Determination of CYP3A activity is usually done with oral midazolam doses of 2-7.5 mg (phenotyping). Because therapeutic doses have pharmacological effects and can cause sedation, especially if CYP3A is inhibited, a microdosing approach for CYP phenotyping was developed to avoid any pharmacological activity. Oral midazolam pharmacokinetics are linear over a 30,000-fold range and 300 ng doses can reliably predict drug interactions with strong CYP3A inhibitors like ketoconazole.