CO 151 denial code

Claim adjustment reason code 151. Often reported as CO 151.

Records do not support number or frequency

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 151 means

The payer decided the documentation does not justify how many services were billed or how often they were done.

The payer accepts that the service can be covered but not in the quantity or frequency billed. It may have paid some units or visits and denied the rest.

Check both the units on the claim and the payer's frequency limits for the service. If the extra services were needed, the record must explain why.

Group codes seen with 151

On a remittance, 151 comes after a group code. These are the ones it is most often reported with:

• CO 151 (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

• More units were billed than the record documents.
• The service was repeated more often than the payer's policy allows without justification.
• The record did not explain the medical need for the extra services.

What to do next

1. Confirm the units billed match what the record shows was performed.
2. Check the payer's frequency limits for the service.
3. Appeal with records that explain the medical need when the extra services were justified.
4. Correct the units and resubmit if the claim was overbilled.

Related codes

• 150 (reason code) Records do not support level of service
• 119 (reason code) Benefit maximum reached
• 50 (reason code) Not medically necessary per the payer