Historically, Medicare had a hard financial "Therapy Cap" (e.g., a $2,300 maximum for PT and Speech combined). While the hard cap was repealed, the threshold still exists, and crossing it requires Modifier KX.
When to use Modifier KX
Once a patient's billed therapy services for the calendar year exceed the annual threshold, Medicare will deny all further claims unless you append Modifier KX to every therapy line item.
Appending KX serves as an attestation by the therapist that: "Yes, the patient has crossed the financial threshold, but continued therapy is strictly medically necessary, and my documentation proves it."
Targeted Medical Review
If you append KX and push the patient's annual total past a second, much higher threshold (historically $3,000), you risk triggering a Targeted Medical Review (audit) by the MAC.
NCCI Edits
Modifier KX does not bypass unbundling edits. You still must use Modifier 59 if you perform two mutually exclusive procedures on the same day.