Top Reasons Medical Claims Get Denied and How to Fix Them

I have spent twelve years working denials. Not reviewing them actually working them. Pulling ERAs, reading CARC codes, writing appeals, getting on hold with payer reps, and tracing each denial back to the exact point in the billing process where it was created. After that many years, one thing is clear: the vast majority of denials are not bad luck. They are predictable, and they are preventable.

According to CMS data published in their Medicare FFS Improper Payment Report, improper payments which include both overpayments and underpayments resulting from claim errors exceeded $31 billion in a single Medicare FFS reporting period. For commercial payers, the American Medical Association’s 2023 Insurance Billing Survey found that physicians spend an average of 15.1 hours per week on prior authorization and billing tasks, a significant portion of which is driven by denial-related rework.

Below are the ten most common reasons medical claims get denied the denial code behind each one, and the specific fix your billing team needs to execute. At the end of each section I have added a prevention protocol, because fixing a denial after the fact recovers some revenue preventing it from happening in the first place recovers all of it.

1. Insurance Eligibility Not Verified Before the Visit

This is the single highest-volume, most preventable denial source I encounter. Eligibility errors account for approximately 23% of all initial claim denials, according to MGMA benchmarking data. A patient whose insurance has lapsed, whose plan has changed, or whose provider is no longer in-network will generate a denial three to five weeks after the visit long after the staff member who registered them has forgotten the encounter.

Denial codes: CO-96 (non-covered service), PR-1 (deductible), CO-4, CO-27 (expenses incurred after coverage terminated).

The fix: Pull real-time eligibility through your clearinghouse before every visit not at initial registration, before every appointment. Eligibility can change monthly, weekly, or even daily for patients on employer-sponsored plans or Marketplace coverage.

✔  Prevention Protocol Eligibility ✔  Verify eligibility 24–48 hours before every scheduled appointment, not just at intake ✔  Confirm in-network status of the rendering provider specifically, not just the practice ✔  Check plan-specific limitations: mental health visit caps, specialty referral requirements, authorization thresholds ✔  Document the verification result and the verification date in the patient account

2. Missing or Invalid Prior Authorization

Prior authorization denials are the most expensive category of denial in dollar terms, because the service has already been delivered when the denial arrives. According to the AMA’s 2023 Prior Authorization Survey, 94% of physicians report that prior authorization causes treatment delays, and 30% say a prior auth delay has led to a serious adverse event for a patient

Denial codes: CO-197 (precertification absent), CO-57 (prior auth required).

The fix: Confirm authorization requirements for every scheduled procedure against the patient’s specific plan not just payer, but plan. The same insurance company can have different authorization requirements across different employer plans. If the service was urgent and auth was not obtainable in advance, submit a retroactive authorization request with clinical documentation immediately after the visit.

✔  Prevention Protocol Prior Authorization ✔  Check authorization requirements at the plan level, not just the payer level ✔  Document the auth number in the patient account before the patient is seen ✔  Set calendar reminders for auth expiration dates most authorizations expire in 30–90 days ✔  For urgent/emergent cases, initiate retroactive auth request within 24 hours of service

3. Claim Submitted After the Timely Filing Deadline

Timely filing denials are entirely administrative and entirely preventable and they are the most frustrating kind of denial to explain to a physician because there is often nothing clinically wrong with the claim. The payer simply received it too late.

Denial code: CO-29 (timely filing limit expired). For a full breakdown of timely filing limits by payer and how to appeal with proof of timely filing, see our dedicated guide: CO-29 Denial Code: Timely Filing Expired.

The fix: If you have clearinghouse confirmation that the claim was submitted within the filing window, submit a timely filing appeal with the clearinghouse report as proof. If the claim was genuinely submitted late, the denial is usually final the only exception is documented payer error or system outage during the filing window.

✔  Prevention Protocol Timely Filing ✔  Know the filing deadline for every payer you bill limits range from 90 days (some commercial payers) to 365 days (Medicare, most Medicaid programs) ✔  Set practice management system alerts when claims approach 30 days without a response ✔  Never let a claim sit unworked past 60 days, regardless of payer ✔  Keep clearinghouse submission reports for every claim as your proof of timely filing

4. Incorrect or Missing Patient Information

Wrong date of birth. Misspelled name. Incorrect insurance ID. Wrong group number. These errors look minor but they produce claim rejections or denials immediately and require manual correction and resubmission adding days or weeks to your payment cycle. Registration errors are the source of roughly 23% of all claim denials per MGMA, placing them alongside eligibility failures as the highest-volume front-end problem.

Denial code: CO-16 (missing or incomplete information), often accompanied by RARC N-codes specifying the exact field.

The fix: CO-16 denials almost always come with a RARC code that identifies the specific missing or incorrect element. Read the RARC first it tells you whether the issue is a wrong insurance ID, a missing date of birth, a name mismatch, or an incomplete address. Correct the specific field, resubmit, and update the patient account so the same error does not recur on future claims. For a complete breakdown see CO-16 Denial Code: Missing or Incomplete Information.

✔  Prevention Protocol Patient Information ✔  Collect a copy of the insurance card (front and back) at every visit plans change ✔  Verify subscriber name exactly as it appears on the card, including middle name or initial if present ✔  Run a clearinghouse eligibility check to confirm the insurance ID is active and matches ✔  Train front desk to flag any discrepancy between what the patient states and what the eligibility check returns

5. Services Bundled by NCCI Edits

National Correct Coding Initiative (NCCI) edits automatically bundle certain pairs of CPT codes together. When two codes that hit an NCCI edit are billed on the same claim, the payer denies or reduces payment on the second code unless a valid modifier proves they were genuinely separate and distinct services.

Denial code: CO-97 (benefit for service included in payment for another service).

The fix: Review the NCCI edit for the code pair. If the services were genuinely separate different anatomical site, different session, or different structure append the correct modifier (modifier 59 or the appropriate X-series modifier: XE, XS, XP, XU) and resubmit with documentation that supports the distinction. If the services were not genuinely separate, the bundle is correct and the claim should be written off. Full resolution steps are at CO-97 Denial Code: Bundled Service Explained.

✔  Prevention Protocol NCCI Edits ✔  Run every claim through a clearinghouse that scrubs NCCI edits before submission ✔  Train coders to check the CMS NCCI edit table when billing two procedure codes on the same date ✔  Never apply modifier 59 without documentation that supports a genuinely distinct service ✔  Conduct quarterly coding audits to identify recurring NCCI-related errors before they reach the payer

6. Medical Necessity Not Established

Payers deny claims for medical necessity when the diagnosis codes on the claim do not, in their view, justify the service billed. This is one of the most appealable denial types but it requires strong clinical documentation to overturn. According to CMS local coverage determination (LCD) guidelines, payers are required to specify which diagnosis codes do and do not support coverage for a given procedure.

Denial code: CO-50 (non-covered service, not deemed medically necessary).

The fix: Review the payer’s LCD or NCD for the procedure code billed. If the clinical documentation supports medical necessity but the wrong diagnosis code was selected, correct the ICD-10 code and resubmit. If the correct diagnosis was used and the service is genuinely clinically necessary, appeal with supporting clinical documentation the treating physician’s notes, referral records, and prior treatment history.

✔  Prevention Protocol Medical Necessity ✔  Reference payer LCDs when selecting diagnosis codes for high-risk procedure types ✔  Ensure clinical documentation explicitly states the indication for the service ✔  For recurring services (therapy, injections, monitoring), maintain a documented treatment plan that justifies ongoing necessity ✔  When in doubt, query the physician for additional documentation before the claim is submitted

7. Duplicate Claim Submission

A duplicate claim denial occurs when an identical or near-identical claim has already been submitted and processed or when the payer’s system flags the new claim as a potential duplicate even if it was not intentionally resubmitted. Duplicate claim denials account for 2–5% of total denials at most practices, but they are uniquely frustrating because the claim is often legitimate it was a resubmission of a previously denied claim that inadvertently hit the payer’s duplicate detection.

Denial code: OA-18 (duplicate claim).

The fix: Verify whether the original claim was processed and paid. If it was, no action is needed. If the original was denied and this is a legitimate resubmission, submit with the note ‘This is not a duplicate original claim denied on [date], denial code [X]. Corrected claim submitted.’ Include the original claim number in the resubmission.

✔  Prevention Protocol Duplicates ✔  Before resubmitting any denied claim, check your practice management system to confirm the original claim status ✔  Use your PMS’s claim tracking to flag corrected claims vs. original submissions ✔  Include original claim reference numbers when resubmitting corrected claims

8. Provider Not Credentialed With the Payer

A rendering provider who has not completed payer enrollment will have every claim denied not because the care was wrong, but because the payer has no record of them as a participating provider. This is especially common with newly hired physicians where the practice assumed credentialing was progressing faster than it actually was.

Denial code: CO-170 (payment denied healthcare not performed by required provider).

The fix: Confirm the provider’s enrollment status directly with the payer. If credentialing is still in process, check whether the payer allows retroactive billing back to the application date upon approval many commercial payers do, Medicare does under certain conditions. If the claim cannot be retroactively billed, this revenue is typically lost. The prevention is starting provider credentialing well before the provider’s first patient ideally 90 days in advance.

✔  Prevention Protocol Credentialing Gaps ✔  Start payer enrollment 90 days before a new provider’s intended start date ✔  Maintain a credentialing status tracker updated weekly for all providers in process ✔  Confirm retroactive billing policy with each payer before the provider sees their first patient ✔  Do not bill under another provider’s NPI as a workaround this creates False Claims Act exposure

9. Incorrect Modifier Usage

Modifier errors are the category that most directly reflects coding quality. Wrong modifier, missing modifier, modifier applied without supporting documentation each generates a specific denial that requires manual review and correction. In my audit work, modifier errors are the most common finding in practices that have never had a formal coding review.

Denial codes: CO-4 (modifier inconsistent with procedure), CO-B7 (provider not certified for procedure).

The fix: Review the payer’s modifier policy for the specific code combination. Resubmit with the correct modifier and documentation that supports its use. A systematic modifier audit even reviewing just 20 random claims per provider almost always surfaces recurring patterns. Our full breakdown of modifier usage rules is in our modifier 59 guide, which covers the X-series modifiers and the most common modifier misapplication scenarios.

✔  Prevention Protocol Modifiers ✔  Conduct a modifier audit on a random 20-claim sample per provider each quarter ✔  Keep a payer-specific modifier policy reference available to all coders ✔  Never apply modifier 25 or 59 automatically documentation must support each use ✔  Review clearinghouse scrub results before release many modifier errors are caught at scrub

10. Coordination of Benefits (COB) Errors

When a patient has more than one insurance plan, claims must be submitted in the correct order primary payer first, then secondary. A claim submitted to the secondary payer without the primary payer’s EOB, or submitted to the wrong payer as primary, will generate a COB-related adjustment or denial.

Denial codes: CO-22 (payment adjusted for other coverage), OA-23 (COB adjustment).

The fix: Identify the correct primary and secondary payer Medicare Secondary Payer (MSP) rules govern situations where Medicare is involved; for commercial-commercial COB, the birthday rule typically applies. Submit the primary claim first, obtain the EOB, then submit to the secondary with the primary’s EOB attached.

✔  Prevention Protocol COB ✔  Ask about secondary insurance at every visit COB situations change ✔  Know Medicare Secondary Payer rules if any of your patients are Medicare-eligible and have employer or VA coverage ✔  Store primary payer EOB in the patient account before submitting to secondary ✔  Flag COB patients in your PMS to prompt the correct claim submission order

How to Turn Denial Data Into a Prevention System

The ten denial reasons above cover the majority of claim denials at most U.S. physician practices. But knowing what they are is different from knowing which ones are hitting your practice, at what frequency, and which upstream process is generating them.

The practices that maintain denial rates below 4% well below the national average of 5–17% depending on payer and specialty do not just work denials faster. They analyze denial patterns by code, by provider, by payer, and by service type, then trace each pattern to its root cause in registration, coding, authorization, or billing workflow.

That root-cause approach is the first thing our denial management team does for every new client engagement not just working the queue, but identifying which of the ten categories above is generating your denials, at what volume, and what the upstream fix looks like. Full process is detailed in our guide: The Denial Management Process in Medical Billing.

“We were writing off about $28,000 per month in denials just treating them as a cost of doing business. Globill identified that 60% of it was timely filing on a single payer where our clearinghouse was submitting to the wrong payer ID. One workflow fix. The write-offs stopped.” Multispecialty Group, New Jersey (6 providers)

The Bottom Line on Claim Denials

None of the ten denial reasons above are inevitable. Every one of them has a specific fix, and more importantly, a specific prevention protocol. The practices that treat denials as data rather than as paperwork consistently collect more revenue per claim, carry lower AR days, and write off less at year end.

If your practice is seeing recurring denials in any of the categories above and you want to know exactly which upstream process is generating them, schedule a free denial analysis with Globill Medical Resources LLC. We will pull your top denial codes by frequency and dollar value, identify the root cause for each, and tell you exactly what the fix looks like before you spend another month writing off recoverable revenue.

Frequently Asked Questions About Claim Denials