CARC denial codes, explained in plain English.

CO-197 means the payer says the required prior authorization, precertification, or notification wasn't obtained before the service.

CO-50 means the payer decided the service wasn't medically necessary under its coverage policy, even though it was performed and documented.

CO-16 means the claim is missing or has invalid information the payer needs to adjudicate it.

CO-45 is the contractual adjustment — the difference between your billed charge and the payer's allowed amount, written off under your contract.

CO-97 means the payer applied a bundling edit and folded one procedure into another, paying only the primary code even though both services were performed.

CO-29 means the claim was received after the payer's filing deadline.

CO-96 means the payer says the service isn't a covered benefit under the patient's plan.

CO/OA-18 means the payer thinks this claim or line was already submitted.

OA-22 means the payer believes another plan is primary, or the patient's COB information is out of date, so it won't pay until the order is resolved.

CO-11 means the payer says the diagnosis code doesn't support the procedure billed — almost always a coding or linkage issue, not a care issue.

CO-151 means the payer denied units above a frequency limit or Medically Unlikely Edit (MUE), paying some units and denying the rest.

CARC B7 means the payer says the rendering provider wasn't eligible or credentialed to perform or bill this service on the date of service.

CO-27 means the payer says the patient's coverage had ended before the date of service.

CO-204 means the payer says the item isn't a benefit under the patient's specific plan.

CO-109 means the claim went to the wrong payer — often a Medicare Advantage or managed plan when the claim was sent to traditional Medicare, or the wrong entity entirely.

CO-119 means the payer says a benefit limit (visits, dollars, or occurrences) for the period has been used up.

CO-4 means the payer says a required modifier is missing from the line, or the modifier billed doesn't match the procedure code.

CO-15 means an authorization exists in the picture but the number on the claim is missing, wrong, or doesn't match the service or provider billed.

OA-23 means this payer adjusted its payment to account for what a prior payer already paid or adjusted.

CO-24 means the payer considers this service already paid for under a capitation or managed-care arrangement, so it won't pay fee-for-service on top.

CO-26 means the payer says the service happened before the patient's coverage started.

CO-54 means the payer denied the assistant surgeon (or additional physician) as not covered for this procedure.

CO-55 means the payer classifies the service, drug, or device as experimental or investigational under its medical policy, so it won't cover it — even when it's the appropriate care.

CO-58 means the payer says the service was billed with a place-of-service that doesn't match what it will pay for that procedure — often an office-vs-facility or site-of-care dispute.

CO/OA-59 means the payer reduced the line under multiple-procedure or concurrent-care rules — most often a multiple procedure payment reduction (MPPR).

CO-107 means the payer needs a related or qualifying claim linked to this one — an add-on code without its primary, or a service that references another claim — and it couldn't find the connection.

CO-167 means the payer says the diagnosis billed isn't covered for this service under its coverage policy — usually an LCD/NCD or medical-policy diagnosis-list issue, not a problem with the care.

CO-170 means the payer won't pay this service when billed by your provider type or specialty.

CO-181 means the CPT/HCPCS code billed wasn't valid on the date of service — usually a code that was deleted, replaced, or not yet active for that year.

CO-182 means the modifier billed wasn't valid on the date of service — typically a modifier that was retired or replaced, or one not yet active for that period.

CO-183 means the payer says the provider named as referring isn't eligible to make that referral — often a missing or wrong referring NPI, or a referrer not enrolled with the payer.

CARC 185 means the payer says the rendering provider wasn't eligible or credentialed to perform this service on the date of service — a close cousin of B7, and one that clusters around new hires.

CO-198 means an authorization existed but the services billed went beyond what it allowed — more visits, units, or a longer date range than the auth covered.

CO-226 means the payer asked the provider for information — records, an itemized bill, a questionnaire — and didn't get it, or got an incomplete response, so it denied the claim.

CO-231 means the payer applied an NCCI mutually-exclusive edit — two codes that normally can't both be billed for the same session — and denied one of them.

CO-234 means the payer treats this procedure as not separately payable — its value is considered included in another service.

CO-236 means the payer says the procedure-and-modifier combination on this claim conflicts with another same-day code under NCCI rules, so it denied one of them.

CO-242 means the payer priced or denied the claim because it read the provider as out-of-network (or not the assigned PCP).

CO-252 means the claim is pended for documentation, not denied on the merits — the payer needs records or an attachment before it will pay.

CO-253 is the roughly 2% sequestration reduction on Medicare and Medicare Advantage payments.

CO-256 means the payer says your managed-care contract doesn't cover this service as billed.

CO-288 means an HMO or POS plan won't pay the specialist claim because the required primary-care referral wasn't on file.

CARC B15 means the payer requires a related qualifying service to have been received and covered first, and it hasn't been.

CARC B16 means the payer denied a new-patient E/M because its records show the patient was seen by your group within the prior three years, so it expects an established-patient code.

CARC A1 is a generic denial that carries no reason on its own — the real explanation is in the accompanying RARC/remark code.