CO-151: more units than Medicare accepts
The payer decided you billed more units of a service than it accepts. Medicare's unit limits decide that, and one number on each limit tells you whether the denial can be fixed.
01What CO-151 means
Claim adjustment reason code 151 says the payer considers that the information submitted doesn't support this many units or this frequency of service. On Medicare claims it usually means the units billed exceeded a Medically Unlikely Edit (MUE): the maximum units of a code that one provider would normally report for one patient on one day. It often comes with remark code N362, "the number of days or units of service exceeds our acceptable maximum".
02How Medicare's unit limits work
CMS publishes MUE values for most codes, updated quarterly. The practitioner table holds thousands of limits. Each limit carries an MUE adjudication indicator (MAI), and that one number decides what happens and whether you can fix it:
| MAI | Type | What happens | Can it be fixed? |
|---|---|---|---|
| 1 | Line edit | The limit applies to each claim line | Sometimes: when the extra units were genuinely separate (a different site or session), they can be billed on separate lines with the right modifiers |
| 2 | Date of service edit: policy | All units of the code on that day are added up and checked against the limit | No. The limit is absolute and not appealable |
| 3 | Date of service edit: clinical | All units on that day are added up and checked against the limit | Yes, by appeal with documentation that the extra units were medically necessary |
When a Medicare practitioner claim exceeds an MUE, the units are not simply trimmed back: Medicare's policy is to deny the line (or, for MAI 2 and 3, the lines for that code on that day). Some other payers reduce the units instead.
03Two cases worth knowing
- One per day. Many evaluation codes, such as the physical therapy evaluations 97161 to 97163, have a limit of one per day. A second evaluation on the same day won't be paid.
- A limit of zero. Some codes have an MUE of 0 in the practitioner setting, which means Medicare doesn't pay them there at all. That isn't a units problem, and cutting units won't fix it.
04How to prevent it
- Check the units on every line against the current MUE, before the claim goes out.
- Know the MAI for the codes you bill most. It tells you in advance whether an overage can ever be paid.
- Split lines only when the services were genuinely separate and the note shows it. Splitting to get around the limit is what payers audit for.
- Watch for codes with a limit of zero in your setting.
05After the denial
- MAI 1: check whether the units were really separate services. If they were, correct the claim with separate lines and the right modifiers.
- MAI 2: the denial stands. Fix the habit that produced it.
- MAI 3: if the extra units were medically necessary, request a redetermination with the documentation.
This guide is general information, not legal or billing advice. Always check the current MUE table and your payer's own rules.
Bring one recent denial. We'll show you the rule behind it.
Your specialty, your payers, your claim. No slides.
Book a 20-minute callContact details only. Nothing about patients. Not ready to talk? Take the 60-second tour.