Physical therapy billing that respects the 8-minute rule.

Physical therapy billing is really a game of counting minutes correctly. Miscount them and you either shortchange yourself on every visit or overbill your way into an audit. The 8-minute rule isn't optional folklore — it's how Medicare and most plans expect you to turn treatment time into billable units. Get it slightly wrong across 40 visits a day and the mistake quietly snowballs before anyone thinks to check.

Timed vs untimed codes and the 8-minute rule

Timed codes such as 97110 (therapeutic exercise), 97112 (neuromuscular reeducation), 97140 (manual therapy), and 97530 (therapeutic activities) are billed in 15-minute units. Under Medicare's 8-minute rule, one unit requires at least 8 minutes of that service; total timed minutes across codes determine how many units you can support. Untimed codes (evaluations, certain modalities billed once per day) do not eat into that minute pool the same way — mixing the logic is a classic new-biller mistake.

If the therapist documents 20 minutes of 97110 and 20 minutes of 97140, that is not automatically four units. Total timed minutes and the distribution across codes matter. We audit unit math weekly until the clinic's error rate stays near zero.

GP modifier, therapy thresholds, and KX

Medicare outpatient therapy claims need the GP modifier for PT services (GO for OT, GN for SLP). When a patient approaches the annual therapy threshold, medically necessary services beyond the threshold require the KX modifier and documentation that supports continued skilled care. Skipping KX is an automatic denial. Applying KX without supporting notes is an audit invitation.

• Calculate timed units from documented minutes, not from the number of exercises listed.
• Append GP on PT claims; confirm discipline modifiers when OT or SLP shares the chart.
• Track cumulative therapy dollars toward the threshold before the visit that crosses it.
• Attach KX only when the record supports medical necessity past the threshold.

Plan of care, certification, and re-evaluation

Medicare still expects a plan of care certified by a physician or allowed practitioner, with recertification on the required cadence. Commercial auth visit counts are a separate trap: treating past the authorized visit without an extension is pure write-off. Progress notes must show skilled care and functional progress; cloned notes that change only the date are a denial and compliance problem.

Auth tracking that protects the schedule

High-volume PT clinics lose more to expired auths than to exotic coding errors. Our physical therapy medical billing workflow counts visits against auth, flags the third-to-last visit, and blocks claim submission for visits past the authorized count until clinical staff extends or the patient is converted to self-pay with consent. Clean claims at 97%+ mean nothing if half the schedule was never authorized.

Evaluations (97161–97163 by complexity) and re-evaluations (97164) have frequency expectations. Rebilling a full eval every time the patient returns from a gap in care without meeting re-eval criteria creates denials. Supervisory rules for assistants vary by payer and state — if the note cannot show who rendered skilled care, the unit will not hold. Get that right and A/R stays under 28 days without weekend heroics.