Medically Unlikely Edits (MUE) in Medical Billing: Unit Limits, MAI 1, 2 and 3, and What to Do

Medically unlikely edits (MUEs) are the unit-of-service limits CMS sets for HCPCS and CPT codes: the maximum number of units a single provider would report for a single patient on a single date of service on the vast majority of correctly coded claims. Exceed the value and the Medicare contractor denies the units.

MUEs are better documented now than when the first version of this article appeared in 2013, and most of what follows is in CMS’s own tables.

Where you see it

Place	What you find
CMS MUE tables	Three quarterly files: practitioner services, facility outpatient hospital services, and DME supplier services. Each row has the code, the MUE value, the MUE Adjudication Indicator (MAI) and the rationale
CMS-1500 box 24G, 837P SV104	The units of service on each line, which is what the edit tests
Remittance advice	Typically CARC 151 (the information submitted does not support this many or this frequency of services) with RARC N362 (the number of days or units exceeds our acceptable maximum); see the reason code column on our annotated remittance
NCCI Policy Manual for Medicare Services, chapter 1, section V	The rules below, revised each January

Not every code has an MUE, and CMS keeps some values confidential. Those are not in the published file and are applied only inside the contractor’s system.

MUE vs NCCI PTP edits

	MUE	PTP edit
Looks at	One code and its units	A pair of codes on the same day for the same patient
Question	Is this many units plausible?	Should these two codes be reported together?
Structure	MUE value plus MAI	Column one code paid, column two code denied, with a modifier indicator (0 never bypassed, 1 bypassed with an appropriate modifier, 9 not applicable)
Bypass	Separate lines with a modifier, MAI 1 only	Modifier 59 or XE, XP, XS, XU and others when the indicator is 1
Published	Quarterly, cms.gov	Quarterly, cms.gov, practitioner and hospital versions

MAI 1, 2 and 3

Before April 1, 2013, every MUE was applied line by line. Since then CMS has been converting codes to date of service edits, and the MAI in the table shows which kind applies to each code.

MAI	Type	How it is applied	Can it be overridden?
1	Claim line edit	Each claim line with the code is tested separately against the MUE value	Yes. Report medically necessary units above the value on separate lines with an appropriate modifier (59 or XE, XP, XS, XU; 76; 77; 91; anatomic modifiers). Each line is adjudicated on its own
2	Date of service edit, policy	Units on all lines for the code and date are summed; if the sum exceeds the value, all units for that code that day are denied	No. The limit comes from statute, regulation or the code descriptor itself; contractors may not override it on processing, reopening or redetermination
3	Date of service edit, clinical	Same summing as MAI 2	On appeal. If medical records show the units were performed, correctly coded and medically necessary, the contractor may bypass the edit on redetermination or a higher-level appeal

Two rules apply to all three. MUE denials may be appealed to the contractor, unlike timely filing denials. And because an MUE denial is a coding denial rather than a medical necessity denial, an Advance Beneficiary Notice does not shift liability: the patient cannot be billed for the denied units.

Radiation oncology values

From the practitioner table effective October 1, 2026:

Code	Service	MUE	MAI
77300	Basic dosimetry calculation	10	3
77334	Treatment devices, complex	10	3
77332	Treatment devices, simple	4	3
77301	IMRT plan	1	3
77280	Simulation, simple	2	3
77387	Guidance for radiation treatment delivery	2	3
77427	Weekly treatment management	1	2
77336	Continuing medical physics consultation	1	2
77470	Special treatment procedure	1	2
96413	Chemotherapy infusion, first hour	1	3
96415	Chemotherapy infusion, each additional hour	8	3

Note the pattern. The codes whose descriptors already define the unit (77427 is per five fractions, 77336 is per five fractions, 77470 is per course) carry MAI 2: two units in one day is impossible by definition and no modifier or appeal will change it. The calculation and device codes carry MAI 3: ten on one day is unusual but not impossible, and an appeal with documentation can be paid.

A worked example

A complex multi-field plan requires 12 basic dosimetry calculations on the same day. The physicist’s worksheet supports all 12. The claim goes out with 77300 at 12 units on one line, 80.00 per unit, 960.00 billed (figures illustrative).

The MUE for 77300 is 10 with MAI 3, a date of service edit. The contractor sums the units for the date, finds 12, and denies all 12, not just the two over the limit: CO-151, N362, 960.00 denied, patient responsibility 0.00. Splitting the claim into a line of 10 and a line of 2 with modifier 76 would not help, because a date of service edit sums across lines before it compares. The remedy is a redetermination request with the plan documentation and the calculation worksheet; with MAI 3 the contractor may bypass the edit and pay all 12.

Contrast 77427. If a claim carries 2 units on one date, the MAI 2 edit denies both and there is nothing to appeal; the correction is to bill each unit on the date its five-fraction block ended.

What to do

• Check units against the current table before you bill. The practitioner table changes quarterly; keep it loaded. The Iridium Suite claim scrubber compares the units on each line with the current MUE value before the claim is released, so a line over the limit is reviewed in the office rather than denied by the contractor.
• For MAI 1 codes, report genuine units above the value on separate lines with the modifier that describes why (anatomic site, repeat procedure, distinct service) and keep the documentation.
• For MAI 2 codes, treat the denial as a coding error. Re-read the descriptor, correct the units or the dates, and resubmit.
• For MAI 3 codes, appeal with records when the units were real and necessary. If they were not, correct and resubmit.
• Never bill the patient for MUE-denied units and do not issue an ABN for them.
• Read the remittance. CARC 151 with N362 is the unit edit; CARC 97 (included in another service) with a different remark code is a PTP edit, which is handled differently.

Related terms

• CPT 77427, weekly radiation treatment management
• Claim adjustment group and reason codes
• Remark codes
• Facility vs non-facility rates

Further reading: CMS: Medicare NCCI Medically Unlikely Edits; NCCI Policy Manual for Medicare Services 2026, chapter 1 (PDF); CMS: Medicare NCCI FAQ library; CMS: Medicare NCCI procedure-to-procedure edits.