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 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| 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. |
| CO-170 | Payment denied for this provider type. | Service outside the provider's payable scope for this payer. | Fix: confirm rendering provider and credentialed specialty; rebill under correct provider if applicable. Prevent: scope rules by payer in the billing SOP. |
| 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. |
| 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). |
| 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. |
| 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. |
| 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. |
| 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. |
| 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.