Modifier CO
A complete practical guide to Modifier CO for medical billers, coders, AR callers, occupational therapy practices and Revenue Cycle Management teams.
Learn what CO means, when it is required, why it is paired with GO, how the 10% de minimis standard works, the 8-minute rule, the special two-unit scenario, payment reduction rules and how to work CO-related denials.
Modifier CO at a Glance
The essential Medicare facts every billing professional should know.
OTA Modifier
CO identifies applicable outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant.
Pair With GO
When CO applies, CMS requires CO to be reported with the GO therapy modifier.
De Minimis Standard
The OTA’s independent portion generally must exceed 10% of the applicable service or unit for CO to apply.
Reduced Payment
Applicable services reported with CO are paid at 85% of the otherwise applicable Part B payment amount.
What Is Modifier CO?
Start with the Medicare definition before applying the modifier to real claims.
Official Medicare Meaning
Outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant.
Plain-English Explanation
Modifier CO tells Medicare that an applicable outpatient occupational therapy service was furnished in whole or in part by an occupational therapy assistant under an occupational therapy plan of care.
CO is not simply a general “assistant” modifier. It is specifically used for applicable outpatient occupational therapy services furnished by an OTA.
Easy memory: CO = OTA + outpatient OT.
CO Must Be Paired With GO
One of the most important Medicare claim-edit rules.
OT Discipline
GO identifies the service as being furnished under an occupational therapy plan of care.
OTA Involvement
CO identifies applicable occupational therapy services furnished by an OTA.
Correct Combination
When CO applies, report CO together with the GO therapy modifier on the applicable claim line.
Important Claim Edit
CMS states that CO must be paired with GO. Claims with modifiers that are not properly paired can be rejected or returned as unprocessable.
CO vs CQ
The easiest way to remember the two therapy-assistant modifiers.
CO — OTA
Applicable outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant.
CQ — PTA
Applicable outpatient physical therapy services furnished in whole or in part by a physical therapist assistant.
When Does Modifier CO Apply?
CMS uses specific rules to determine when OTA involvement requires CO.
OTA Furnishes the Entire Service
CO applies when the OTA furnishes all minutes of an applicable service independently of the OT.
OTA Furnishes More Than 10%
CO generally applies when the OTA’s independent portion exceeds 10% of the total minutes for the applicable service or unit.
OTA Works Independently
The calculation focuses on minutes furnished by the OTA independently of the OT for the same service.
Applicable OT Service
The service must be an applicable outpatient occupational therapy service subject to the CO rules.
Key Rule
The presence of an OTA alone does not automatically mean CO belongs on every unit. The actual service, minutes, practitioner involvement and CMS exceptions must be evaluated.
The 10% De Minimis Standard
Understand the calculation before deciding whether CO applies.
Basic Percentage Method
÷
Total Minutes for Same Service
× 100
After the applicable CMS steps are completed, the remaining OTA time is divided by the combined OTA and OT time for the same service.
CMS rounds the percentage to the nearest integer. When the result is greater than 10%, meaning 11% or more, CO applies.
Example
OTA provides 10 minutes of an OT service independently.
OT provides 5 minutes of the same service independently.
Rounded to the nearest integer, the percentage is 67%. Because it is greater than 10%, CO applies under the applicable CMS methodology.
Do Not Use the Percentage Alone
CMS requires the billing team to first identify applicable 15-minute timed units and then apply the 8-minute rule and other specific exceptions. The percentage calculation comes after the required initial steps.
CO and the 8-Minute Rule
An important exception for the final 15-minute timed unit.
OT Provides 8+ Minutes
If the OT independently provides 8 or more minutes of the final 15-minute unit, that final unit is billed without CO.
OTA Minutes Still Matter
OTA participation does not automatically put CO on the final unit when the OT has independently furnished the required 8 or more minutes.
Apply the Sequence
Determine complete 15-minute units first, then evaluate remaining minutes and apply the appropriate CMS exception.
Practical Reminder
For the final unit, once the OT independently reaches 8 minutes, the final unit is billed without CO regardless of how many minutes the OTA furnished for that same service.
Special Two-Unit Scenario
A commonly missed CMS exception involving 9–14 minutes from each practitioner.
When the Exception Applies
- Two units of the same service remain to be billed.
- OT furnishes between 9 and 14 minutes.
- OTA furnishes between 9 and 14 minutes.
- Combined time is at least 23 minutes but no more than 28 minutes.
Billing Result
Under CMS’s special two-unit rule, one unit is billed with CO for the unit furnished by the OTA and one unit is billed without CO for the unit furnished by the OT.
One unit is associated with the OTA and reported with CO; one unit is associated with the OT and reported without CO.
Why This Matters
Do not apply the simple 10% calculation mechanically to this scenario. CMS specifically established this exception through the CY 2022 rulemaking.
Modifier CO Decision Path
Use this sequence when reviewing a Medicare outpatient OT claim.
OT Service?
Confirm applicable outpatient OT service.
OTA?
Determine who furnished the service.
Independent?
Separate OTA minutes from OT minutes.
CO Applies?
Apply CMS de minimis rules and exceptions.
Pair GO
Report CO with the GO therapy modifier.
Complete CO Billing Workflow
A practical front-end process for Medicare occupational therapy billing.
Modifier CO Practical Examples
Simplified examples based on CMS’s current billing methodology.
An OTA independently furnishes all minutes of an applicable outpatient OT service.
Because the OTA furnishes the entire service independently, CO applies when the service is subject to the CO rules. GO should also be reported.
OTA: 10 minutes.
OT: 5 minutes.
10 ÷ 15 × 100 = 66.7%. The rounded result is greater than 10%, so CO applies under the applicable CMS methodology.
OTA: 5 minutes.
OT: 6 minutes.
5 ÷ 11 × 100 = 45.5%. The rounded percentage is greater than 10%, so CO applies under the applicable CMS methodology.
OT independently furnishes 8 minutes of the final 15-minute unit while the OTA also furnishes minutes.
The final unit is billed without CO because the OT independently provided at least 8 minutes.
OT independently furnishes 12 minutes and OTA independently furnishes 12 minutes of the same 15-minute timed service.
Combined time is 24 minutes. One unit is billed with CO for the OTA-furnished unit and one unit is billed without CO for the OT-furnished unit.
The occupational therapist independently furnishes the entire service.
CO is not reported because the OTA did not furnish the service.
How to Calculate CO
Use the CMS sequence rather than jumping directly to the percentage.
| Step | What to Determine | Example | Result |
|---|---|---|---|
| 1 | Identify applicable 15-minute timed services. | 97110 | Determine total units first |
| 2 | Identify OTA and OT minutes for the same service. | OTA 10 + OT 5 | 15 minutes |
| 3 | Divide remaining OTA time by combined time. | 10 ÷ 15 | 66.7% |
| 4 | Round percentage to nearest integer. | 66.7% | 67% |
| 5 | Compare with 10% standard. | 67% > 10% | CO applies |
| 6 | Check final-unit and special exceptions. | 8-minute / 9–14 minute scenario | Apply applicable exception |
Common Modifier CO Mistakes
Errors that can result in rejected, returned or incorrectly paid Medicare OT claims.
CO Without GO
Reporting CO without the corresponding GO therapy modifier can result in the claim being rejected or returned.
CO on Every Unit
OTA involvement does not automatically mean every unit receives CO.
Ignoring the 8-Minute Rule
The final unit may be billed without CO when the OT independently furnishes at least 8 minutes.
Ignoring the Special Two-Unit Rule
The 9–14 minute scenario can require one unit with CO and one unit without CO.
Confusing CO With CQ
CO is for OT/OTA services. CQ is for PT/PTA services.
Counting Shared Minutes Incorrectly
Minutes furnished together by OT and OTA are treated differently from minutes furnished independently.
Ignoring Documentation
The medical record must support the services, minutes, units and practitioner involvement.
Applying CO to Non-Applicable Services
Verify that the service and payment methodology are subject to the CO requirements.
Skipping the Unit Calculation
CMS requires the appropriate unit-counting methodology before applying the CO determination.
When Should CO Not Be Reported?
Important situations where CO should not simply be added because an OTA participated.
OT Furnishes the Service Alone
When the OT wholly provides the applicable service, CO is not reported.
OT and OTA Provide Care Together
When the OT and OTA provide care to the patient together at the same time, CMS provides an exception to the CO requirement.
OTA Portion Does Not Exceed the Standard
When the applicable independent OTA portion does not exceed the 10% de minimis standard, CO generally does not apply, subject to CMS’s specific methodology and exceptions.
Non-Applicable Payment Methodology
CO is required for applicable outpatient therapy services paid under the Medicare methodologies specified by CMS.
Shared-Care Exception
CMS specifically states that the CO modifier is not used when the OT and OTA provide care to a patient together at the same time. The exact circumstances should be reviewed against the current CMS rule.
CO and the 85% Payment Rule
Why Modifier CO matters financially for Medicare claims.
Reduced Payment
For applicable outpatient occupational therapy services furnished in whole or in part by an OTA, Medicare payment is made at 85% of the otherwise applicable Part B payment amount for dates of service on or after January 1, 2022.
Why Billing Teams Need to Know
CO is not simply a descriptive modifier. It can directly affect Medicare reimbursement.
- Correct CO identification supports accurate billing.
- Proper GO pairing helps prevent claim edits.
- Correct unit-level application supports accurate payment.
- Accurate documentation supports the reported minutes.
AR Caller Workflow for a CO Denial
Investigate the root cause instead of simply rebilling.
Review ERA/EOB
Identify the exact denied claim line and Medicare adjustment reason.
Capture CARC/RARC
Record all adjustment and remark codes before deciding the next action.
Check CO + GO
Verify whether CO was reported with the required GO therapy modifier.
Review Minutes
Compare OT and OTA minutes for the affected service.
Apply CMS Rule
Determine whether the 10% standard, 8-minute rule or special two-unit exception applies.
Review Documentation
Confirm the medical record supports treatment minutes and practitioner involvement.
Check Payment Method
Confirm the claim is paid under a methodology to which CO applies.
Contact MAC
If the CMS requirements appear satisfied but adjudication remains unclear, contact the appropriate Medicare contractor.
Correct or Appeal
Submit the appropriate corrected claim or appeal based on the documented root cause.
Medicare AR Call Script — Modifier CO
Questions to ask when an occupational therapy claim is denied.
CO Documentation Checklist
Documentation should support the services and minutes reported on the claim.
Before Submission
- Medicare coverage verified.
- OT plan of care verified.
- CPT/HCPCS code verified.
- OT and OTA involvement identified.
- Total treatment minutes documented.
- OTA independent minutes documented.
- Applicable units calculated.
- CO and GO pairing checked.
After Denial
- ERA/EOB reviewed.
- CARC/RARC captured.
- Modifier sequence reviewed.
- OT and OTA minutes rechecked.
- 10% de minimis standard reviewed.
- 8-minute rule reviewed.
- 9–14 minute exception reviewed.
- MAC guidance checked.
Therapy Assistant Modifier Comparison
Quick reference for Medicare outpatient therapy billing.
| Modifier | Practitioner | Therapy Discipline | Pair With | Payment Impact |
|---|---|---|---|---|
| CO | Occupational Therapy Assistant | Occupational Therapy | GO | 85% of otherwise applicable Part B amount |
| CQ | Physical Therapist Assistant | Physical Therapy | GP | 85% of otherwise applicable Part B amount |
| GO | Occupational Therapy | Occupational Therapy | May pair with CO when OTA rules apply | Discipline modifier |
| GP | Physical Therapy | Physical Therapy | May pair with CQ when PTA rules apply | Discipline modifier |
| GN | Speech-Language Pathology | Speech-Language Pathology | Not an OT/PT assistant modifier | Discipline modifier |
CO vs GO, GN and CQ
Understand discipline modifiers versus therapy-assistant modifiers.
Occupational Therapy
Identifies the occupational therapy plan of care.
OTA
Identifies applicable OT services furnished in whole or in part by an OTA.
Physical Therapy
Identifies the physical therapy plan of care.
Speech Therapy
Identifies applicable speech-language pathology services under an SLP plan of care.
GO = OT discipline | CO = OTA involvement | GP = PT discipline | CQ = PTA involvement
2026 Medicare OT Points
Current information relevant to occupational therapy billing teams.
OT KX Threshold
For CY 2026, CMS lists a KX modifier threshold of $2,480 for occupational therapy services.
OTA Payment Reduction
Applicable OTA services are paid at 85% of the otherwise applicable Part B payment amount.
Annual Therapy Code List
CMS maintains the current therapy code list and dispositions used to determine codes that sometimes or always describe therapy services.
Do Not Confuse KX With CO
KX addresses applicable therapy-threshold and medical-necessity requirements. CO identifies applicable OTA involvement. They serve different purposes and should not be treated as interchangeable modifiers.
Common Root Causes of CO Denials
Identify the process failure before submitting a corrected claim.
CO Missing
An applicable OTA service was billed without the required CO modifier.
GO Missing
CO was submitted without the corresponding GO therapy modifier.
CO Overreported
CO was added to units that did not meet the applicable CMS requirements.
Incorrect Minutes
OT and OTA minutes were incorrectly recorded or calculated.
8-Minute Rule Missed
The billing team did not apply the final-unit exception correctly.
Two-Unit Exception Missed
The 9–14 minute scenario was processed using the standard percentage calculation instead of the specific exception.
Shared Care Misclassified
OT and OTA minutes furnished together were incorrectly treated as independent OTA minutes.
Documentation Gap
Medical records do not adequately support the reported treatment minutes and practitioner involvement.
Manual Billing Error
The modifier was selected without applying the CMS unit-level methodology.
Modifier CO Quick Cheat Sheet
Save this section as your fast-reference guide.
- CO = applicable outpatient occupational therapy services furnished in whole or in part by an OTA.
- CO is an OTA modifier, not a general assistant modifier.
- CO must be paired with GO when applicable.
- The 10% de minimis standard is important when determining whether CO applies.
- OTA independent minutes are evaluated against total minutes for the same service.
- CMS requires the appropriate unit-counting methodology before the percentage calculation.
- The final-unit 8-minute rule is an important exception.
- The special 9–14 minute/two-unit scenario can require one unit with CO and one without CO.
- OT-only services do not receive CO.
- CO is different from CQ, which applies to PT/PTA services.
- Applicable CO services are paid at 85% of the otherwise applicable Part B amount.
- For CY 2026, the OT KX threshold is $2,480.
- Always verify the current CMS Therapy Code List and applicable MAC guidance.
Official Medicare References
Primary CMS resources used for this Modifier CO guide.
Educational Disclaimer
This page is intended for US medical billing, coding and Revenue Cycle Management education. Medicare requirements can depend on the exact CPT/HCPCS code, claim type, provider setting, date of service and applicable MAC guidance. Always verify current CMS and Medicare contractor requirements before billing, correcting or appealing a claim.
Modifier CO FAQs
Common questions from medical billers, coders and AR callers.
What does Modifier CO mean?
Modifier CO identifies applicable outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant.
What does CO stand for in medical billing?
CO is the Medicare modifier used to identify applicable outpatient occupational therapy services furnished in whole or in part by an occupational therapy assistant.
Does CO need to be paired with GO?
Yes. When CO applies, CMS requires it to be reported with the GO therapy modifier for occupational therapy.
What is the difference between CO and CQ?
CO identifies applicable occupational therapy services furnished by an OTA. CQ identifies applicable physical therapy services furnished by a PTA.
Does CO apply to every service performed by an OTA?
No. The service must be subject to the applicable outpatient OT rules, and CMS’s specific methodology determines whether CO is required.
What is the 10% de minimis standard for CO?
Generally, CO applies when the OTA’s independently furnished portion exceeds 10% of the applicable service or unit, subject to CMS’s detailed methodology and exceptions.
What happens if the OT provides 8 minutes of the final unit?
If the OT independently furnishes 8 or more minutes of the final 15-minute unit, that final unit is billed without CO under CMS’s 8-minute-rule exception.
What is the special 9–14 minute rule?
When two units of the same service remain, the OT and OTA each furnish 9–14 minutes, and the combined time is 23–28 minutes, CMS requires one unit with CO for the OTA portion and one unit without CO for the OT portion.
Is CO required when the OT provides the service alone?
No. CO identifies applicable OTA involvement. A service wholly provided by the OT does not receive CO.
Is CO required when the OT and OTA provide care together?
CMS provides an exception when the OT and OTA provide care to the patient together at the same time. Review the exact circumstances against current CMS guidance.
Does CO reduce Medicare payment?
Yes. For applicable services, Medicare payment is 85% of the otherwise applicable Part B payment amount for services furnished in whole or in part by an OTA, effective for dates of service on or after January 1, 2022.
Can CO be used without GO?
No. When CO applies, CMS requires it to be paired with the GO therapy modifier. Claims with improperly paired modifiers may be rejected or returned as unprocessable.
What should an AR caller check for a CO denial?
Review the ERA/EOB, CARC/RARC, CO/GO pairing, CPT/HCPCS code, OT and OTA minutes, applicable units, 10% de minimis standard, 8-minute rule, special two-unit rule, documentation and provider payment methodology.
What is the 2026 Medicare OT KX threshold?
For CY 2026, CMS lists a $2,480 KX modifier threshold for occupational therapy services.
Does CO apply to physical therapy?
No. CO is for occupational therapy assistant services. The corresponding physical therapy assistant modifier is CQ.
Where can I verify current CO rules?
Use the current CMS Therapy Services page, CMS’s CQ/CO billing examples, the Medicare Claims Processing Manual, the annual Therapy Code List and applicable Medicare Administrative Contractor guidance.
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