What are common reasons for medical claim denials and how to appeal them?

What are the most common reasons medical claims are denied?

Most medical claims are denied for five fixable reasons: eligibility and coverage errors, coding and modifier errors, missing or invalid prior authorization, duplicate claims and late filing. In our claim audit, eligibility and coverage errors caused 24% of denials, coding and modifier errors caused 21%, and missing or invalid prior authorization caused 17%. Duplicates made up 9% and timely filing 6%. Everything else, from coordination of benefits to non-covered services, made up the last 23%.

The chart splits denials across 61,400 audited claims: 24% eligibility and coverage, 21% coding and modifiers, 17% prior authorization, 9% duplicates, 6% timely filing and 23% all other reasons.

Medical claim denial reasons, with what each one looks like on the remit

• Eligibility and coverage. Coverage ended, the plan changed, the member ID is wrong, or another payer is primary. Shows up as CARC 27 (expenses after coverage ended), 22 (coordination of benefits) or 109 (send to the correct payer).
• Coding and modifiers. The diagnosis does not support the procedure, a modifier is missing or wrong, or a bundled service was billed separately. Shows up as CARC 11, 4 or 97.
• Prior authorization. No authorization, an expired one, or one that does not match the service billed. Shows up as CARC 197 or 15.
• Medical necessity. The payer decided the service was not necessary under its policy. Shows up as CARC 50. These need clinical records, not a corrected claim.
• Missing information. A field is blank or invalid, such as the referring NPI. Shows up as CARC 16 with a remark code that names the missing item.
• Duplicates. The same claim was sent twice. Duplicate claim denials made up 9% of denials, mostly from resubmitting instead of correcting.
• Timely filing. The claim reached the payer after its deadline. Medicare allows one calendar year from the date of service, and many commercial contracts allow far less.

Denials concentrate. In the average practice we reviewed, the top three denial reasons accounted for 58% of denied dollars. Fix three root causes and you remove more than half of the problem.

What is the difference between denied claims and rejected claims?

A rejected claim never entered the payer’s system, so you fix it and resubmit it. A denied claim was processed and refused, so you correct it, request a reopening or appeal it. Rejections come back from the clearinghouse or the payer’s front-end edits, usually within a day or two, for problems like an invalid member ID or missing NPI. Denials come back on the 835 electronic remittance advice (ERA) with a claim adjustment reason code (CARC).

The difference matters because the wrong fix creates a new denial. Resending a denied claim as a new claim triggers a duplicate denial. Appealing a simple clerical error to Medicare wastes a level of appeal: CMS requires minor errors, such as a transposed code, to go through a reopening, not an appeal.

What do claim denial codes like CO-16, CO-50 and CO-197 mean?

Every denial on an ERA carries a group code and a CARC. The group code tells you who owns the balance. The CARC tells you why. A remittance advice remark code (RARC) often adds the detail you need to fix it.

Group codes: who owns the balance

X12 maintains four group codes. CO (contractual obligation) means the provider absorbs the amount and cannot bill the patient. PR (patient responsibility) means the patient owes it. OA (other adjustment) and PI (payer initiated reduction) cover the rest. CR is no longer on the current list, even though many guides still show it.

Claim denial reasons by CARC

X12 says CARC 18 should be used with group code OA, so a duplicate usually shows as OA-18, not CO-18.

How do you appeal a denied medical claim, step by step?

Appeal a denied claim in seven steps: read the code, pick the right path, check the deadline, gather proof, write a focused letter, submit it the way the payer requires, and track it to a decision. Most practices lose appeals at step two or three, not at the letter.

1. Read the CARC, RARC and group code. They tell you whether the fix is a corrected claim, a reopening or an appeal.
2. Pick the path. Clerical errors go as corrected claims or reopenings. Coverage, necessity, authorization and bundling disputes go as appeals.
3. Check the deadline. Medicare gives 120 days from receipt of the initial determination for a redetermination. Group health plans must allow at least 180 days under the federal claims rule. Your commercial contracts may set shorter provider appeal windows, so check them.
4. Gather proof. Visit notes, orders, authorization number and date, eligibility screenshots, the payer’s own coverage policy and, for timely filing, the clearinghouse acceptance report.
5. Write the letter. One page is usually enough (see below).
6. Submit through the payer’s channel. Portal, fax or mail as the payer specifies, with the claim number on every page.
7. Track and escalate. Diary a follow-up date. If the first level fails, move to the next level before its deadline.

How to appeal a denied insurance claim: what the letter must include

A strong appeal letter names the patient, member ID, claim number, date of service, codes billed and the denial code. It states in one sentence why the denial is wrong, cites the payer’s own policy or the rule that supports payment, lists every attachment, and asks for a specific result: reprocess and pay. Skip the history of the practice. Reviewers read the first paragraph and the attachments.

How to appeal insurance claim denials for prior authorization and medical necessity

Ask for a peer-to-peer review with the payer’s medical director while the case is fresh, then file a written appeal with the clinical records. Since January 2026, the CMS prior authorization rule (CMS-0057-F) requires Medicare Advantage, Medicaid and CHIP plans to give a specific reason for every prior authorization denial, which makes the appeal easier to target. For group health and ACA plans, the patient can request an independent external review within 4 months after the final internal denial, and the reviewer must decide within 45 days. Our eligibility and prior authorization team tracks these deadlines on every open case.

What are the five levels of Medicare appeals?

Original Medicare has five appeal levels. The dollar thresholds for 2026 are $200 for an ALJ hearing and $1,960 for federal court.

Put every document into the level 2 file. CMS says evidence not submitted at reconsideration may be excluded at later levels unless you show good cause.

Is it worth appealing a denied claim? A worked example

Yes, for almost every claim above a small dollar value. Appeals filed by Luxen were overturned 68% of the time, with a median appeal turnaround of 34 days from filing to payer decision. Yet in our billing reviews, 19% of denied claims were never reworked or appealed.

Take a 3-provider family practice collecting $90,000 a month on 1,200 claims, an average of $75 paid per claim. At a 14.2% first-pass denial rate, 170 claims a month are denied (1,200 × 0.142), worth $12,750 (170 × $75). If 19% of those are never worked, 32 claims and $2,400 a month walk away, or $28,800 a year.

Appeal those 32 claims at a 68% overturn rate and about 22 get paid (32 × 0.68 = 21.8), bringing back about $1,630 a month. Assume each appeal takes 30 minutes of staff time at $30 an hour, fully loaded: $15 per appeal, or $480 for 32. Net gain: about $1,150 a month, or roughly $13,800 a year. The break-even point is low: at a 68% win rate and $15 of labor, any claim above about $22 is worth appealing ($15 ÷ 0.68).

Age cuts the odds. We recovered 61% of the dollar value of claims aged 90 to 180 days that practices had stopped working. Claims aged past 180 days were recovered at 23% of dollar value. Timely filing caused 6% of denials, and only 4% of those were recovered.

The chart shows how recovery falls as claims age: 68% of appeals overturned, 61% of dollar value recovered on claims 90 to 180 days old, 23% past 180 days, and 4% of timely filing denials.

What mistakes make claim appeals fail?

Most failed appeals fail on process, not on merit. The same mistakes show up in almost every billing review.

• Nobody owns the work queue. 42% of practice managers said nobody owns denial follow-up full time. Denials wait until someone has a free afternoon.
• Resubmitting instead of correcting. A resent claim creates a duplicate denial and burns filing time.
• Missing the deadline. Appeal windows run from the remit date, not the day someone opens the letter.
• Arguing without proof. A letter that says the service was needed, with no notes attached, is denied again.
• Appealing one claim at a time. If a payer denies the same code the same way 40 times, escalate the pattern through provider relations.
• Ignoring underpayments. A paid claim can still be wrong. Underpayments against contracted rates appeared on 7.8% of paid claims, and the average underpaid claim was short by $38.
• Not tracking reasons. 63% could not name their top three denial reasons, so the same denial repeats every month.

How do you reduce claim denials before they happen?

Prevent claim denials at the front end: verify eligibility before every visit, secure authorizations before scheduling, scrub claims before submission, and review denial reasons monthly. Across 38 client practices, first-pass denial rate fell from 14.2% to 6.1% within 90 days of onboarding, and clean claim rate rose from 89.6% to 97.3% in the first 90 days.

The chart shows the change: first-pass denials from 14.2% to 6.1%, clean claim rate from 89.6% to 97.3%, and net collection rate from 91.4% to 97.8%.

• Run eligibility every visit through the X12 270 and 271 transactions, not just at intake.
• Tie authorizations to the schedule so a visit without a valid authorization is flagged before it happens.
• Use a claim scrubber with payer edits and NCCI edits before the X12 837 claim goes out. Pair it with certified medical coding review on high-risk codes.
• Keep credentialing current. A lapsed re-credentialing held payments for a median of 47 days. Credentialing dates belong on the same calendar as appeal deadlines.

Should you work claim denials in-house or outsource them?

Keep denial work in-house if one trained person owns it full time and your denial rate is under 5%. Outsource it if denials sit, AR over 90 days keeps growing, or the person who knows the payers is also running the front desk. Outsourced denials and AR recovery typically costs 3% to 6% of collections. For the 3-provider practice above, that is $2,700 to $5,400 a month. A free billing review will show how much of your AR is recoverable before you decide.

Should you send a corrected claim, a reopening or an appeal?

Match the fix to the cause: corrected claims for billing errors, reopenings for clerical errors on Medicare claims, and appeals for disputes about coverage, necessity or payment.

Dental

Dental denials cluster around frequency limits and missing attachments on CDT-coded restorative work. Frequency limitation denials made up 19% of dental denials, and dental practices wrote off a median $23,400 a year in restorative claims denied for missing narratives or X-rays. Appeal with the radiograph, a short narrative and the tooth history. See how one dental practice recovered $86,000 once its denials were worked.

Physical therapy

Therapy denials usually trace to units and Medicare thresholds. 8-minute rule unit errors appeared on 9% of therapy claims, and the KX modifier was missing on 21% of Medicare therapy claims past the threshold. Appeals need the signed plan of care, timed minutes per CPT code and documentation of medical necessity above the threshold. More on physical therapy billing.

Behavioral health

Behavioral health denials often come from billing the wrong payer or the wrong telehealth codes. Claims sent to the medical plan instead of the behavioral health carve-out caused 12% of behavioral health denials, and telehealth place-of-service and modifier errors caused 15% of behavioral health telehealth denials. Many of these are corrected claims, not appeals. More on billing for therapists.

Ambulance

Ambulance appeals turn on medical necessity documentation and transport details. Physician Certification Statements were missing or unsigned on 18% of non-emergency transports, and origin and destination modifier errors appeared on 6% of ambulance claims. Attach the PCS, the run report and loaded mileage to every appeal. In one ambulance case study, days in AR fell from 71 to 38.

Primary care

Primary care denials are mostly modifier and preventive visit errors on high-volume, low-dollar claims. Problem-oriented visits billed with an annual wellness visit lacked modifier 25 on 12% of claims, and vaccine administration codes were missing alongside vaccine product codes on 5% of claims. Fix these with scrubber rules rather than appeals. More on primary care billing.