TPE removes large-molecular-weight substances such as harmful antibodies from the plasma. It is usually carried out using an automated blood cell separator to ensure fluid balance and maintain a normal plasma volume. This may require the insertion of a femoral or jugular line to allow adequate blood flow. Typically, 30–40 mL/kg of plasma (1–1.5 plasma volumes) are removed at each procedure and replaced with isotonic 4.5 or 5.0% human albumin solution (some services substitute 25–50% of replacement volume with 0.9% saline). Exchange with fresh frozen plasma (FFP) is reserved for the replacement of ADAMTS13 in thrombotic thrombocytopenic purpura (see below) or to replace clotting factors. A one plasma volume exchange removes about 66% of an intravascular constituent and a two plasma volume exchange approximately 85%. TPE is normally combined with disease modifying treatment, such as immunosuppressive drugs, for the underlying condition.