CO-151 Denial Code: Billed units or frequency exceed the payer's limit

What CO-151 means

CO-151 is a units/frequency denial. The payer paid up to its allowed quantity and denied the excess, or denied the line because the volume exceeds policy. For Medicare this often reflects a Medically Unlikely Edit (MUE), which caps the units of a HCPCS/CPT code per patient per day. Sometimes the units were keyed incorrectly; sometimes the documentation genuinely supports more units than the edit allows, in which case the excess may be reported with the correct documentation.

Common causes

• Units billed exceed the MUE value for the code (per patient, per day).
• A data-entry error inflated the unit count.
• Frequency exceeds a payer policy limit (for example, allowed visits or tests per period).
• Bilateral or multiple services were reported as raw units instead of with the correct modifier.

How to fix it

• Confirm the units actually performed and documented versus the units billed.
• If keyed wrong, submit a corrected claim with the right units.
• If the documentation supports more units than the MUE allows, the path depends on the edit's MAI: MAI 1 (claim-line) - report the excess on separate lines with the appropriate modifier; MAI 3 (date-of-service) - submit with documentation of medical necessity; MAI 2 (date-of-service) edits are absolute and cannot be exceeded.
• For bilateral or multiple services, report with the correct modifier rather than inflated units.

How to prevent it

• Scrub units against current MUE values before submission.
• Use the correct anatomic/bilateral modifiers instead of raw unit counts.
• Build frequency-limit checks for policy-capped services (therapy, labs, imaging).

Appeal letter template

Fill in the bracketed fields, attach your supporting documentation, and send through the payer's appeal channel. This is a starting point — adjust to your payer's requirements.

Re: Units/Frequency Appeal (CO-151) - Claim [Claim Number]

Patient: [Patient Name]   |   Member ID: [Member ID]
Date(s) of Service: [DOS]   |   Code: [CPT/HCPCS]   |   Units billed: [Units]

To Whom It May Concern:

Claim [Claim Number] line for [CPT/HCPCS] was denied/reduced under CO-151 for frequency/units. The documentation supports [Units] units on [DOS] because [clinical rationale]. We request reconsideration and payment of the medically reasonable units.

Supporting records are enclosed.

Sincerely,
[Your Name], [Practice Name]   |   [Phone]   |   [NPI/TIN]