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CO-151 Denial Code: Frequency or Units Not Supported

CO-151 means the payer adjusted the claim because it decided the information submitted does not support this many services or this frequency of service. The service may be covered, but not as often, or in as many units, as billed.

Quick facts

Code
CO-151 (CARC 151)
Status
Active In use since October 31, 2002; last modified January 27, 2008.
Code set
Claim Adjustment Reason Codes (CARC)
Group codes
  • CO (Contractual Obligation): The provider absorbs the denied units or services. The patient generally cannot be billed unless a valid advance notice was signed beforehand.
  • PR (Patient Responsibility): Used when the patient was informed in advance that services beyond the frequency limit would be their responsibility, or when the plan makes excess services a member liability.
Official description
Payment adjusted because the payer deems the information submitted does not support this many/frequency of services.
X12 Claim Adjustment Reason Codes, retrieved September 25, 2026
Last verified
against the official X12 list.

What CO-151 means

CARC 151 reads payment adjusted because the payer deems the information submitted does not support this many/frequency of services. The payer accepts that the service can be covered, but not in the quantity or at the interval you billed. It denies the excess, or the whole line, depending on how it applies the limit.

Frequency limits come from several sources: coverage policies (“once every 12 months”), per-day unit limits, per-episode limits, and payer utilization rules. The remark code usually tells you which applies, and the fix depends on whether the claim was simply miscounted or the patient truly needed more.

Example: a screening test covered once per year is performed again ten months after the last one. The payer denies it with CO-151 and remark M90. Unless the patient signed an advance notice, the practice writes it off.

Common causes

  • A service repeated sooner than the payer’s policy allows, such as annual or every-other-year services.
  • Units in box 24G above the per-day or per-encounter limit.
  • Another provider performed the same service for the patient within the limit period.
  • Units reported incorrectly, such as minutes reported instead of units, or a multi-day total on one line.
  • A higher frequency justified by a change in the patient’s condition, but not documented or linked to the right diagnosis.
  • Duplicate lines for the same service on the same date without the modifiers that show they were distinct.

How to fix it

  1. Read the remark code to identify the limit (per day, per year, per episode) and check the payer’s policy.
  2. Check your units and dates. If the units or dates were wrong, send a corrected claim (resubmission code 7 in box 22).
  3. If services were distinct (for example, bilateral or separate sessions), correct the claim with the payer’s required modifiers, such as RT/LT, 50, 76, or 91 where appropriate.
  4. If more services were medically necessary, appeal with records showing why: a new diagnosis, complication, failed treatment, or high-risk status that the policy recognizes.
  5. If the limit applies and there was no advance notice, write off the denied amount. If the patient signed a valid notice, bill them per the payer’s rules.

For per-day unit limits specifically, see our guide to MUE denials and units of service.

How to prevent it

  • Check service history and the date of the last covered service before scheduling services with frequency limits.
  • Load payer frequency and unit rules into your claim scrubber. A Claims Validator can flag units and intervals that exceed common limits.
  • Document the clinical reason whenever a service must be repeated early.
  • Use advance notices when a service will likely exceed a frequency limit.
  • Train staff on unit conversion for timed services.

Specialty notes

PT/OT practices commonly hit CO-151 on timed services where units exceed what the payer allows per visit, or where the total treatment time documented does not support the units billed. Behavioral health practices may see it when psychotherapy is billed more often per week than a payer’s policy allows without additional documentation. DME suppliers see it on replacement supplies billed ahead of the allowed interval.

Remark codes that may appear with CO-151

  • N640 (Exceeds number/frequency approved/allowed within time period.): The service exceeds the number or frequency approved or allowed within the time period.
  • M90 (Not covered more than once in a 12 month period.): Not covered more than once in a 12-month period.
  • N362 (The number of Days or Units of Service exceeds our acceptable maximum.): The number of days or units of service exceeds the acceptable maximum.
  • N435 (Exceeds number/frequency approved /allowed within time period without support documentation.): Exceeds the allowed frequency without supporting documentation, so documentation may reverse it.
  • M86 (Service denied because payment already made for same/similar procedure within set time frame.): Payment was already made for the same or a similar procedure within a set time frame.
  • PR-119 (Benefit maximum for this time period or occurrence has been reached.): Benefit maximum for the period reached, a plan limit rather than a clinical frequency rule.
  • CO-150 (Payer deems the information submitted does not support this level of service.): The information does not support the level of service, rather than the number.
  • CO-198 (Precertification/notification/authorization/pre-treatment exceeded.): Precertification or authorization exceeded, when units go beyond what was approved.
  • CO-273 (Coverage/program guidelines were exceeded.): Coverage or program guidelines were exceeded.
  • OA-18 (Exact duplicate claim/service (Use only with Group Code OA except where state workers' compensation regulations requires CO)): Exact duplicate claim, which can look like a frequency problem.

CO-151 FAQ

Is CO-151 the same as an MUE denial?

Not exactly. Medically Unlikely Edits limit units per day, and payers may report them with various codes. CARC 151 is broader: it covers any frequency or quantity the payer thinks the documentation doesn't support, including per-year limits.

Can I appeal CO-151?

Yes, if the extra services were medically necessary and documented. Send records explaining why the patient needed them more often, such as a change in condition or a complication.

Can I bill the patient for services over the frequency limit?

Only with a valid advance notice signed before the service, where the payer and your contract allow it. Otherwise, under CO, the amount is a write-off.

How do I find a payer's frequency limits?

Check the payer's medical and reimbursement policies, and for Medicare the relevant NCDs, LCDs, and edits. The remark code often names the limit applied.