Physical Therapy Billing: The 8-Minute Rule, KX Modifier, and PTA Compliance in 2026

Published May 22, 2026 · 10 min read · By RCMAXIS Revenue Cycle Team

Physical therapy billing runs on precise time documentation. Unlike most medical specialties where billing is event-based, PT billing for therapeutic procedures ties directly to minutes spent — and the rules governing how those minutes translate into billable units are exacting. The APTA's 2025 Practice Survey found that 41% of physical therapy claim denials involve 8-minute rule calculation errors, while Medicare's aggressive post-payment audit program for therapy services has resulted in over $220 million in overpayment recoveries from PT practices in the last two years alone.

At RCMAXIS, we bill for outpatient PT clinics, hospital-based therapy departments, and multispecialty practices with integrated rehabilitation services. Here is the definitive guide to PT billing compliance in 2026.

1. The 8-Minute Rule: How Timed Units Work

Medicare and most payers following Medicare guidelines use the "8-minute rule" to determine how many units of a timed procedure code are billable. One unit equals 15 minutes of direct skilled care. The 8-minute rule specifies the minimum time required to bill any unit at all, and governs how remaining minutes are rounded.

The Rule Applied

• To bill 1 unit: 8–22 minutes of timed service
• To bill 2 units: 23–37 minutes
• To bill 3 units: 38–52 minutes
• To bill 4 units: 53–67 minutes

When multiple timed codes are provided in a single session, calculate the total timed minutes first, determine the total billable units from that total, then allocate units to the highest-time services. This "total time approach" prevents inflated unit counts from rounding up each individual service separately.

Example: Three Timed Services in One Session

Therapeutic exercise (97110): 20 min + Manual therapy (97140): 15 min + Neuromuscular re-ed (97112): 10 min = 45 total timed minutes = 3 billable units. Allocate: 97110 gets 2 units (30 min), 97140 gets 1 unit (15 min), 97112 does not reach 8 additional minutes so it is not billed as an additional unit — but the time is accounted for in the total.

2. Timed vs. Untimed Codes

Not all PT procedure codes are timed. Untimed codes are billed once per session regardless of how long they take. Mixing up timed and untimed status is a common billing error, particularly for evaluation codes and functional assessments.

Common Timed PT Codes (15 min per unit)

• 97110: Therapeutic exercise — strengthening, ROM, endurance training
• 97112: Neuromuscular reeducation — balance, coordination, kinesthetic sense
• 97116: Gait training
• 97140: Manual therapy — joint mobilization, soft tissue mobilization, manipulation
• 97150: Therapeutic activities, group (2+ patients) — billed per patient, 1 unit per session regardless of duration
• 97530: Therapeutic activities — functional tasks, dynamic activities
• 97535: Self-care/home management training

Common Untimed PT Codes (bill once per session)

• 97161–97163: PT evaluation (low, moderate, high complexity) — one per episode of care or re-evaluation
• 97164: PT re-evaluation
• 97750: Physical performance test or measurement
• 97760: Orthotic management and training, initial encounter
• 97010: Hot/cold pack application — untimed, bill once per session
• 97018: Paraffin bath — untimed
• 97022: Whirlpool therapy — untimed

3. KX Modifier: Documenting Medical Necessity Beyond the Threshold

Medicare applies an annual therapy threshold — in 2026, $2,330 for PT and speech-language pathology combined, and $2,330 for occupational therapy. Services beyond this threshold are not automatically denied; they are subject to enhanced medical necessity review. The KX modifier signals to Medicare that the therapist attests the services are medically necessary and documentation supports continued treatment beyond the threshold.

KX Modifier Requirements

• Append KX to every timed and untimed therapy code once the patient's cumulative therapy charges exceed the threshold for the calendar year
• The plan of care must be updated and re-certified by the supervising physician or NPP
• Documentation must show the patient continues to make measurable functional progress toward specific goals — not maintenance of current status
• Measurable progress means objective functional measures: walk speed, ROM in degrees, strength in MMT grades, functional independence measure scores

Do not append KX unless documentation truly supports ongoing skilled care. CMS audits KX-flagged claims for documentation supporting medical necessity — submitting KX without compliant documentation is a compliance risk, not just a billing error.

4. CQ and CO Modifiers for PTAs and OTAs

Effective January 1, 2022, CMS implemented a 15% payment reduction for services furnished in whole or in part by a physical therapist assistant (PTA) or occupational therapist assistant (OTA). The CQ modifier (for PTA services) and CO modifier (for OTA services) must be appended to identify which services were provided by an assistant.

CQ Modifier Rules

• Append CQ to any timed service code where a PTA furnished more than 10% of the total timed service minutes for that code during the session
• If a PT and PTA split the service — PT provided 8 minutes, PTA provided 7 minutes of a 15-minute unit — the more-than-10% threshold means CQ applies
• Untimed codes furnished by a PTA also require CQ
• Evaluations (97161–97164) are always performed by the PT and never receive CQ

5. Skilled vs. Maintenance Therapy: The Documentation Line

Medicare only covers physical therapy that requires the skilled knowledge, judgment, and expertise of a licensed physical therapist. Services that a non-skilled caregiver or the patient themselves could safely perform are maintenance therapy — not covered by Medicare in most settings.

Documenting Skilled Need

• Explain why a skilled PT is required — not just what was done, but why it required clinical expertise to perform safely
• "Patient performed strengthening exercises" is not skilled documentation. "Patient required verbal cueing and manual facilitation to maintain neutral lumbar alignment during quadriceps strengthening secondary to proprioceptive deficits from lumbar radiculopathy" is skilled documentation
• Progress notes must show objective measurement — standardized outcome tools (FOTO, OPTIMAL, LEFS) provide defensible evidence of functional progress
• When a patient plateaus (no measurable functional improvement over two to three sessions), document the clinical reason continued skilled care is necessary or transition to discharge planning

6. Modalities: When They Are and Are Not Separately Billable

Passive modalities (hot pack, ultrasound, electrical stimulation) are frequently overbilled as standalone services when Medicare bundles them into the therapeutic procedure performed at the same visit. Understanding which modalities are separately billable and which are included in other codes prevents both overbilling and lost revenue from failing to capture legitimate charges.

Modality Billing Rules

• 97010 (hot/cold pack): Not separately billable by Medicare when provided as a preparatory service before manual therapy or therapeutic exercise — bill only if it is the primary and only service that session
• 97014 (electrical stimulation, unattended): Separately billable, untimed, once per session
• 97032 (electrical stimulation, attended): Timed, 15 minutes per unit; requires the therapist to be in constant attendance
• 97035 (ultrasound): Timed, separately billable
• 97039 (unlisted modality): Use with caution; requires a narrative description and is subject to individual consideration by payer

RCMAXIS manages PT billing for outpatient clinics and hospital-based therapy programs, with built-in 8-minute rule calculation validation, CQ/CO modifier review, and KX threshold tracking integrated into our billing workflow. Our reporting dashboards give PT practice managers real-time visibility into units billed per visit, denial trends, and threshold utilization. Connect with our specialty billing team to learn how we eliminate PT billing compliance risk.