PR 51 denial code

Claim adjustment reason code 51. Often reported as PR 51 or CO 51.

Pre-existing condition not covered

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 51 means

The payer excluded the service because it treats the condition as one the patient had before coverage started.

The payer applied a pre-existing condition exclusion or waiting period to this claim. Many plans can no longer use these exclusions, but some types of coverage still do.

If the plan type should not apply such an exclusion, or the patient had prior creditable coverage, the denial may be worth challenging. A policy reference at the service line, when the payer sends one, names the rule behind the decision.

Group codes seen with 51

On a remittance, 51 comes after a group code. These are the ones it is most often reported with:

• PR 51 (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.
• CO 51 (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.

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 patient's plan type still allows pre-existing condition exclusions or waiting periods.
• The payer did not have the patient's prior coverage history on file.
• The diagnosis on the claim matched a condition the payer recorded before the coverage start date.

What to do next

1. Confirm the type of plan and whether it can apply pre-existing condition exclusions.
2. Ask the patient for proof of prior coverage if the plan gives credit for it.
3. Appeal with that proof or with the plan's own terms if the exclusion looks wrong.
4. Check the group code before moving any balance to the patient.

Related codes

• 50 (reason code) Not medically necessary per the payer
• 26 (reason code) Service before coverage began
• 179 (reason code) Waiting period not met