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CO-56 Denial Code: Not Proven to Be Effective

CO-56 means the payer hasn't deemed the procedure or treatment 'proven to be effective,' so it won't cover it. The service may be established in practice, but the payer's policy finds the evidence insufficient for this condition or use.

Quick facts

Code
CO-56 (CARC 56)
Status
Active In use since January 1, 1995; last modified July 1, 2017.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider absorbs the amount unless the denial is overturned or the patient accepted financial responsibility in advance.
  • PR (Patient Responsibility): Used when the patient is responsible, typically after a valid advance notice or waiver. The patient can then be billed.
Official description
Procedure/treatment has not been deemed 'proven to be effective' by the payer. Usage: Refer to the 835 Healthcare Policy Identification Segment (loop 2110 Service Payment Information REF), if present.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-56 means

CARC 56 says the procedure or treatment has not been deemed “proven to be effective” by the payer. It’s similar to CO-55, but the emphasis is different. Instead of calling the service experimental, the payer is saying the evidence doesn’t convince it the service works well enough, for this condition or in this way, to be covered.

This language often appears in policies for services that are widely offered but debated, such as certain injections, devices, therapies, or testing approaches. The usage note refers to the 835 Healthcare Policy Identification segment, where the payer may name the policy.

Common causes

  • Payer policy classifies the service as unproven for the patient’s diagnosis.
  • Service billed for a non-covered indication even though it’s covered for others.
  • Treatment approach or frequency the policy doesn’t consider effective, such as repeated treatments beyond a covered number.
  • Newer versions of established procedures that the payer hasn’t evaluated separately.

How to fix it

  1. Get the payer’s policy for the service and date of service, including its covered indications and evidence summary.
  2. Check whether the patient meets any covered indication. If so, correct the diagnosis coding and send a corrected claim with resubmission code 7 in box 22.
  3. Appeal with evidence. Include peer-reviewed studies, specialty guidelines, and a clinician’s letter explaining why the treatment is effective for this patient.
  4. Show individual response where relevant, such as documented improvement from earlier treatments.
  5. Pursue external review if the plan is subject to it and internal appeals fail.
  6. Bill the patient only when the payer reports PR or a valid advance agreement exists.

How to prevent it

  • Review medical policies for services with a history of this denial.
  • Request predetermination or prior authorization when offered.
  • Discuss coverage risk with patients in advance and document their decision.
  • Track which payers deny which services so scheduling staff can flag them. See preventable medical claim denials.

Specialty notes

Pain management, integrative medicine, and some behavioral health treatments, such as newer neuromodulation approaches, frequently encounter “unproven” policies. Coverage varies widely between payers and changes over time.

Remark codes that may appear with CO-56

  • N623 (Not covered when deemed unscientific/unproven/outmoded/experimental/excessive/inappropriate.): Not covered when deemed unscientific, unproven, outmoded, experimental, excessive, or inappropriate.
  • N130 (Consult plan benefit documents/guidelines for information about restrictions for this service.): Directs you to the plan's benefit documents for the restriction.
  • CO-55 (Procedure/treatment/drug is deemed experimental/investigational by the payer.): Experimental or investigational, used when the payer sees the service as still under study.
  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): Not medically necessary for this patient, a patient-level decision rather than a policy about the treatment.
  • CO-188 (This product/procedure is only covered when used according to FDA recommendations.): Covered only when used according to FDA recommendations.
  • PR-204 (This service/equipment/drug is not covered under the patient's current benefit plan): Not covered under the patient's benefit plan.

CO-56 FAQ

What's the difference between CO-55 and CO-56?

CO-55 labels a service experimental or investigational, suggesting it's still being studied. CO-56 says the payer doesn't consider it proven effective, which can apply to established services the payer thinks lack enough evidence for this use. Appeals for both rely on clinical evidence.

What evidence helps overturn CO-56?

Peer-reviewed outcome studies, specialty society guidelines and position statements, other payers' coverage decisions, and documentation of this patient's response to prior treatments.

Can I bill the patient?

Only if the payer reports PR or the patient signed an advance agreement the payer recognizes.

Should I stop offering a service after CO-56 denials?

Not necessarily. Coverage differs by payer, so track which plans deny it and discuss costs with affected patients before treatment.