Claim denials in medical billing: common causes and fixes

A claim denial is the payer saying no. One or more lines on the claim get refused, a reason code is attached, and the money stops moving until someone figures out why. It is not a typo that bounced at the front door. It is a verdict delivered after the payer actually looked at the claim.

The good news: many denials are fixable, and many of the big causes are preventable. A claim denial happens when a payer refuses to reimburse the provider for a billed service after adjudication, usually with a reason code explaining why. Denials are different from rejections, which happen before adjudication, when the claim never reaches the payer's decision desk at all.

Soft denials vs hard denials

Not every denial deserves the same effort. The first question is whether the denial can be worked at all.

Sorting soft from hard is the first step in every denial workflow. Soft denials are the ones worth staff time. A practice that works hard denials as if they were soft spends hours correcting claims that can only pay if the appeal succeeds, while the soft denials that could pay sit unworked. Industry research from MGMA found that roughly 65% of denied claims are never reworked at all, which is the more expensive version of the same mistake.

Reading a denial: reason codes and the remittance

The payer's answer arrives on a remittance. Electronic versions come as an 835 file (the ERA); paper versions arrive as an explanation of benefits (EOB). Either way, each denied line carries two pieces of information: a group code and a reason code.

The group code says who the payer thinks is responsible. CO (contractual obligation) means the payer is pointing at its own contract or rules. PR (patient responsibility) means the payer is pointing at the patient. The reason code says why the line was refused. Common ones include CO-197 (authorization required), CO-11 (diagnosis inconsistent with procedure), CO-96 (non-covered charges), CO-18 (duplicate claim), and PR-1 (deductible not met).

Read the two together and the fix usually names itself. A CO-197 says the authorization was missing: get the authorization documented and appeal, or write it off if authorization was never obtained and cannot be retroactively approved. A PR-1 says the patient owes the balance under their plan: that balance moves to the patient's account, not back to the payer. Misreading the remittance is how denied lines get written off as contractual adjustments when they were actually collectible.

The 8 most common causes, and the fix for each

Denials cluster into a small number of causes. A Tebra survey of 140 billing professionals ranked eligibility and coverage issues at the top (46%), followed by documentation requests (42%), duplicates or coordination of benefits (37%), and missing prior authorization (30%). Coding edits and modifier issues came in at 28%. The pattern is worth noticing: the biggest causes are front-end misses, not coding mistakes. Most of these denials start before the claim exists.

1. Eligibility and coverage gaps

The patient's coverage was not active on the date of service, or the plan on file was wrong. This is the most cited cause of denials in the Tebra survey, and it is almost entirely preventable. The fix is verification before the visit: check active coverage and benefits in real time, not after the claim comes back denied. For established patients, re-verify on a schedule, because coverage lapses mid-treatment are common.

2. Missing or invalid prior authorization

The payer required approval before the service and never got it. Imaging, surgeries, and specialty services are the usual triggers. This one stings because the care was already delivered; the only question is whether the bill ever gets paid. The fix is a pre-visit authorization checklist tied to payer rules: know which services each payer requires authorization for, get it before the date of service, and attach the authorization number to the claim. Some payers allow retroactive authorization within a window, but it is never a plan.

3. Coding and modifier errors

Wrong, outdated, or mismatched codes, or modifiers that do not pair with the payer's rules. Modifier -25 and modifier -59 get this wrong often, because payers have specific ideas about what counts as a separately billable service. The fix is unglamorous: keep coding software current, train coders on payer-specific edits, and audit samples of claims before they go out. Regular internal audits catch patterns like overused modifiers before payers do.

4. Missing or incomplete documentation

The documentation does not justify the service as medically necessary. Common in physical therapy, behavioral health, and diagnostic testing: the service happened, but the notes do not support why. The fix starts with templates that prompt for the required elements (diagnosis, treatment plan, progress notes, signatures), and continues with claim-time checks that documentation is attached before submission. If the payer asked for records, send what they asked for, not what was convenient.

5. Duplicate claims

The same service was billed twice, by accident or because someone resubmitted before the first claim finished processing. Payers flag duplicates automatically, and they deny them, adding rework to the pile. The fix is process: check claim status before resubmitting, and use billing software that flags potential duplicates. Resubmitting before the first claim finishes processing creates a duplicate denial on top of the original problem.

6. Coordination of benefits errors

The patient has two plans and the claim went to the wrong one first. COB rules decide which payer is primary, and when the wrong payer gets the claim, it denies. The fix is collecting secondary insurance at intake and keeping the COB information current. When the primary payer's remittance passes the remaining balance to the secondary, that sequence has to be followed in order.

7. Missed timely-filing windows

Every payer sets its own deadline for claim submission, often 90 days to a year from the date of service. Miss it and the claim denies no matter how clean it was. This is a hard denial by definition: the window closed, the claim cannot be reworked. The fix is a filing calendar with alerts per payer, and enough staffing on the front end that claims do not sit waiting to go out. Internal delays, waiting on documentation, an initial rejection nobody fixed, are what push claims past the deadline.

8. Credentialing and NPI issues

The rendering provider was not credentialed with the payer, or the NPI or tax ID on the claim was wrong. Practices with multiple providers and service locations are especially exposed, because credentialing data goes stale quietly. The fix is keeping enrollment current and auditing NPI and tax ID data before it causes denials. Provider credentialing is the next post in this series and will cover the process in detail.

Causes at a glance

How to work a denial

Fixing denials is a routine, not a scramble. The workflow has four steps.

Triage first. Sort soft from hard. Soft denials go to billing staff for correction. Hard denials go to leadership for the appeal-or-write-off decision. This is the step the MGMA finding is about: most denied claims never get worked, and many of the ones that do get worked were never collectible.

Decide the path. If the claim was wrong, correct it and resubmit. The corrected claim goes back through normal processing as if it were new. If the claim was right and the payer refused anyway, appeal: write up the case, attach the documentation, and ask for reconsideration. Appeals take longer and need their own tracking, separate from the resubmission queue.

Watch the clocks. Two clocks run during denial work. The timely-filing clock started at the date of service, and it does not pause while a denial sits on someone's desk. The appeal clock starts when the denial arrives, and payers set their own appeal deadlines. A fixable denial that dies because a window closed is a loss the practice caused itself.

Feed the findings back. Track denials by reason code, payer, provider, and location. A dashboard that shows the same CO-197 spiking for one procedure points at a broken authorization step, not a payer conspiracy. Share the pattern with the front desk and the coders, because the fix almost always lives upstream of billing. Denial management as a discipline is this loop run continuously.

Preventing denials: the upstream playbook

Prevention starts before the claim exists, which is where the biggest causes live.

Before the visit. Verify insurance eligibility in real time. Get prior authorization for every service the payer requires it for. Confirm referrals are in place. Collect secondary insurance. This is where the Tebra survey says most denials are born.

Before the claim goes out. Scrub against payer-specific edits, not just generic formatting checks. The edits that matter are the ones that vary by payer: referral numbers on file, required attachments, services the plan excludes. Audit a sample of claims regularly; many revenue-cycle guides suggest 5 to 10 percent of submissions, which catches pattern errors like overused modifiers before payers do.

After the denial arrives. Triage, correct or appeal, track, and feed the pattern back upstream. Every recurring denial is a broken step somewhere earlier in the process. Fix the step, not just the claim.

The through-line is simple. Denials look like a billing problem, but most of them start upstream of billing. Practices that verify coverage, get authorizations, and scrub claims well spend far less time working denials than practices that treat denial management as cleanup. For the full journey a claim takes from visit to payment, see how medical billing works; the front-end pieces sit inside the revenue cycle, where denials are usually born. Our methodology explains how the firms in this directory are evaluated.