Insights · Reference

Top denial reason codes (CARC) and how to fix each one

What the most common Claim Adjustment Reason Codes mean, why they happen, how to correct the claim, and what to change so you see them less often. Search by code or keyword.

By the Accancy team12 min read · 16 codes

01How to read a denial

Every adjustment on an electronic remittance (835) or EOB comes with a group code and a reason code. Read them together; the same reason code can mean something different depending on who is responsible.

CO

Contractual obligation. The provider is responsible; generally can’t be billed to the patient.

PR

Patient responsibility. Can be billed to the patient, such as deductible or coinsurance.

OA

Other adjustment. Doesn’t fit CO or PR; often coordination with other payers.

PI

Payer-initiated reduction. The payer’s adjustment, not covered by contract terms.

Many denials also carry a Remittance Advice Remark Code (RARC), such as N290 or M51, which adds detail. Always read the RARC before you rework a claim; it often tells you exactly what is missing.

i

Codes are maintained by X12 and updated regularly

Descriptions below are plain-English summaries. Payers also apply their own policies, so check the official code list and the payer’s guidance before you resubmit or appeal.

02The codes

Grouped by where in the revenue cycle the problem usually starts. Shown with their most common group code.

Showing 16 of 16 codes
16CO-16

Claim is missing information or has errorsThe claim lacks information needed to adjudicate it, or contains a submission or billing error.

Eligibility & registration

Common causes

  • Missing or invalid data element (NPI, address, DOB, referring provider)
  • Incomplete fields the payer requires for this service

How to fix

  • Read the RARC to find the exact missing item
  • Correct the field and resubmit as a corrected claim

Prevent it

  • Add scrubber edits for the payer’s required fields
  • Verify provider and patient data at registration
18CO-18

Exact duplicate claim or serviceThe payer already received a claim for the same patient, date, provider and service.

Timely filing & admin

Common causes

  • Claim resubmitted while the original was still processing
  • A corrected claim sent as new instead of a replacement

How to fix

  • Check status of the original claim first
  • If correcting, resubmit with the correct frequency code and original claim number

Prevent it

  • Check claim status before resubmitting
  • Use replacement/void frequency codes for corrections
22CO-22

Another payer may be primary (coordination of benefits)The payer believes this care may be covered by another payer.

Eligibility & registration

Common causes

  • Wrong payer order on file
  • Patient’s COB information out of date with the payer

How to fix

  • Confirm primary coverage with the patient and payers
  • Bill the primary, then submit to secondary with the primary’s EOB

Prevent it

  • Ask about other coverage at every visit
  • Run eligibility checks that return COB data
27CO-27

Coverage had ended before the date of serviceThe service was provided after the patient’s coverage terminated.

Eligibility & registration

Common causes

  • Insurance changed and wasn’t updated
  • Eligibility not checked before the visit

How to fix

  • Verify eligibility on the date of service
  • Bill the correct current payer, or the patient if uninsured

Prevent it

  • Real-time eligibility checks before every appointment
  • Prompt staff to scan new insurance cards
31CO-31

Patient can’t be identified as the payer’s memberThe payer can’t match the patient to a member record.

Eligibility & registration

Common causes

  • Name, DOB or member ID mismatch
  • Typos or a dependant billed under the wrong subscriber

How to fix

  • Compare demographics to the insurance card and payer portal
  • Correct and resubmit

Prevent it

  • Scan cards and verify details at check-in
  • Use eligibility responses to correct names and IDs
197CO-197

Pre-certification or authorization was missingThe service required prior authorization, notification or pre-certification that wasn’t obtained.

Authorization

Common causes

  • Service on the payer’s prior-auth list
  • Auth obtained but for a different service, date or provider

How to fix

  • Check whether a valid auth exists and add the number
  • Where allowed, request a retro-authorization, then appeal

Prevent it

  • Maintain payer-specific auth requirements
  • Confirm auth before scheduling high-cost services
15CO-15

Authorization number missing or invalidAn authorization was required but the number on the claim was missing, invalid or didn’t match.

Authorization

Common causes

  • Auth number not entered on the claim
  • Auth expired or for a different CPT or date range

How to fix

  • Locate the correct auth and resubmit with it
  • If expired, request an extension or appeal with documentation

Prevent it

  • Link auths to appointments in your PM system
  • Track auth start and end dates and visit counts
4CO-4

Modifier missing or inconsistent with procedureThe procedure code doesn’t match the modifier used, or a required modifier is missing.

Coding

Common causes

  • Missing laterality, professional/technical or distinct-service modifier
  • Modifier not valid for that CPT/HCPCS code

How to fix

  • Review the code and documentation, apply the correct modifier
  • Resubmit as a corrected claim

Prevent it

  • Scrubber edits for modifier-to-code validity
  • Coder education on payer modifier policies
11CO-11

Diagnosis doesn’t support the procedureThe diagnosis code is inconsistent with the procedure billed.

Coding

Common causes

  • Diagnosis not linked to the right line
  • Unspecified or incorrect diagnosis for the service

How to fix

  • Check documentation and correct diagnosis pointers
  • Code to the highest supported specificity and resubmit

Prevent it

  • Diagnosis-to-procedure edits in your scrubber
  • Feedback to providers on documentation specificity
97CO-97

Service is bundled into another serviceThe benefit for this service is included in another service already paid.

Coding

Common causes

  • NCCI procedure-to-procedure edit
  • Separately billing a component of a global or comprehensive service

How to fix

  • Confirm whether the service was truly separate and distinct
  • If supported, add the appropriate modifier and resubmit or appeal; if not, adjust

Prevent it

  • Keep NCCI edits current in your claim scrubber
  • Educate coders on global periods and bundling
236CO-236

Code combination not allowed (NCCI)This procedure or procedure/modifier combination isn’t compatible with another on the same day.

Coding

Common causes

  • Two codes that can’t be billed together without a modifier
  • Incorrect modifier used to bypass an edit

How to fix

  • Review documentation for a separate, distinct service
  • Apply the correct modifier if supported; otherwise remove the line

Prevent it

  • Pre-submission NCCI edits
  • Periodic coding audits on frequently denied pairs
50CO-50

Not deemed medically necessaryThe payer doesn’t consider the service medically necessary based on the information provided.

Coverage & necessity

Common causes

  • Diagnosis doesn’t meet the payer’s coverage policy
  • Documentation doesn’t show why the service was needed

How to fix

  • Review the payer’s coverage policy (LCD/NCD for Medicare)
  • Appeal with supporting clinical documentation where appropriate

Prevent it

  • Check coverage policies for high-risk services
  • Obtain an ABN or waiver where required before the service
96CO-96

Non-covered chargeThe service isn’t covered under the patient’s plan. Read with the RARC for specifics.

Coverage & necessity

Common causes

  • Excluded benefit under the plan
  • Service billed to the wrong payer or product

How to fix

  • Check RARC and plan benefits
  • Bill the patient if a valid waiver was signed, or appeal if covered

Prevent it

  • Verify benefits for elective or uncommon services
  • Use financial-responsibility forms where permitted
119CO-119

Benefit maximum reachedThe benefit maximum for this period or occurrence has been met.

Coverage & necessity

Common causes

  • Visit limits reached for therapy or similar services
  • Annual benefit cap met

How to fix

  • Confirm the count with the payer
  • Bill secondary or patient as allowed, or appeal with justification for additional visits

Prevent it

  • Track visit counts against benefit limits
  • Inform patients before limits are reached
29CO-29

Time limit for filing has expiredThe claim was received after the payer’s timely filing deadline.

Timely filing & admin

Common causes

  • Claim held too long or lost
  • Initial submission rejected and not reworked in time

How to fix

  • Find proof of timely filing (clearinghouse acceptance report)
  • Appeal with that proof; otherwise adjust per policy

Prevent it

  • Work clearinghouse rejections daily
  • Track payer-specific filing limits in your system
1PR-1

Deductible amountThe amount is the patient’s deductible responsibility. Not a denial to rework, but often misrouted.

Timely filing & admin

Common causes

  • Patient hasn’t met their annual deductible

How to fix

  • Post correctly as patient responsibility
  • Bill the patient or secondary insurance

Prevent it

  • Estimate patient responsibility before the visit
  • Collect at time of service where appropriate

03A simple denial workflow

  1. Triage daily. Sort new denials by reason code, payer and dollar value, and route each to the right owner.
  2. Read the full remit. Group code, CARC and RARC together, plus any payer letter.
  3. Decide: correct, appeal or adjust. Correct and resubmit for data errors; appeal when the claim was right; adjust off only with a documented reason.
  4. Act within the deadline. Resubmission and appeal windows vary by payer; track them for every open denial.
  5. Log the root cause. Record what caused each denial so patterns become visible.
  6. Close the loop. Share patterns with front desk, coding and charge capture monthly.

For a fuller approach, see our guide on how to reduce claim denials.

04Prevent, don’t just rework

Working denials recovers revenue. Preventing them saves the work in the first place. Most of the codes above trace back to three places:

1

Front end

Real-time eligibility checks, correct payer order, and complete demographics at registration.

2

Authorizations

A payer-specific list of services that need auth, and a check before every scheduled service.

3

Coding & scrubbing

Up-to-date claim edits for NCCI, modifiers, frequency and diagnosis-to-procedure matching.

!

Track your top five

For most organizations, a handful of codes make up the majority of denied dollars. Fixing the root cause of your top five usually beats working every denial a little faster.

This reference is general information for revenue cycle teams. CARC and RARC descriptions are summarised; refer to the current X12 code lists and individual payer policies before correcting, resubmitting or appealing a claim.

Let us work the backlog. And find what’s causing it.

Accancy helps providers organise denial and A/R work queues, investigate non-payment and follow up on every account.

Discuss denials & A/R support

Keep reading

GuideHow to choose an RCM partnerTen things to evaluate, the questions to ask, red flags and a scorecard to compare partners.ChecklistRADV audit preparation checklistA phase-by-phase checklist for Medicare Advantage plans, from year-round documentation to submission.

Related services

Denials & A/RDenial review, aged A/R follow-up and appeal support for hospitals and physician groups. Organised work queues, root-cause tracking and clear next actions.Revenue cycleOutsourced revenue cycle support for hospitals and physician groups: billing support, claims follow-up and payment posting, delivered inside your systems.