Every code below includes what it actually means, why it happens, and what to do about it — not just the official description. Type a code or keyword to filter instantly.
Every time an insurance payer processes a claim, it sends back an Explanation of Benefits (EOB) or Electronic Remittance Advice (ERA). Embedded in that remittance is a set of standardized codes — called Claim Adjustment Reason Codes (CARC) — that explain exactly why each service line was paid at the amount it was, reduced, or denied entirely. These are what the industry colloquially calls "claim reason codes" or "denial codes."
CARC codes never appear alone. They always appear as a group code + reason code pair. The group code comes first and determines who is financially responsible for the adjustment. A CO prefix means the provider absorbs the amount and cannot bill the patient — it's a contractual obligation. A PR prefix means the amount is the patient's responsibility — deductibles, copays, and coinsurance. OA covers other adjustments (commonly used in coordination of benefits situations). CR marks a correction or reversal of a prior payment.
The most important insight for billers: the same CARC number can mean completely different things depending on the group code. CARC 96 as CO-96 means the provider cannot bill the patient (non-covered, contractual write-off). CARC 96 as PR-96 means the patient can be billed for the non-covered service. Read the group code before the number.
RARC codes (Remittance Advice Remark Codes) are supplemental codes that provide additional specificity, especially alongside CO-16. Where a CARC tells you a claim was denied for lacking information, the accompanying RARC tells you which information is missing — the ordering provider NPI, the diagnosis code, the attachment. Always read RARC codes before taking action on a CO-16 denial.
Use this reference to look up any code — filter the table below by code number or keyword. Click any linked code for a full guide with fix steps, appeal language, and prevention systems. The most common codes in practice — CO-45, CO-16, CO-97, CO-50, PR-1 — each have dedicated pages with detailed fix workflows.
Every adjustment arrives as a group code + reason code pair (e.g., CO-45). The group code tells you who eats the money:
Provider absorbs it. Cannot be billed to the patient. Your contract created this adjustment.
Billable to the patient — deductibles, copays, coinsurance, non-covered services with notice.
Neither provider contract nor patient responsibility — commonly COB-related situations.
Reverses a previous adjudication. Watch these — they often precede recoupments.
| Code | What It Means (Plain English) | Why It Happens | How to Fix & Prevent |
|---|---|---|---|
| CO-4 | Procedure code and modifier don't match, or a required modifier is missing. | Missing anatomical modifier, missing 26/TC split, or modifier invalid for the code. | Fix: add/correct the modifier and resubmit. Prevent: scrubber rule flagging modifier-required codes before submission. |
| CO-6 | Procedure/revenue code conflicts with the patient's age. | Age-specific code billed for wrong-age patient (e.g., pediatric code for adult). | Fix: verify DOB and code; correct whichever is wrong. Prevent: demographic validation at charge entry. |
| CO-8 | Procedure conflicts with the provider's taxonomy/specialty. | Provider type not eligible to bill this service per payer rules. | Fix: confirm taxonomy on file with payer matches the service. Prevent: credentialing file audit; correct taxonomy at enrollment. |
| CO-11 | Diagnosis doesn't support the procedure billed. | Dx-procedure mismatch; medical necessity edit failed. | Fix: review documentation; correct coding if supported; appeal with records if payer policy is wrong. Prevent: LCD/NCD and payer policy checks for flagged codes. |
| CO-16 | Claim lacks information or has a submission error — the catch-all "something's missing" code. | Missing NPI, invalid member ID, absent required attachment — always arrives with a RARC telling you what. | Fix: read the paired RARC code, supply the missing element, resubmit (this is a rejection-fix, not an appeal). Prevent: front-end claim edits; CO-16s are nearly 100% preventable. Full CO-16 guide → |
| CO-18 | Exact duplicate of a claim already processed. | Auto-rebilling before adjudication finished, or true double entry. | Fix: confirm the original's status first — if it paid, no action; if it denied, fix the original reason. Prevent: claim-status checks before rebilling; disable blind auto-rebill. |
| CO-22 | Another payer should be billed first (coordination of benefits). | Patient has other primary coverage per payer's COB file. | Fix: bill the primary, then resubmit with the primary EOB. If COB info is outdated, have the patient update it with the payer. Prevent: COB question at every eligibility check. Full CO-22 guide → |
| CO-23 | Prior payer's adjudication impacts this payment. | Secondary claim where primary payment/adjustments already consumed the allowable. | Fix: usually informational on secondaries — verify the math against both contracts. Prevent: post primary EOBs accurately before secondary submission. |
| CO-27 | Coverage terminated before the date of service. | Patient's policy ended; eligibility wasn't checked or changed after check. | Fix: re-verify coverage; if truly termed, bill the patient or their new plan. Prevent: real-time eligibility at every visit — the classic front-desk leak. |
| CO-29 | Timely filing limit expired. | Claim submitted after the contract's filing window. | Fix: appeal only with proof of timely original submission (clearinghouse acceptance report). Otherwise it's a write-off — and cannot be billed to the patient. Prevent: filing-deadline work queues; see our state filing tables. Full CO-29 guide → |
| CO-45 | Charge exceeds the contracted/fee schedule rate — the standard contractual write-off. | Normal on every paid claim; the gap between billed and allowed. | Fix: none needed when the allowed amount is right. Audit: compare allowed vs. your loaded fee schedule — CO-45 is where underpayments hide. Full CO-45 guide → |
| CO-50 | Service deemed not medically necessary by the payer. | Documentation doesn't meet the payer's coverage policy, or policy wasn't checked. | Fix: appeal with clinical notes, letter of medical necessity, relevant LCD/NCD citations; request peer-to-peer if denied again. Prevent: check coverage policies before high-dollar services. Full CO-50 guide → |
| CO-59 | Payment reduced because multiple procedures were performed in the same surgical session. | Multiple surgery rules applied: primary pays 100%, second pays 50%, third+ pays 25%. | Check: verify primary is ranked highest-value, confirm no Modifier 51 exempt codes were reduced, and confirm procedures were truly same-session. Appeal only if reduction was incorrectly applied. Full CO-59 guide → |
| CO-96 | Charge denied as non-covered per the plan. | Service excluded from the benefit package. | Fix: verify the exclusion is real (plans err); if valid and patient signed notice, bill patient. Prevent: benefit verification beyond eligibility — coverage ≠ eligibility. Full CO-96 guide → |
| CO-97 | Payment for this service is bundled into another service already paid. | NCCI edit: the code pair isn't separately payable without a valid modifier. | Fix: check NCCI — if distinct service, add 59/XS/XE/XP/XU and resubmit with documentation. If bundling is correct, write off. Prevent: NCCI-aware scrubbing. Full CO-97 guide → |
| CO-109 | Wrong payer — claim belongs to a different contractor/plan. | Sent to the wrong Medicare contractor, wrong plan within a family, or carved-out benefit (e.g., behavioral health). | Fix: identify the correct payer from the eligibility response and redirect. Watch your filing clock. Prevent: capture plan and carve-out details at verification. Full CO-109 guide → |
| CO-110 | Billing date precedes the date of service. | Data entry error in DOS or submission date. | Fix: correct the date field and resubmit. Prevent: date-logic claim edits. |
| CO-119 | Benefit maximum reached for this period. | Visit/unit/dollar cap exhausted (very common in therapy and chiropractic). | Fix: verify the payer's count vs. yours; if exhausted, bill patient with notice. Prevent: track benefit caps in the PM system; warn patients as limits approach. Full CO-119 guide → |
| CO-140 | Patient name and health ID number do not match payer records. | Demographic mismatch — member ID or name on the claim differs from what the payer has on file. | Fix: pull the insurance card, call payer to confirm exact name and ID on file, correct the claim data, and resubmit. Prevent: scan card and run eligibility at every visit. Full CO-140 guide → |
| CO-151 | Units/frequency exceed what the payer allows for this service. | MUE (medically unlikely edit) or frequency limit hit. | Fix: verify units billed; if clinically justified, appeal with documentation and appropriate modifier. Prevent: MUE tables in the scrubber. Full CO-151 guide → |
| CO-167 | Diagnosis not covered by this payer. | Dx excluded under plan policy. | Fix: review for more specific/accurate coding supported by documentation; appeal if policy misapplied. Prevent: payer dx-coverage lists for your top procedures. Full CO-167 guide → |
| CO-170 | Payment adjusted because pre-certification/authorization was not obtained. | Service rendered without required prior authorization on file. | Fix: call payer's retro-auth line immediately (window: 30–60 days from DOS); for emergencies, appeal citing federal prudent-layperson standard. Prevent: payer-specific auth-required list + pre-service verification 24 hrs before DOS. Full CO-170 guide → |
| CO-177 | Claim submitted in wrong currency. | CUR segment in the 837 transaction contains a non-USD currency code; nearly always a billing system configuration error. | Fix: locate the CUR segment in the raw 837 (Loop 2000B for 837P), correct payer profile to USD, resubmit as corrected claim (frequency code 7). Prevent: verify currency default = USD after every billing system upgrade. Full CO-177 guide → |
| CO-180 | Patient has not met the required deductible / spending commitment (capitated or zero-deductible plan). | Usually a payer error on commercial plans — CO-180 with group code CO signals a provider write-off, but most deductible amounts should be PR-1 (patient responsible). Correct code is CO-180 only for capitated/100%-employer-paid plans. | Fix: verify plan type — if commercial PPO/HMO, this may be a payer error (should be PR-1); call payer. If truly CO-180, write off. Prevent: check deductible group code on every EOB — CO-180 vs PR-1 distinction has direct billing impact. Full CO-180 guide → |
| CO-181 | Procedure code is inconsistent with the revenue code (UB-04 / institutional claims only). | The CPT/HCPCS in FL 44 does not belong in the revenue code category listed in FL 42 of the UB-04 — e.g., a radiology CPT under a pharmacy revenue code. | Fix: correct either the revenue code or the procedure code (whichever is wrong) and resubmit as a corrected institutional claim (bill type frequency digit 7). Prevent: charge description master (CDM) validation that enforces revenue code ↔ procedure code pairs. Full CO-181 guide → |
| CO-182 | Procedure modifier is inconsistent with the procedure code. | Modifier applied that doesn't belong with the CPT — wrong split-billing modifier, modifier not on approved list for this code, or a required separate modifier (59/X-modifier) missing. Compare CO-4 (missing modifier) vs CO-182 (wrong modifier applied). | Fix: check NCCI modifier indicator and payer policy; remove or replace the incorrect modifier; add correct modifier if missing; resubmit. Prevent: modifier-specific scrubber rules updated quarterly with NCCI releases. Full CO-182 guide → |
| CO-183 | The referring provider is not eligible to refer the service billed. | Referring provider not enrolled in PECOS (Medicare), not a participating provider in the HMO's network, wrong NPI entered in Box 17, or not the patient's designated PCP in a gated plan. | Fix: verify referring NPI in NPPES; check PECOS enrollment (pecos.cms.hhs.gov) for Medicare; for HMO plans confirm the referral came from the patient's designated PCP; correct NPI or obtain valid referral and resubmit. Prevent: referring provider PECOS/network verification before every specialist appointment. Full CO-183 guide → |
| CO-184 | The prescribing/ordering provider is not eligible to order or prescribe the service billed. | Most common on DME and home health claims: the ordering physician is not enrolled in PECOS as an ordering/referring provider. Also fires for wrong NPI in the ordering provider field, lapsed enrollment, OIG exclusion, or scope-of-practice restriction. | Fix: verify ordering provider NPI in NPPES; check PECOS at pecos.cms.hhs.gov; also check OIG exclusion list; correct NPI or obtain new order from PECOS-enrolled physician. DME suppliers: verify PECOS enrollment before dispensing — CO-184 on a dispensed item means the supplier absorbs the cost. Full CO-184 guide → |
| CO-185 | Claim denied — psychiatric venue of care not authorized or not an approved setting. | The specific behavioral health setting (inpatient psych, PHP, IOP, RTC) was not authorized or is not an approved venue under the plan. Common causes: level-of-care mismatch (IOP authorized, PHP delivered), facility not in BH network, claim sent to medical plan instead of MBHO carve-out, or a MHPAEA parity violation where psychiatric settings are restricted more than comparable medical settings. | Fix: identify specific denial reason — facility credentialing, LOC mismatch, or wrong payer (reroute to MBHO). For LOC mismatches: appeal with clinical documentation supporting the level delivered. Parity: evaluate MHPAEA violation if comparable medical settings aren't subject to the same restriction — file a parity appeal citing CAA 2021. Full CO-185 guide → |
| CO-197 | No prior authorization on file for a service that required it. | Auth never obtained, expired, or doesn't match the service/units/provider billed. | Fix: if auth existed, appeal with the auth number and approval letter. Retro-auth is possible with some payers within tight windows. Prevent: auth tracking tied to scheduling — the #1 preventable denial. See the PA guide. Full CO-197 guide → |
| CO-198 | Authorization exists but its limits were exceeded. | More units/visits delivered than approved. | Fix: request additional units before continuing care; appeal overage with clinical justification. Prevent: real-time auth-unit tracking against scheduled visits. |
| CO-204 | Service not covered under the current benefit plan (CO variant). | Plan exclusion where contract puts it on the provider. | Fix: verify exclusion; check whether a covered alternative code accurately describes the service. Prevent: benefit checks for non-routine services. |
| CO-226 | Information requested from the billing provider wasn't received in time. | Records request ignored or missed. | Fix: submit the requested documentation and reopen. Prevent: a work queue for payer correspondence with owner and deadline. |
| CO-236 | Procedure combination not compatible per coding rules on the same day. | NCCI procedure-to-procedure conflict across the claim. | Fix: review the pair; modifier if truly distinct, else write off the column-2 code. Prevent: same-day compatibility edits pre-submission. |
| CO-252 | An attachment/documentation is required before adjudication. | Payer needs notes, invoice, or certificate to process. | Fix: send exactly what the paired RARC specifies via the payer's attachment channel. Prevent: attach known-required documents at first submission (common in DME). Full CO-252 guide → |
| PR-1 | Deductible amount — patient owes it. | Annual deductible not yet met. | Fix: bill the patient. Prevent (surprises): collect estimated deductible at time of service based on eligibility response. Full PR-1 guide → |
| PR-2 | Coinsurance amount — patient owes it. | Patient's percentage share per plan. | Fix: bill promptly — patient collection odds fall ~20% every 30 days. See patient collections guide. Full PR-2 guide → |
| PR-3 | Copayment amount — patient owes it. | Fixed visit copay. | Fix/Prevent: collect at check-in, full stop. A copay uncollected at the desk costs more to chase than it's worth. Full PR-3 guide → |
| PR-204 | Service not covered by the patient's current plan — patient responsible. | Benefit exclusion; patient had no coverage for this service. | Fix: bill patient if proper notice was given (ABN for Medicare). Prevent: benefit verification + signed financial notice before non-covered services. Full PR-204 guide → |
| OA-18 | Duplicate claim (other-adjustment variant). | Same as CO-18 where responsibility is unassigned. | Fix: trace the original claim's outcome before any resubmission. |
| OA-23 | Prior payer adjudication impact (other-adjustment variant). | Secondary-claim math after primary processing. | Fix: verify secondary allowed amounts against contract; usually informational. |
No CARC codes match your filter.
Remark codes ride along with reason codes (especially CO-16) to tell you the specific missing or problematic element:
| Code | What It Means | What To Do |
|---|---|---|
| M15 | Separately billed services should have been bundled. | Review NCCI; rebill bundled or modifier if distinct. |
| M20 | Missing/invalid HCPCS code. | Verify the code is active for the DOS; check annual HCPCS updates. |
| M51 | Missing/invalid procedure code(s). | Confirm code validity and format; watch deleted codes after Jan 1 / Jul 1 updates. |
| M76 | Missing/incomplete/invalid diagnosis. | Check dx specificity (no truncated ICD-10) and pointer linkage. |
| M79 | Missing/invalid charge amount. | Correct the charge field; often a claim-format issue from the PM system. |
| M81 | Diagnosis must be coded to the highest specificity. | Replace 3-character category codes with full-specificity ICD-10 codes. |
| M127 | Missing patient medical record documentation. | Submit records through the payer's documentation channel; calendar the deadline. |
| N29 | Missing documentation/orders/notes supporting the service. | Attach the specific document type the payer's policy requires (orders, CMN, notes). |
| N30 | Patient ineligible for this service. | Re-run eligibility; confirm benefit category and program (esp. Medicaid programs). |
| N56 | Wrong procedure code for the service documented. | Recode from documentation, not from habit; verify with coding references. |
| N130 | Consult plan benefit documents for coverage rules. | Pull the plan's coverage policy; determines whether appeal has any basis. |
| N179 | Additional information requested from the member. | Claim stalls on the patient — notify them; COB questionnaires are the usual culprit. |
| N265 | Missing/invalid ordering provider identifier. | Add the ordering provider's NPI; verify PECOS enrollment for Medicare orders. |
| N286 | Missing/invalid referring provider identifier. | Correct referring NPI; required for many specialist and diagnostic claims. |
| N290 | Missing/invalid rendering provider identifier. | Rendering NPI absent or doesn't match credentialed file — check enrollment records. |
| N362 | Units billed exceed allowed frequency. | Verify units against MUE limits; appeal with documentation if clinically justified. |
| N381 | See contractual agreement for payment terms. | Pull your contract — this is the payer saying the rate/rule is contractual; audit the math. |
| N522 | Duplicate of a claim already in process. | Stop rebilling; check status of the in-process claim and work that one. |
No RARC codes match your filter.
Descriptions are written in plain English by the ABA editorial team, not reproduced from official code sets. CARC and RARC codes are maintained by X12; consult the payer's remittance advice and the official X12 lists for authoritative definitions. Fix guidance reflects common payer behavior and our sourcing methodology — individual payer policies control. Updated July 2026.
Codes tell you what happened. The denial management guide shows you how to make them stop.