fees matter. A billing company paid a percentage of collections gets paid whether it prevents denials or just reworks them. Our denial management services are built around fixing the cause, for one flat fee. If you run billing in-house, our guide on denial management for small practices shows how to set up a simple tracking process.
Get a free denial audit
Not sure which codes are costing you the most? We’ll review a sample of your recent remittances, group your denials by code and cause, and show you which three changes would recover the most money. You get the report whether or not you work with us.
What are the most common denial codes in medical billing?
The most common are CO-16 (missing information), CO-97 (bundled service), CO-197 (no prior authorization), CO-22 (another payer may be primary), CO-29 (timely filing expired), CO-50 (medical necessity), PR-27 (coverage ended), CO-252 (documentation needed) and duplicate claims (OA-18 or CO-18). CO-45 also appears constantly, but it's usually a contract adjustment, not a denial.
What is the difference between CO and PR denial codes?
CO means contractual obligation: the amount is the provider's responsibility and can't be billed to the patient. PR means patient responsibility: the amount, such as a deductible or coinsurance, can be billed to the patient.
What is a CARC and a RARC?
A CARC (Claim Adjustment Reason Code) explains why a claim or line wasn't paid in full. A RARC (Remittance Advice Remark Code) adds detail, such as which field is missing. They work together with the group code.
