CO-56 Denial Code: Not Proven Effective

The official definition

“Procedure/treatment has not been deemed proven to be effective by the payer”

That is the verbatim definition of CARC 56 from the X12 Claim Adjustment Reason Code set, the standardized codes every insurer uses on EOBs and remittance advices. The letters in front of the number are the group code. CO: Contractual Obligation: an adjustment between the provider and the insurer. An in-network provider should not bill you for CO amounts.

What it means in plain English

A close cousin of the experimental denial: the insurer says the treatment hasn't been proven to work. The dispute is about evidence, which means the appeal is about evidence: published studies, clinical guidelines, and your doctor's documentation of why this treatment fits your case.

What to check on your EOB

• The insurer's stated basis: request the medical policy or criteria they applied.
• Whether the treatment carries FDA approval or appears in recognized clinical guidelines for your condition.

What to do next

1. Request the insurer's evidence standard for 'proven effective' and the policy they cited.
2. Build the counter-file with your doctor: published studies, specialty-society guidelines, and a letter of medical necessity.
3. Appeal internally, then pursue external review. Independent reviewers apply clinical standards, not the payer's policy.

Who's responsible

You, with your doctor's support. Most denials carry a clear owner. Knowing whether the fix belongs to you, your doctor, or the billing office is half the battle. If it's the provider's error, you should not be paying for it.

Want the fundamentals first? Start with how to read an EOB and the 7 most common billing errors. This page is general information about standardized denial codes, not legal or medical advice.