CARC CO-11 means a payer denied or reduced a claim line because the diagnosis reported doesn’t fit the procedure billed, such as a knee procedure linked to a hip diagnosis or a right-side procedure billed with a left-side code. Group code CO assigns the balance to the provider rather than the patient.

CARC CO-16 means a payer couldn’t pay a claim or line because information was missing or the claim had a submission or billing error. A remittance advice remark code (RARC) names the specific problem, and the usual fix is to correct the data and resubmit rather than appeal.

CARC CO-4 means a payer reduced or denied a claim line because the procedure code doesn’t agree with the modifier billed with it, or because a modifier the code needs is missing or invalid. Group code CO assigns the balance to the provider, so the practice, not the patient, absorbs it.

CARC CO-50 means a payer treated a service as non-covered because it did not consider the service medically necessary. Group code CO assigns the balance to the provider, so the practice can’t bill the patient unless the payer assigns it to patient responsibility, for example after a valid Advance Beneficiary Notice in Original Medicare.

CARC CO-97 means a payer didn’t pay a service separately because it treats that service as already paid through another service or procedure it has processed. It is the usual result of NCCI bundling edits and global surgical periods, and group code CO keeps the balance with the provider.