CO 55 denial code

Claim adjustment reason code 55. Often reported as CO 55 or PR 55.

Experimental or investigational service

Unofficial reference. This is our own plain-English explanation, not the official code text. Payer rules differ. Verify with the payer and the official X12 code list. AlphaPrefix is not affiliated with X12, CMS or any payer. Data last updated Nov 11, 2026.

What 55 means

The payer considers the procedure, treatment or drug experimental or investigational, so the plan excludes it.

The payer's policy classifies this service as not yet proven or accepted for the patient's condition, so the plan excludes it. Drugs used outside their approved indications and newer procedures are common examples.

Clinical trial costs and newly accepted treatments may be handled differently, and policies change. Group code CO or PR depends on the contract and on whether the patient agreed to pay in advance. When the payer includes a policy reference on the line, read that policy before correcting or appealing.

Group codes seen with 55

On a remittance, 55 comes after a group code. These are the ones it is most often reported with:

• CO 55 (Contractual Obligation): The provider's side of the ledger. The reduction comes from the payer contract or a rule the provider has to follow, and the patient normally cannot be billed for it.
• PR 55 (Patient Responsibility): The amount is assigned to the patient. Deductibles, coinsurance and copays are the everyday examples. Whether you can collect it still depends on your contract and the patient's plan.

The payer chooses the group code for each adjustment, so the same reason code can appear with another group code on your remittance. Read the two together. More about group codes.

Common causes

• The procedure or device is new and not yet covered in the payer's medical policy.
• A drug was given for a use the payer does not accept for the diagnosis billed.
• The service was part of a clinical trial and not billed with the trial indicators the payer expects.

What to do next

1. Read the payer's medical policy for the service and note the effective date.
2. Appeal with published evidence and clinical records if the service is accepted practice for the condition.
3. Check whether clinical trial billing rules apply and correct the claim if trial indicators were missing.
4. Look for a signed patient agreement before moving any balance to the patient.

Related codes

• 56 (reason code) Effectiveness not proven per the payer
• 50 (reason code) Not medically necessary per the payer
• 39 (reason code) Authorization was requested and denied