Medical Billing Denial Codes Explained — CARC Code Library

How to Read Denial Codes

Every denied claim comes back with a Claim Adjustment Reason Code (CARC) that explains why the payer didn't pay. The two-letter prefix tells you who is responsible: CO (Contractual Obligation) means the provider absorbs the adjustment and the patient cannot be billed; PR (Patient Responsibility) means the balance can go to the patient (deductibles, copays, coinsurance); OA (Other Adjustment) and PI (Payer Initiated) cover situations where neither applies cleanly. Alongside the CARC, Remittance Advice Remark Codes (RARCs) add the specific detail — a CO-16 is meaningless until you read the remark code that says which field was missing.

The guides below cover the highest-volume denial codes in US medical billing. Each one explains what the code actually means, the real-world reasons it fires, a step-by-step fix, appeal wording you can adapt, and the front-end changes that stop it from recurring. We add new codes every week.

Every denial follows the same path: read the codes, classify the cause, then correct, appeal, or bill accordingly.

Denial Code Guides

CO-45

Charge Exceeds Fee Schedule / Maximum Allowable

CO-45 means the amount you billed is higher than the payer's contracted or allowed amount for that service. The difference is a contractual adjustment…

CO-16

Claim Lacks Information or Has Submission Error

CO-16 means the claim is missing information the payer needs to process it, or contains a submission/billing error. It is not a judgment on medical ne…

CO-18

Exact Duplicate Claim or Service

CO-18 means the payer believes this exact claim or service line was already submitted — same patient, provider, CPT code, and date of service. One of …

CO-22

Care May Be Covered by Another Payer (Coordination of Benefits)

CO-22 means the payer believes another insurance is primary for this patient, and you billed the wrong payer first — or the patient's coordination of …

CO-29

Timely Filing Limit Expired

CO-29 means the claim arrived after the payer's filing deadline — the window between the date of service and the date the payer must receive the claim…

CO-50

Non-Covered Service — Not Deemed Medically Necessary

CO-50 means the payer decided the service was not medically necessary under its coverage policy — usually because the diagnosis codes on the claim do …

CO-97

Service Bundled Into Another Adjudicated Service

CO-97 means the payer considers this service part of another service already paid — the benefit is 'included in the payment/allowance for another serv…

CO-109

Claim Not Covered by This Payer/Contractor

CO-109 means you sent the claim to the wrong payer or the wrong contractor — the entity that received it has no responsibility for this patient or thi…

CO-167

Diagnosis Not Covered

CO-167 means the payer does not cover services for the diagnosis submitted on the claim. Unlike CO-50 (service not necessary), CO-167 targets the ICD-…

CO-4

Procedure Code Inconsistent with Modifier (or Required Modifier Missing)

CO-4 means the CPT/HCPCS code and the modifier on the claim don't work together — either the modifier used is invalid for that code, or a required mod…

CO-11

Diagnosis Inconsistent with the Procedure

CO-11 means the diagnosis code and the procedure code on the claim don't logically match — the payer's edits flag the combination as clinically incons…

PR-1

Deductible Amount

PR-1 is not a denial — it's the payer telling you the allowed amount was applied to the patient's unmet deductible. The claim processed correctly; the…

PR-2

Coinsurance Amount

PR-2 reports the patient's coinsurance — their percentage share of the allowed amount after the deductible. Like PR-1, it isn't a denial; it's the pay…

PR-3

Copayment Amount

PR-3 reports the patient's copayment — the flat per-visit amount their plan assigns. It's the simplest cost-sharing code on any remittance, and the op…

PR-96

Non-Covered Charge — Patient Responsibility

Same reason as CO-96, opposite liability. The PR group code assigns the balance to the patient — but you can only bill them if you obtained valid advance notice first…

CO-96

Non-Covered Charges

Code 96 means the service isn't covered — excluded by the plan, not a benefit, or non-covered in this circumstance. Critically, it arrives as either C…

CO-197

Precertification / Authorization Absent

CO-197 means the service required prior authorization and the payer has no record of one — or the authorization on file doesn't match what was billed.…

CO-151

Frequency / Units Exceed Payer Limits

CO-151 means the payer decided the quantity was too much — more units, more visits, or more frequency than its policy supports for this patient in thi…

CO-B7

Provider Not Certified / Eligible for This Service on This Date

CO-B7 means the payer's records say the rendering provider wasn't eligible to be paid for this service on this date — an enrollment, credentialing, or…

CO-119

Benefit Maximum Reached

Code 119 means the patient exhausted a capped benefit — the plan covered a set number of visits or dollars for this service category in this period, a…

CO-24

Charges Covered Under a Capitation Agreement / Managed Care Plan

CO-24 means the payer believes this patient's care is covered under a capitation arrangement or managed care plan — so the fee-for-service claim you s…

CO-27

Expenses Incurred After Coverage Terminated

CO-27 means the payer's records show the patient's coverage ended before the date of service. The plan isn't disputing the care — it's saying the pati…

CO-23

Impact of Prior Payer Adjudication (Secondary Claims)

Code 23 — usually seen as OA-23 — is the least understood code on secondary remittances, because it isn't a denial at all. It reports the impact of th…

PR-204

Service Not Covered Under the Patient's Current Benefit Plan

PR-204 means the patient's plan simply doesn't include this service, equipment, or drug as a benefit — and the PR prefix assigns the balance to the pa…

CO-146

Diagnosis Was Invalid for the Date of Service

CO-146 means the diagnosis code on the claim wasn't valid on the date the service was performed. ICD-10 codes are living things — added, revised, and …

CO-181

Procedure Code Was Invalid on the Date of Service

CO-181 is CO-146's procedural twin: the CPT or HCPCS code billed wasn't valid on the date of service. CPT updates every January 1, HCPCS quarterly — c…

CO-252

Attachment or Documentation Required to Adjudicate

CO-252 means the payer won't decide the claim until it sees supporting documentation — an operative note, invoice, primary EOB, certificate of medical…

CO-256

Service Not Payable Per Managed Care Contract

CO-256 means your managed care contract itself is the reason for non-payment — the payer is pointing at terms you agreed to: services outside your con…

CO-15

Missing or Invalid Authorization Number

CO-15 means the payer couldn't match your claim to a valid prior authorization — the number is missing, mistyped, or doesn't cover what you billed. Ofte…

CO-8

Procedure Inconsistent With Provider Type/Specialty

CO-8 means the payer's edit system decided the procedure billed doesn't match the provider's specialty on file — usually a taxonomy or enrollment data …

CO-170

Payment Denied for Service by This Type of Provider

CO-170 means the payer's policy excludes this provider type from being paid for this service — common in chiropractic, PT, and NP billing…

CO-236

Procedure Combination Not Compatible (NCCI)

CO-236 means two codes on the claim can't be billed together per NCCI edits — when a modifier unlocks payment, when it doesn't, and how to prevent it…

OA-23

Impact of Prior Payer Adjudication

OA-23 reports the amount the primary payer already handled on a secondary claim — usually informational, not a denial. When it's normal, and when the …

CO-5

Procedure Inconsistent With Place of Service

CO-5 means the CPT code and the place of service on the claim don't go together — a facility-only code with an office POS, or a telehealth POS mismatch…

CO-6

Procedure Inconsistent With Patient's Age

CO-6 means the code billed conflicts with the patient's age — age-banded preventive E/M codes, pediatric vs adult vaccines, or a simple DOB typo at regi…

CO-13

Date of Death Precedes Date of Service

CO-13 means the payer's records show the patient died before the service date — very often the patient is alive and an SSA or payer record error is to …

CO-B15

Qualifying Service Not Received or Covered

CO-B15 means an add-on code arrived without its payable primary procedure — the qualifying service is missing, denied, or split onto another claim…

PR-49

Routine/Preventive Exam Not Covered

The payer processed the service as non-covered routine care. When to recode as diagnostic, when to appeal under ACA preventive rules, and when it's truly the patient's…

CO-59

Multiple/Concurrent Procedure Rules

Usually the multiple-procedure payment reduction, not a true denial — how the 100%/50% math works and which mis-rankings to appeal…

CO-A1

Claim Denied — Check Remark Codes

The catch-all denial whose real reason lives in the remark codes — decoding MA04, N130, N56 and fixing the root cause…

CO-B9

Patient Enrolled in Hospice

The denial that follows a missed hospice election — the GV/GW rebill paths and the eligibility habit that prevents it…

CO-253

Sequestration — the Medicare 2%

Not a denial at all — the mandatory federal payment cut, the exact math, and why you never bill patients for it…

CO-234

Procedure Not Paid Separately

A bundling-family denial — when modifier 25 or 59 recovers it, when it's a true write-off, and how the remark codes tell you which…

B11

Claim Routed to the Wrong Payer

Often OA-B11 with remark N418 — an administrative routing issue, not a denial. How to find the correct payer and rebill without a duplicate…

CO-186

Level of Care Change Adjustment

Usually inpatient paid as observation — a utilization-review outcome, not a coding error. When to appeal with clinical criteria and how to prevent it…

CO-204

Not Covered Under Current Benefit

The CO twin of PR-204 — same reason, opposite outcome. Why the group code decides who pays, and when you can still bill the patient…

More denial codes are added weekly. Fighting a code you don't see here? Ask us directly — or let our team work your denials for you.

Frequently Asked Questions

What is a CARC code?

A Claim Adjustment Reason Code — the standardized code payers use on remittances to explain why a claim or service line was adjusted or denied. CARCs are maintained by X12 and used industry-wide, so CO-45 means the same thing from every payer.

What's the difference between CO and PR codes?

CO (Contractual Obligation) adjustments are the provider's responsibility — you cannot bill the patient for them. PR (Patient Responsibility) amounts — deductibles, copays, coinsurance — can be billed to the patient. Reading the prefix correctly protects you from both compliance problems and lost revenue.

Are denial codes the same for Medicare and commercial payers?

The codes themselves are standardized, but payers apply them differently — the same missing-modifier problem might return CO-4 from one payer and CO-16 with a remark code from another. That's why each guide here covers payer-specific behavior.

Denial Code Library