Every remittance carries a Claim Adjustment Reason Code — a CARC — and most practices read it as a rejection. It is better understood as a diagnosis. The code tells you which part of the process failed, and the fix for each is different.
Here are the ten that account for the bulk of denial volume in an independent practice.
CO-16 — Claim lacks information
The most common and least specific code on the list. Something required was missing or invalid, and the accompanying Remittance Advice Remark Code (RARC) tells you what. Read the RARC, not the CARC.
In practice this is usually a missing modifier, an absent referring provider NPI, or a timed service billed without documented start and stop times.
Fix: correct and resubmit as a corrected claim, not a new one. A fresh submission of the same claim generates CO-18.
CO-18 — Duplicate claim or service
Either the claim really was sent twice, or it looks like a duplicate because laterality or distinct-service modifiers were omitted. Two identical procedure lines on the same date with no LT/RT or 59/XU will be read as one claim submitted twice.
Fix: check whether the second line is genuinely distinct before appealing. If it is, add the correct modifier and resubmit.
CO-22 — Coordination of benefits
The payer believes another plan is primary. This is a front-desk problem wearing a billing costume.
Fix: confirm the primary payer at eligibility verification, bill the primary first, then submit the secondary with the primary's remittance attached.
CO-27 — Expenses incurred after coverage terminated
The plan had ended by the date of service. Almost always preventable: coverage terminates far more often than patients realise, particularly in January and after a job change.
Fix: verify eligibility before the visit. Once the patient has gone, this balance is very hard to collect.
CO-29 — Timely filing limit expired
The claim was submitted past the payer's deadline. This one is usually final, and it is the reason unworked A/R is not a passive loss.
Fix: there is rarely a fix after the fact. Some payers accept proof of timely original submission. The real answer is a follow-up cadence that reaches every claim before its window closes.
CO-50 — Not deemed medically necessary
The diagnosis submitted does not support the procedure under the payer's coverage policy. This is a policy determination, not a coding error, and it is appealable.
Fix: read the payer's Local Coverage Determination or medical policy bulletin, confirm the documentation supports the covered indication, and appeal with the clinical record. See how to appeal a CO-50.
CO-96 — Non-covered charge
The service is excluded under the plan. Distinct from CO-50: this is not about necessity, it is about what the contract covers at all. Cosmetic procedures, some screenings outside their frequency, and services carved out to another administrator all land here.
Fix: check whether the benefit sits with a carve-out administrator before appealing. Often the claim was simply sent to the wrong payer.
CO-97 — Included in another service
The payer bundles this service into one already paid. NCCI edits drive most of these.
Fix: determine whether the bundle is correct. If the services were genuinely distinct, the appropriate X modifier — not a reflexive 59 — plus documentation supports an appeal. If the bundle is correct, the fix belongs in coding, not appeals.
CO-197 — Precertification or authorisation absent
The service required authorisation and did not have one. Difficult to overturn after the fact, and in many contracts the balance cannot be billed to the patient either.
Fix: some payers permit retroactive authorisation in defined circumstances. Pursue it where policy allows. Otherwise this is a scheduling process problem — see what a CO-197 really costs.
CO-B7 — Provider not certified or eligible
The rendering provider was not enrolled with this payer, or was not enrolled on that date. Common with new hires and after a missed revalidation.
Fix: confirm the effective date, and hold claims for a provider whose enrolment is pending rather than submitting them into a denial. Credentialing lapses can invalidate months of claims retroactively.
The pattern matters more than the claim
Working denials one at a time keeps the lights on. Working them by category tells you what to change. If CO-27 is a third of your denial volume, the answer is not a better appeals process — it is verifying eligibility before the visit.
That is the difference between denial management and denial administration.