Every practice knows the feeling. A claim goes out clean. Weeks pass. Then the remittance advice comes back with three letters and a number that ruin your day: CO-16, CO-97, and CO-29. Whatever it is, it means the money isn’t coming yet, and maybe not at all.
Medical claim denials are not a small problem anymore. Initial denial rates now sit around 11.8% industry-wide, and more than 4 in 10 providers report denial rates above 10%. That’s up sharply from just a few years ago, when 30% of providers said the same thing. Every one of those denials has a code attached to it. And a handful of codes show up again and again, quietly draining revenue from practices that never quite figure out why.
Read the blog to explore the primary denial codes in medical billing and the most effective ways to prevent claim denials.
What Is a Claim Denial in Medical Billing?
Healthcare claim denials happen when an insurance payer receives a claim, processes it, and decides not to pay. This is different from a rejection, which happens earlier. A claim rejection in medical billing means the claim never even made it into the payer’s adjudication system. It bounced back because of a formatting error, a missing field, or a clearinghouse issue.
A medical billing denial means the payer looked at the claim and made a decision, and that decision was no. A denied claim usually needs a real fix: a corrected code, added documentation, or a formal appeal.
Recent data shows hospitals lose an average of around $5 million a year to denials, which works out to roughly 5% of net patient revenue. At the national level, estimates put total denied claims at somewhere near $262 billion a year. Reworking each one costs money too—anywhere from $25 to $181 per claim. Medical claim resubmission depends on the source and the complexity of the case.
What Is a Denial Code in Medical Billing?
A denial code, formally called a Claim Adjustment Reason Code (CARC), is a standardized code that a payer attaches to a claim to explain why the payment was reduced or denied. These codes are maintained centrally so that every payer in the country uses the same numbering system. It means a CO-16 from Aetna means the same basic thing as a CO-16 from Medicare.
Codes usually show up with a prefix, called a group code, that tells you who’s financially responsible for the balance. CO means Contractual Obligation—the provider agreed to this adjustment under their payer contract, and the patient can’t be billed for it. PR means Patient Responsibility—like deductibles, copays, and coinsurance, which can be passed to the patient. OA covers other adjustments that don’t fit neatly into either bucket.
Alongside the CARC, you’ll often see a Remittance Advice Remark Code (RARC), which adds detail. If CO-16 tells you something is missing, the RARC tells you what. Reading only the CARC and ignoring the RARC is one of the most common reasons billers waste time guessing at a fix instead of just applying it.
The Top 10 Denial Codes and What They Mean
The top claim denials codes in medical billing include eligibility issues, missing data, duplicate submissions, non-covered services, and authorization failures.
CO-16 — Missing or Incomplete Information
CO-16 is the single most common denial code in medical billing.
What is the CO-16 denial code?
A CO-16 denial code means the claim is missing a required field, or something on it doesn’t match what the payer has on file. This could be a missing NPI, an incorrect patient ID number, a blank date of birth, or a mismatched insurance ID.
When does it happen?
The CO-16 denial code in medical billing happens most often on high-volume days. When front-desk staff are rushing registration, or when a patient’s insurance information changed and nobody caught it. It also shows up when EHR fields auto-populate incorrectly and nobody double-checks before the claim goes out.
How to prevent CO-16 denial code?
Prevention starts at registration. Verify every field against the insurance card and the payer’s own eligibility system, not just what’s already in the chart. A claim scrubber that flags blank or mismatched fields before submission catches most CO-16s before they ever leave your office.
CO-109 / CO-27 — Eligibility or Coverage Expired
This kind of claim denial happens because the insurance plan was inactive on the date of service.
What is the CO-109 denial code?
CO-109 means the claim was sent to a payer that isn’t responsible for it. It usually happens because the patient’s coverage had already ended, changed, or moved to a different plan on the date of service.
What is the CO-27 denial code?
CO-27 is closely related and specifically points to expired coverage. A common real-world version of this: a claim goes to traditional Medicare, but the patient actually enrolled in a Medicare Advantage plan, so the wrong payer gets billed entirely.
When does it happen?
This happens when eligibility isn’t checked at every single visit, not just at the first one. Coverage lapses, plans change, and employer-sponsored insurance ends more often than people expect.
How to prevent CO-109 / CO-27 denial code?
The fix is real-time eligibility verification, done at every visit, not just intake. Confirm the payer, the plan, and the effective dates before the appointment even happens, and keep payer ID information current in your billing system.
CO-18 — Duplicate Claim
This usually happens by mistake when the same service was sent to the payer more than once.
What is the CO-18 denial code?
CO-18 means the payer already has a claim on file for the exact same service, same patient, same provider, and same date. It’s their way of saying, “We’ve seen this already.” See our full CO 18 denial code guide for fixes.
When does it happen?
It usually happens by accident. A claim gets resubmitted before the original has finished processing. A clearinghouse glitch sends it twice, or two departments bill the same encounter without realizing it. Legitimate repeat services on the same day, done without the right modifier, can also trigger this by mistake.
How to prevent the CO-18 denial code?
Prevention comes down to visibility. Track claim status before resubmitting anything, and never resend a claim just because you haven’t heard back yet. Give it time to clear the system first. If a service really was repeated on purpose, use the correct modifier so the payer can tell the difference.
CO-197 / CO-15 — Lack of Prior Authorization
In this denial code, the service was performed before getting required pre-approval.
What is the CO-197 / CO-15 denial code?
CO-197 and CO-15 both point to the same root problem: the service needed pre-approval, and it either wasn’t obtained, wasn’t obtained in time, or doesn’t match what was actually performed. This is one of the costliest denials to fix after the fact, because most payers won’t retroactively approve a service that’s already been delivered.
When does it happen?
It happens most in specialties like imaging, surgery, and certain high-cost medications, where payers have expanded prior authorization requirements. It also happens when a scheduled procedure changes at the last minute and nobody updates the authorization to match.
How to prevent the CO-197 / CO-15 denial code?
The only real prevention is discipline on the front end. Confirm authorization requirements before scheduling, not after. Build a payer-specific authorization checklist into your workflow. If a procedure changes, update the authorization the same day.
CO-11 — Diagnosis Inconsistent with Procedure
The CO-11 denial code in medical billing shows the ICD-10 code does not match or support the CPT procedure code.
What is the CO-11 denial code?
CO-11 shows up when the ICD-10 diagnosis code billed doesn’t logically support the CPT procedure code. Payers use coverage determinations that link specific diagnoses to specific procedures, and if the pairing isn’t on that approved list, the claim gets kicked back.
When does it happen?
This happens when coders default to a general or outdated diagnosis code instead of the most specific one available, or when clinical documentation doesn’t clearly connect the diagnosis to the treatment performed.
How to prevent the CO-11 denial code?
Prevention means training coders to code to the highest level of specificity and cross-checking diagnosis-procedure pairs against payer coverage policies before submission. When in doubt, the clinical note should make the connection explicit so the coder has something concrete to work from.
CO-50 — Medical Necessity
When the payer feels the clinical notes do not justify the provided treatment.
What is the CO-50 denial code?
CO-50 means the payer doesn’t believe the clinical documentation justifies the treatment billed. This is judged against the payer’s Local Coverage Determination (LCD) or National Coverage Determination (NCD), which spell out exactly what conditions and documentation support a given service.
When does it happen?
It happens when documentation is thin, generic, or doesn’t clearly explain why this particular treatment was necessary for this particular patient. Medical necessity denials are also expensive—the average dollar amount tied to a single one has climbed sharply in recent years, making this one of the higher-stakes codes on this list.
How to prevent the CO-50 denial code?
Prevention comes down to documentation quality. Clinical notes need to spell out symptoms, prior treatments tried, and the specific reasoning behind the chosen treatment. Coders and clinicians should be checking notes against LCD/NCD criteria before the claim goes out, not after a denial arrives.
CO-96 — Non-Covered Services
In this medical claim denial, the patient’s specific policy does not include the billed care.
What is the CO-96 denial code?
CO-96 means the specific service billed simply isn’t part of the patient’s plan benefits. This is different from CO-50—it’s not about whether the treatment was medically justified. It’s about whether the plan covers that category of service at all.
When does it happen?
It happens when a service is assumed to be covered because it’s covered under most plans, without actually checking this patient’s specific policy. Cosmetic procedures, certain therapies, and some preventive services are common triggers.
How to prevent the CO-96 denial code?
The fix is benefit verification, not just eligibility verification. Confirm the specific service is a covered benefit under this patient’s plan before treatment. Let the patient know upfront if it isn’t, so there are no surprises later.
CO-29 — Timely Filing Limit Exceeded
It occurs when the claim arrives past the payer’s strict deadline.
What is the CO-29 denial code?
CO-29 means the claim arrived after the payer’s deadline for submission. These deadlines vary a lot—anywhere from 90 days to a full year depending on the payer. Once the window closes, this is one of the few denials that’s almost never appealable.
When does it happen?
It happens when claims sit in a queue too long, when back-end staff are backlogged, or when a claim bounces back for correction and the resubmission clock isn’t tracked closely enough.
How to prevent the CO-29 denial code?
Prevention is entirely about tracking. Know each payer’s specific filing deadline, not a generic assumption, and build alerts into your billing system for claims approaching that window. Treat any claim over 30 days old without payer response as a priority follow-up, not a routine one.
CO-22 — Coordination of Benefits
This kind of claim denial happens when the primary and secondary insurance details are mixed up or missing.
What is the CO-22 denial code?
CO-22 happens when a patient has more than one insurance plan and the payer can’t tell which one is primary and which is secondary, or the claim was sent to the wrong one first.
When does it happen?
It happens most with patients who have coverage through a spouse’s employer plan alongside their own or with older patients who have both Medicare and a supplemental plan. If this information isn’t updated at every visit, the payer’s system won’t know which claim to process first.
How to prevent the CO-22 denial code?
Prevention means asking about other insurance at every visit, not just at intake, and confirming the coordination of benefits order directly with each payer when a patient reports multiple plans.
CO-97 — Bundling Issues
When the separate codes were billed for medical services, they should be grouped into one single code.
What is the CO-97 denial code?
CO-97 means the payer considers one or more billed procedures to already be included in the payment for another procedure billed the same day. In other words, you billed separately for something the payer expects to be bundled into a single code.
When does it happen?
This happens when National Correct Coding Initiative (NCCI) edits aren’t checked before submission, or when a procedure that’s genuinely separate and distinct isn’t marked with the appropriate modifier to show that.
How to prevent the CO-97 denial code?
Prevention means running claims through NCCI edit checks before submission and applying modifiers like -59 or the X-series modifiers correctly when two procedures really were distinct, not just billing them side by side and hoping for the best.
What Causes Medical Billing Denials?
Some of the top reasons for medical billing denials are detailed here:
Registration and Eligibility Errors
A wrong policy number, an outdated address, or a name that doesn’t match what’s on file with the payer—these small errors are behind a huge share of denials.
Coding and Documentation Mistakes
Mismatched diagnosis and procedure codes, missing modifiers, and clinical notes that don’t clearly support the treatment given all fall here.
Authorization and Policy Rules
Payers keep expanding what requires prior approval, and practices that don’t keep pace get hit with CO-197 and CO-15 denials.
Administrative and Timing Issues
Late submissions, duplicate claims, and unresolved coordination-of-benefits issues fall here.
How to Prevent Medical Billing Denials
Prevention works in layers, and each layer catches something the one before it might miss.
Ensure Front-End Accuracy
Verify eligibility and benefits at every single visit, not just the first one. Collect and confirm patient demographic data carefully at check-in. Secure prior authorizations before the service is scheduled, and confirm them again if anything changes.
Avoid Coding and Documentation Errors
Match every diagnosis code to the procedure it supports, and code to the highest level of specificity available. Keep coding staff updated on payer-specific policy changes and LCD/NCD criteria, since these shift regularly. Apply modifiers correctly and consistently, especially for bundled or repeated services.
Optimize Back-End Workflow
Run every claim through a scrubber before it leaves the building, checking for missing fields, mismatched codes, and NCCI edit conflicts. Monitor filing deadlines closely, payer by payer, since they aren’t standardized. Track denial trends over time by payer and by code, because patterns usually point straight to the root cause, whether that’s a training gap or a specific payer’s shifting rules.
How Does Denial Management Help Reduce Medical Billing Denials?
Denial management isn’t just about fixing individual claims. Professional denial management services help in building a feedback loop. A good denial management process tracks every denial by code, by payer, and by root cause, so patterns show up before they become expensive habits.
Practices that actively manage denials—instead of just reworking them one at a time—tend to see fewer repeat denials over time. It helps to get faster turnaround on appeals and a much smaller share of claims that get written off entirely.
Conclusion
Denial codes aren’t random. They’re the payer’s way of telling you, in a very specific language, exactly what went wrong. The top 10 denial codes covered here are CO-16, CO-109/CO-27, CO-18, CO-197/CO-15, CO-11, CO-50, CO-96, CO-29, CO-22, and CO-97. Remember every single one of them is preventable with the right front-end checks, documentation habits, and back-end tracking. If denials are draining your revenue, panaHEALTH’s medical billing services and revenue cycle management services prevent them at the front end, and our denial management services recover what is already denied.
FAQs
Start by reading the CARC and RARC codes together to understand exactly what the payer flagged. Compare the claim against the patient’s chart, insurance information, and any authorization on file. If the issue is a clerical error, correct it and resubmit as a clean claim. If it involves medical necessity or coverage disputes, gather supporting documentation and file a formal appeal within the payer’s deadline.
Prevention works best in layers: verify eligibility and benefits at every visit. Collect accurate patient data at registration, secure prior authorizations before scheduling, code claims to the highest specificity with proper documentation, scrub claims before submission, and track denial trends by payer.
Review the denial reason carefully and confirm it’s actually appealable—some, like most timely filing denials, generally aren’t. Gather the clinical documentation, coverage policy references, and any other evidence that supports the claim. Write a clear appeal letter tying that documentation directly to the payer’s own coverage criteria, and submit it within the payer’s specific appeal window, since these deadlines vary by payer and by plan.
