Medical claims get denied for a short list of recurring reasons: the patient's coverage or details don't match the payer's records, the claim is missing information, it was sent twice or too late, the codes don't fit together, or the payer wanted proof of medical necessity or an authorization the claim can't show. Each denial arrives on your remittance with a code that names the cause. Fix the step that produced it, and that denial stops coming back.
It is not a small problem. In a 2024 Premier survey of hospitals and health systems, nearly 15% of claims sent to private payers were denied on first submission, and more than half of those denials were later overturned and paid. That second number matters: many denials are money you are owed, and you only collect it if someone works them.
How to read a denial on your remittance
Every denied line on an electronic remittance (the 835, or ERA) carries three pieces:
- A group code that says who owes the amount: CO (contractual obligation, so the practice can't bill the patient), PR (patient responsibility), OA (other adjustment) or PI (payer-initiated reduction).
- A Claim Adjustment Reason Code (CARC), a number from the national list kept by X12, that says why the line wasn't paid.
- Often a Remittance Advice Remark Code (RARC) that adds the detail, such as which piece of information was missing.
So "CO-16" means a contractual adjustment for reason 16. The group code in front can change with the payer and the situation. The number tells you the cause.
The common denials, code by code
Code | What the payer is saying | Where it usually starts |
|---|---|---|
CO-16 | Information missing or a billing error | Registration, charge entry |
CO-27, CO-26, CO-31 | Not covered on that date, or patient not found | Eligibility |
OA-18 | Exact duplicate claim | Resubmitting without checking status |
CO-29 | Filing time limit expired | Claims sitting unsent |
CO-11, CO-4 | Diagnosis or modifier doesn't fit the procedure | Coding |
CO-97 | Paid as part of another service | Bundling rules |
CO-50 | Not medically necessary, in the payer's view | Documentation, coding |
CO-197 | Authorization or precertification absent | Scheduling |
CO-16: the claim lacks information or has a billing error
This is the catch-all. X12 requires a remark code with it, and the remark code tells you what was missing: a referring provider, a date, a valid NPI, a subscriber detail. Read the remark code before you touch the claim.
Fix at the source: look at which remark codes repeat over a month. If the same field keeps coming back, the gap is in a registration screen or a charge-entry habit, and that is where to fix it.
CO-27, CO-26 and CO-31: coverage and patient details
CO-27 means the service happened after the patient's coverage ended. CO-26 means it happened before coverage began. CO-31 means the payer can't identify the patient as its member, usually because of a wrong member ID, a misspelled name or the wrong subscriber.
Fix at the source: check eligibility before every visit, not only the first one. Patients change jobs, plans and Medicare status between appointments. Scan the card again when the patient says anything changed, and confirm the name and date of birth exactly as the plan has them. Our eligibility verification page has a tool, "What a check before the visit covers", that walks through each item.
OA-18: exact duplicate claim
The payer already has this claim. It usually happens when someone resends a claim because it looks unpaid, without checking whether the payer has it in process.
Fix at the source: check claim status (on the payer's portal or with an electronic status inquiry) before resending anything. If a claim truly needs changing, send it as a corrected claim, not as a new one.
CO-29: the time limit for filing has expired
Every payer sets a deadline for receiving a claim. For Medicare it is 12 months from the date of service. Commercial payers set their own in their contracts, and many are much shorter. A claim denied for late filing is rarely recoverable.
Fix at the source: keep a list of each payer's filing limit, and run a weekly report of visits with no claim sent. A claim that missed the deadline usually sat in a queue nobody owned. Keep the clearinghouse's acceptance report for every claim: if a payer says a claim arrived late when it didn't, that report is your proof on appeal.
CO-11 and CO-4: codes that don't fit together
CO-11 means the diagnosis doesn't support the procedure billed. CO-4 means the modifier doesn't fit the procedure code: the wrong modifier, or one that doesn't belong on that code.
Fix at the source: code from the note, not from habit, and link each procedure to the diagnosis that justifies it. A claim scrubber that checks diagnosis-to-procedure and modifier rules before the claim goes out catches many of these while they are still cheap to fix.
CO-97: paid as part of another service
The payer considers this service included in another one it has already paid. Medicare's rules for which codes bundle together are its National Correct Coding Initiative edits, and many commercial payers follow similar rules.
Fix at the source: when two services really were separate, the documentation has to show it, and the claim needs the right modifier: modifier 25 for a significant, separately identifiable visit on the day of a procedure, or 59 for a distinct procedural service. When they weren't separate, the denial is correct and shouldn't be appealed.
CO-50: not medically necessary
The payer decided the record doesn't show the service was needed. For Medicare, local and national coverage policies list which diagnoses support many tests and procedures.
Fix at the source: check the coverage policy for services that are often questioned, and make sure the note states why the service was needed, in the provider's own words. When the record does support the service, a CO-50 denial is worth appealing with those notes attached.
CO-197: authorization or precertification absent
The plan required an authorization, precertification or notification before the service, and the claim can't show one.
Fix at the source: check authorization requirements when the visit is scheduled, not when it is billed, record the authorization number and its dates, and recheck when the service date moves or the number of visits grows. Some payers accept a retroactive authorization; where they don't, an appeal is the only route.
Where to start this month
You don't need to fix everything at once. Pull the last 90 days of denials and sort them by reason code. Then:
- Take the three codes that cost you the most, in dollars, not count.
- For each, trace five claims back to the step where the problem started.
- Change that step: a registration field, a scheduling check, a coding habit.
- Watch the same codes next month.
To put a number on it first, the denial cost calculator works out what denials cost your practice in a year from your own volume. For running the work week to week, read 5 essential denial management tips.
Where Beeline fits
Beeline is a US company with a dedicated billing team. Our denial management work reads each denial's codes, corrects, resubmits or appeals the claim, and traces repeat denials back to the step that caused them. The page's tool, "The denial decoder", explains what each common denial code means and how a denial with that code is usually worked. If you'd like us to look at your own denials first, start with a free consultation.
Frequently Asked Questions
What is the most common reason medical claims get denied?
It differs by practice and payer mix, but missing or incorrect information (CO-16), coverage and patient-detail problems (CO-27, CO-31) and coding mismatches (CO-11, CO-4) are among the most common. Your own remittances will tell you which ones lead for you.
What's the difference between a denied claim and a rejected claim?
A rejected claim failed the clearinghouse or payer checks before processing, usually for missing or invalid data, so it is corrected and sent again. A denied claim was processed and the payer refused to pay it, so it needs a correction, a reconsideration or an appeal.
How long do I have to appeal a denied claim?
It depends on the payer. For Medicare, the first appeal (a redetermination) must be filed within 120 days of receiving the initial determination. Commercial payers set their own limits, commonly between 30 and 180 days, in their contracts and provider manuals.
Should every denial be appealed?
No. A denial that was correct, such as a true duplicate or a service that really was bundled, should be closed, not appealed. Appeal when the record supports the claim, and fix the claim when the error was yours.
How often should my practice review its denials?
Look at new denials every week so none miss an appeal deadline, and review the trend by reason code and payer every month. That monthly review is where repeat causes show up.