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CO-55 Denial Code: Experimental or Investigational

CO-55 means the payer classified the procedure, treatment, or drug as experimental or investigational, so it doesn't cover it. Appeals typically rely on clinical evidence and guidelines showing the service is accepted standard care for the patient's condition.

Quick facts

Code
CO-55 (CARC 55)
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 cost unless the denial is overturned or the patient agreed in advance to pay under the payer's rules.
  • PR (Patient Responsibility): Used when the patient is responsible, for example after signing an advance agreement acknowledging the service may not be covered.
Official description
Procedure/treatment/drug is deemed experimental/investigational 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-55 means

CARC 55 says the procedure, treatment, or drug is deemed experimental or investigational by the payer. The payer’s medical policy has concluded that the service isn’t established medical practice for the patient’s condition, so the plan excludes it.

Payers make these decisions through their own technology assessment processes, and policies change as evidence grows. A treatment one payer considers experimental may be covered by another, or covered by the same payer a year later. The usage note points to the 835 Healthcare Policy Identification segment, where the payer can name the policy used.

Common causes

  • New procedures or technologies that payers haven’t yet adopted into coverage.
  • Off-label drug use outside the payer’s accepted indications.
  • Category III or emerging technology codes for tracking new services, which payers often don’t cover.
  • Services for conditions outside the payer’s covered indications, even when covered for others.
  • Clinical trial services billed without the required trial identifiers.

How to fix it

  1. Read the payer’s policy that applies to the service and date of service.
  2. Check whether it covers specific indications. If the patient meets them, send documentation or a corrected claim with the right diagnosis codes (resubmission code 7 in box 22).
  3. Build an appeal with peer-reviewed literature, specialty guidelines, FDA status, and a letter of medical necessity tied to the patient’s clinical situation.
  4. Request peer-to-peer review where offered.
  5. Report clinical trial information if the service was part of a qualifying trial.
  6. Consider external review if internal appeals are exhausted and the plan is subject to external review rules.
  7. Bill the patient only if the payer reports PR or a valid advance agreement exists.

How to prevent it

  • Check medical policies before scheduling new or emerging procedures.
  • Request prior authorization or predetermination where available so coverage is decided up front.
  • Inform patients in writing when a service may be considered experimental, following payer rules.
  • Track policy updates for technologies your practice offers.

Remark codes that may appear with CO-55

  • N623 (Not covered when deemed unscientific/unproven/outmoded/experimental/excessive/inappropriate.): Not covered when deemed unscientific, unproven, outmoded, experimental, excessive, or inappropriate.
  • MA50 (Missing/incomplete/invalid Investigational Device Exemption number or Clinical Trial number.): The Investigational Device Exemption or clinical trial number is missing or invalid.
  • CO-56 (Procedure/treatment has not been deemed 'proven to be effective' by the payer.): Not proven to be effective, a close cousin of the experimental denial.
  • CO-114 (Procedure/product not approved by the Food and Drug Administration.): The procedure or product isn't approved by the FDA.
  • CO-188 (This product/procedure is only covered when used according to FDA recommendations.): Covered only when used according to FDA recommendations, relevant for off-label use.
  • CO-50 (These are non-covered services because this is not deemed a 'medical necessity' by the payer.): A general medical necessity denial.

CO-55 FAQ

How do I appeal an experimental denial?

Identify the payer's policy, then submit peer-reviewed studies, specialty society guidelines, FDA status, and a letter of medical necessity explaining why the service is appropriate for this patient. Ask for a peer-to-peer review if available.

What about clinical trials?

Many plans and Medicare cover routine costs for patients in qualifying clinical trials, even when the investigational item itself isn't covered. Report the trial information the payer requires.

Can the patient be billed?

If the payer reports PR or the patient signed a valid advance agreement allowed by the payer, generally yes. Under CO without such an agreement, the provider absorbs the cost.

Will a prior authorization protect me from CO-55?

An approved authorization usually means the payer reviewed the service against its policy, which reduces the risk. However, an approval does not guarantee payment, so keep the approval letter in case you need it for an appeal.

Why does one payer cover a service another calls experimental?

Each payer runs its own evidence review and updates policies on its own schedule. Coverage can differ between payers and change over time.