A fall off study is when they do tests to see how your body responds and uses factor when it enters your body. They usually check your factor level by drawing blood before infusing medicine into you than drawing so often after having it infused in you. They drawed blood than infused me with a 100% dose and than drew at 15 minutes after infusion and than 1 hr, 2 hrs, 4 hrs, and 6 hrs after infusion. They can do any amount the doctor chooses to do. Some do 12 and 24 hrs checks also. This showed them what my half life was and how long the factor lasts in me when I treat myself. This helped them figure out how much medicine I needed for when I had surgery. They figured out how much medicine I needed infused in an hour to keep my factor lever close to the 100% level throughout the day and night.