CO 56 denial code

Claim adjustment reason code 56. Often reported as CO 56 or PR 56.

Effectiveness not proven per the payer

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 56 means

The payer will not cover the service because it does not consider it proven to work for the patient's condition.

This is close to an experimental denial but rests on effectiveness: the payer has decided the evidence does not show the treatment works well enough for this use. It is a coverage decision, not a coding error.

Appeals usually need clinical literature and records specific to the patient. The line may carry a payer policy identifier, which is the quickest way to see which rule was applied.

Group codes seen with 56

On a remittance, 56 comes after a group code. These are the ones it is most often reported with:

• CO 56 (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 56 (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 payer's medical policy lists the treatment as unproven for the diagnosis billed.
• The service was billed with a diagnosis the payer does not accept for that treatment.
• The treatment is effective for some conditions, but the payer limits coverage to others.

What to do next

1. Read the payer's medical policy for the service and the conditions it covers.
2. Check whether the diagnosis on the claim reflects the patient's actual condition.
3. Appeal with peer reviewed evidence and the patient's records if the treatment is appropriate.
4. Confirm the group code and any patient agreement before moving the balance.

Related codes

• 55 (reason code) Experimental or investigational service
• 50 (reason code) Not medically necessary per the payer
• 11 (reason code) Diagnosis does not support the procedure