The AT modifier is used in chiropractic billing to indicate that a service was "active treatment" or "acute treatment." This modifier is used when chiropractors provide treatment for an acute or active condition that is expected to improve, such as a recent injury or acute exacerbation of a chronic condition.
When the AT modifier is used, it indicates to the insurance carrier that the service provided was not maintenance care or preventative care but rather active treatment for a condition that requires more frequent and intensive treatment.