CO 303 denial code

Claim adjustment reason code 303. Often reported as CO 303.

QMB: prior payer cost sharing 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 303 means

The payer will not cover the prior payer's cost sharing for a Qualified Medicare Beneficiary, and the patient is not billed for it.

Qualified Medicare Beneficiaries are people in a Medicaid program that covers Medicare cost sharing. Federal rules generally bar providers from billing them for Medicare deductibles, coinsurance, and copays.

This code is only used with group code CO, so the remaining cost sharing is the provider's, not the patient's. Compare it with code 275, which uses PR for patients without this protection.

Group codes seen with 303

On a remittance, 303 comes after a group code. These are the ones it is most often reported with:

• CO 303 (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 is a Qualified Medicare Beneficiary and Medicaid did not pay the cost sharing.
• The state Medicaid payment for the crossover claim was zero or limited under its rules.
• The secondary payer follows a lesser of policy for dual eligible patients.

What to do next

1. Confirm the patient's QMB status in eligibility.
2. Make sure the cost sharing is not on a patient statement, and remove it if it is.
3. Post the amount according to your policies for QMB cost sharing, using the CO group code as a guide.

Related codes

• 275 (reason code) Prior payer's patient share not covered
• 1 (reason code) Amount applied to the deductible
• 2 (reason code) Patient's coinsurance share