Claim denial codes, explained.
What each common denial code means, what usually causes it, and the steps to fix and prevent it. Educational summaries, not billing advice. Payer rules vary.
- CO-4Modifier issueThe payer says the procedure code is inconsistent with the modifier on the claim, or a modifier the code needs is missing.
- CO-11Diagnosis mismatchThe payer says the diagnosis code on the claim does not support the procedure billed.
- CO-16Missing informationThe claim is missing information or has a billing or submission error, so the payer could not process it.
- CO-18Duplicate claimThe payer says this claim or service duplicates one it already received.
- CO-22Other payer primaryThe payer believes another insurer should pay first under coordination of benefits.
- CO-27Coverage had endedThe payer says the patient's coverage had ended before the date of service.
- CO-29Timely filing expiredThe payer says the claim reached it after the filing deadline.
- CO-45Above fee scheduleThe billed charge was higher than the payer's allowed amount, so the difference is written off as a contractual adjustment.
- CO-50Medical necessityThe payer decided the service was not medically necessary under its coverage policy.
- CO-96Non-covered chargeThe payer says the service or charge is not a covered benefit under the patient's plan.
- CO-97Bundled serviceThe payer says this service is already included in the payment for another service billed the same day.
- CO-109Wrong payerThe payer says the claim belongs to a different payer or contractor.
- CO-197Authorization missingThe payer denied the claim because a required prior authorization or notification was not obtained.