CO Modifier in Medical Billing: Outpatient Occupational Therapy Assistant Guidelines
A quick note before anything else, because it’s the single most important fact in this guide; the AR modifier and the Medicare Physician Scarcity Area (PSA) bonus it supported are no longer active. The bonus program officially sunset for dates of service after June 30, 2008. If you’re coding a current claim, AR isn’t the modifier you’re looking for, no matter what a training manual, template, or older billing reference might still say.
That said, understanding what AR was, why it existed, and why it disappeared is genuinely useful for anyone working in medical billing today. It explains part of the history behind the Health Professional Shortage Area bonus program that’s still very much active, it matters for anyone auditing or researching historical claims, and it helps clear up a real source of confusion between AR and its still-active cousin, the AQ modifier. This guide covers all of that clearly, without presenting expired billing guidance as if it were current.
The CO modifier identifies outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant (OTA), and it triggers a mandatory 15 percent reduction to the Medicare payment for that service line. It’s required whenever an OTA provides more than 10 percent of a given unit of service, a threshold known as the de minimis standard. Below that threshold, the supervising occupational therapist can bill the full rate without CO attached at all. This guide walks through exactly how that 10 percent line gets calculated, when CO applies, how it pairs with the GO modifier, and where these claims most often go wrong.
What Is the CO Modifier?
CO is a HCPCS Level II modifier that stands for Outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant. It’s appended to the CPT code for the specific therapy service, alongside the GO modifier that already identifies the claim as occupational therapy, whenever an OTA’s involvement in delivering that service crosses a defined minimum threshold.
The modifier exists because of a fairly specific piece of legislation. Section 53107 of the Bipartisan Budget Act of 2018 directed CMS to implement a reduced payment rate for physical therapy and occupational therapy services delivered by assistants rather than by the licensed therapist. CMS phased the requirement in over several years: the modifier itself became mandatory on claims for dates of service on or after January 1, 2020, but the actual 15 percent payment reduction didn’t take effect until January 1, 2022. Since then, CO has directly determined whether a given service line pays at the full physician fee schedule rate or at 85 percent of it.
Why the CO Modifier Matters
Before this policy existed, Medicare paid outpatient therapy services at the same rate regardless of whether the treating therapist or a supervised assistant actually delivered the care. CMS’s rationale for changing that, reflected in the CY 2022 Physician Fee Schedule final rule, was that assistant-delivered care represents a different resource cost than therapist-delivered care, and payment should reflect that difference.
For OT practices, that shift changed the financial math around how OTAs get used day to day. A service delivered entirely by the occupational therapist bills at the full rate. The same service delivered even partly by an OTA, once past the 10 percent threshold, bills at 85 percent. That’s a real, permanent difference in revenue per unit, not a one-time adjustment, and it’s exactly why accurate time tracking and correct modifier application matter so much in OT billing.
It’s worth being precise about what actually gets reduced, too. The 15 percent cut applies to Medicare’s Part B payment share, the 80 percent Medicare covers, not to the patient’s 20 percent coinsurance. The patient’s out-of-pocket portion isn’t affected by whether CO is on the claim.
The 10 Percent De Minimis Standard
This is the single most important number in the entire CO modifier framework, and it’s worth understanding in detail because it determines whether the modifier applies at all.
CMS defines a service as “furnished in whole or in part” by an OTA once the OTA’s contribution reaches more than 10 percent of that billed unit of service. This is commonly called the de minimis standard, and it applies to both timed and untimed therapy codes.
| OTA Contribution to the Unit | CO Modifier Required? | Payment Impact |
|---|---|---|
| 10% or less of the unit | No | Full rate, billed under the therapist |
| More than 10% of the unit | Yes | 15% reduction, 85% of the applicable rate |
| 100% of the unit, OTA alone | Yes | 15% reduction |
An important nuance sits inside that first row: the 10 percent threshold is measured against the billed unit, not against the entire therapy session for that date of service. A patient might receive several different CPT codes in one visit, and each code gets evaluated against the de minimis standard independently, based on how the minutes for that specific code broke down between the OT and the OTA.
How to Calculate De Minimis: The Percentage Method
CMS’s guidance walks through this using a straightforward percentage calculation. When an OT and an OTA both contribute minutes toward the same billed unit, divide the OTA’s minutes by the total combined minutes for that unit, then multiply by 100.
Example: An OTA provides 5 minutes of a therapeutic activity, and the supervising OT provides 6 minutes of the same activity, for a combined 11 minutes going toward one billed unit.
5 (OTA minutes) ÷ 11 (total minutes) = 0.45
0.45 x 100 = 45 percent
Since 45 percent is well above the 10 percent threshold, CO is required on that unit, and the 15 percent reduction applies.
There’s an important rule buried in how minutes get attributed when the OT and OTA work together on the same service: minutes that the OT and OTA provide together, at the same time, toward the same CPT code are counted entirely as therapist minutes, not split proportionally. The de minimis calculation only comes into play when the OTA’s contribution is separate and distinguishable from the OT’s, which is most common when an OTA independently delivers part of a session, or an entire unit, on their own.
CO vs. CQ: Occupational Therapy vs. Physical Therapy
CO and CQ are built on identical logic, and understanding one makes the other immediately familiar. The only real difference is which discipline and which base modifier each one pairs with.
| Feature / Metric | CO (Occupational Therapy) | CQ (Physical Therapy) |
|---|---|---|
| Assistant type | Occupational Therapy Assistant (OTA) | Physical Therapy Assistant (PTA) |
| Pairs with base modifier | GO | GP |
| De minimis threshold | More than 10% of the billed unit | More than 10% of the billed unit |
| Payment reduction | 15% reduction, paid at 85% of the fee schedule rate | 15% reduction, paid at 85% of the fee schedule rate |
| Effective date for the reduction | January 1, 2022 | January 1, 2022 |
| Legal basis | Section 53107 of the Bipartisan Budget Act of 2018 | Section 53107 of the Bipartisan Budget Act of 2018 |
Because they operate identically, a practice running both OT and PT services under one roof can generally apply the same training, workflow, and time-tracking logic to both, just watching for which discipline, and which assistant credential, applies to each specific claim line.
Where the CO Modifier Applies
CO isn’t universal across every setting that bills Medicare for occupational therapy. It applies specifically to services paid under the Medicare Physician Fee Schedule.
| Setting | CO Modifier Applies? |
|---|---|
| Outpatient hospital departments | Yes |
| Rehabilitation agencies | Yes |
| Skilled nursing facilities, billing under Part B | Yes |
| Home health agencies, billing under Part B | Yes |
| Comprehensive Outpatient Rehabilitation Facilities (CORFs) | Yes |
| Private practice OT clinics | Yes |
| Critical Access Hospitals | No, since CAHs aren't paid under the Physician Fee Schedule |
Supervision Requirements for OTAs
Supervision rules matter alongside CO because they determine whether an OTA’s involvement was even compliant in the first place, separate from the payment reduction question. As of the CY 2026 Physician Fee Schedule final rule, general supervision of OTAs in outpatient private practice settings is now permanently authorized, meaning the supervising occupational therapist does not need to be physically on-site while the OTA delivers care, but does need to be available for real-time consultation if needed.
| Supervision Level | What It Requires |
|---|---|
| General supervision (private practice, current standard) | OT available for real-time consultation, not required to be on-site |
| Direct supervision (some other settings and payer contexts) | OT present in the office suite, though not necessarily in the same room |
Supervision level and the CO modifier are related but separate compliance questions. Getting the CO calculation right doesn’t excuse inadequate supervision documentation, and vice versa. Both need to be correct independently for the claim to hold up under review.
CO Modifier Documentation Requirements
| Documentation Element | Why It Matters |
|---|---|
| Minute-by-minute breakdown of OT vs. OTA time per CPT code | This is the raw data the de minimis calculation depends on |
| Clear identification of which provider delivered which portion of each service | Confirms whether the 10% threshold was crossed for that specific unit |
| Supervision documentation appropriate to the setting | Confirms the OTA's involvement met the applicable supervision standard, separate from the CO calculation |
| GO modifier present alongside CO | CO doesn't stand alone; it needs the base occupational therapy modifier on the same line |
| Plan of care reflecting OTA involvement, where applicable | Supports that assistant-delivered care was part of an established, therapist-directed plan |
The minute-by-minute documentation is the piece that determines everything else. Without a clear record of exactly how many minutes the OT contributed versus the OTA for each specific code, there’s no reliable way to run the de minimis calculation, and a billing team is left guessing whether CO should be on the claim at all.
Common CO Modifier Billing Mistakes
Common CO modifier billing mistakes include reporting it for services not furnished by an occupational therapy assistant, overlooking required supervision rules, or attaching it to the wrong therapy codes. These errors can trigger claim denials, payment reductions, and reimbursement delays.
| Mistake | What's Actually Happening |
|---|---|
| Applying CO to every unit whenever an OTA was present at all during the visit | Ignores the de minimis threshold; some units may not actually cross 10% |
| Failing to apply CO when the OTA's contribution genuinely exceeded 10% | Results in overbilling, since the service should have paid at 85%, not the full rate |
| Miscounting combined OT/OTA minutes as split rather than as therapist minutes | Minutes delivered together, at the same time, on the same code count entirely toward the OT |
| Missing the 8-minute rule interaction | Applying CO to a unit the OT's own time already qualified for independently under the 8-minute midpoint |
| Submitting CO without the GO modifier | CO needs to accompany the base occupational therapy modifier on the claim line |
| Applying CO/CQ logic to a Critical Access Hospital claim | The reduction doesn't apply in a cost-based payment setting |
Best Practices for Billing the CO Modifier
For accurate CO modifier billing, verify that the service was furnished in whole or in part by an occupational therapy assistant and apply the modifier only when CMS requirements are met. Maintain clear documentation of the OTA’s involvement, review applicable therapy codes, and confirm modifier accuracy before claim submission.
- Track OT and OTA minutes separately and precisely for every code, every session: The entire de minimis calculation depends on having that raw data available.
- Run the percentage calculation per unit, not per visit: A single session with multiple CPT codes may need CO on some lines and not others.
- Check the 8-minute rule before defaulting to CO: If the OT’s own time already supports the final unit independently, CO may not be needed even with an OTA involved.
- Confirm CO is paired with GO on every applicable claim line: One without the other is an incomplete claim.
- Keep supervision documentation current with whatever standard applies in your specific setting: General supervision rules for private practice OTAs changed as recently as CY 2026, so don’t assume older guidance still applies without checking.
- Audit a sample of OT claims periodically against session notes: This catches both over- and under-application of CO before a payer catches it first.
- Train front-line documentation habits, not just billing habits: The accuracy of the entire CO calculation starts with how clearly OTs and OTAs log their own minutes in the note.
Compliance Considerations
CO carries a fairly clear compliance profile because the rule itself is mechanical: cross 10 percent of a unit, the modifier is required, full stop. There isn’t much room for clinical judgment the way there is with something like an anesthesia direction modifier. That said, the compliance risk here shows up in a specific, predictable place: practices that consistently under-apply CO, whether through imprecise time tracking, incomplete documentation, or simply not running the calculation at all, are effectively billing at the full rate for services that should have paid at 85 percent.
Frequently Asked Questions About the CO Modifier
What does the CO modifier mean in medical billing?
CO indicates that an outpatient occupational therapy service was furnished in whole or in part by an occupational therapy assistant, and it triggers a mandatory 15 percent reduction to the Medicare payment for that service line.
When is the CO modifier required?
CO is required whenever an OTA’s contribution to a specific billed unit of service exceeds 10 percent, a threshold known as the de minimis standard. If the OTA’s involvement is 10 percent or less, CO isn’t required and the full rate applies.
How much does the CO modifier reduce payment?
Services billed with CO are reimbursed at 85 percent of the otherwise applicable Physician Fee Schedule rate, a 15 percent reduction, applied to Medicare’s Part B payment share specifically, not to the patient’s coinsurance.
Does the CO modifier apply to every OT setting?
No, it applies to services paid under the Medicare Physician Fee Schedule, including outpatient hospitals, rehabilitation agencies, SNFs and home health agencies billing under Part B, CORFs, and private practices. It does not apply to Critical Access Hospitals, which are paid on a cost basis.
What's the difference between CO and CQ?
CO applies to occupational therapy assistant involvement and pairs with the GO modifier. CQ applies to physical therapy assistant involvement and pairs with the GP modifier. Both use the identical 10 percent de minimis threshold and 15 percent payment reduction.
If an OT and OTA work together on the same code at the same time, how are the minutes counted?
Minutes that the OT and OTA provide together, simultaneously, toward the same CPT code are counted entirely as therapist minutes, not split between them. The de minimis calculation applies specifically when the OTA’s contribution is separate and distinguishable from the OT’s own time.
Can the OT's own time exempt a unit from needing CO, even if an OTA was also involved?
Yes, if the OT’s own minutes are sufficient on their own to independently satisfy the 8-minute rule for a given unit, that unit doesn’t require CO, regardless of whether an OTA also participated in the broader session.
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