CO-56 denial code
Procedure has not been deemed proven to be effective
How to fix it
Appeal against the payer's published coverage criteria with supporting evidence.
How to prevent it
Review medical policy in advance for services that sit at the edge of accepted practice.
In practice
A patient receives a treatment supported by early clinical literature but not yet in wide use. The plan returns CO-56, classifying it as not proven effective.
This determination rests on the payer's own medical policy, which sets out what evidence the plan requires before it considers a service established. That policy is published, and reading it is the difference between an appeal that engages and one that asserts.
A viable appeal cites the specific evidentiary standard in the policy and shows the published literature meets it. Where the plan requires randomised trial evidence and only case series exist, the appeal will not succeed and the honest answer is to tell the patient before treatment rather than after.
What sits behind it
The neighbouring codes describe adjacent positions. CO-55 covers experimental and investigational services, CO-56 covers services considered unproven, and CO-114 covers items lacking FDA approval for the indication. Payers apply them inconsistently and the practical approach is the same for all three.
Because these determinations follow the evidence base, they change over time. A service denied as unproven in one year may be routinely covered two years later once trial results publish, which makes it worth rechecking policy before assuming a standing denial still stands.
For any service sitting near this boundary, the conversation belongs before delivery. A patient who understands the plan considers a treatment unproven, and who agrees in writing to accept the cost, is in a very different position from one who receives a bill for a service they assumed was covered.
Related codes
Terms used here — Medical Necessity · Appeal · Patient Responsibility
How we handle it — Denial Management · Prior Authorization · Patient Support
Primary sources
The rules behind CO-56, at the bodies that publish them.
- Medicare Coverage Database (LCD/NCD) (opens in a new tab)
Centers for Medicare & Medicaid Services — Searchable national and local coverage determinations. The direct answer to whether a diagnosis supports medical necessity for a given procedure.
- Medicare Benefit Policy Manual (opens in a new tab)
Centers for Medicare & Medicaid Services — What Medicare covers and under what conditions, as distinct from how a claim is processed. The starting point for any coverage or medical necessity question.
- Advance Beneficiary Notice of Noncoverage (ABN) (opens in a new tab)
Centers for Medicare & Medicaid Services — The form and the rules for issuing it. Whether a non-covered service can be billed to the patient usually turns on whether a valid ABN was obtained beforehand.
Looking for a different code? Search all 190 CARC and RARC codes
Questions about CO-56
Engaging the plan's published evidentiary standard directly. Find the medical policy, identify what level of evidence it requires, and show the literature meets it. An appeal asserting clinical benefit without addressing the standard the plan actually applies will be upheld against you.
CO-55 covers services the payer classifies as experimental or investigational; CO-56 covers those it considers unproven in effectiveness. The distinction is fine and payers apply the two inconsistently, so the appeal strategy is effectively identical for both.
Yes, and it is worth rechecking. Coverage policy follows the evidence base, so a service denied as unproven can become routinely covered once further trials publish. Before treating a standing denial as settled, confirm the current version of the policy still says what it said.
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