Safety issues The study examinations/tests are considered to be safe. No invasive procedures (besides intravenous peripheral catheters) are involved. During the study tests, two 'diagnostic agents' (i.e. inulin and PAH) need to be administered; both agents are inert and have no side effects. The total amount of drawn blood will be 500 mL during a total period of 16 weeks. Side effects are not expected because the blood volume taken per visit is relatively small, especially in comparison with regular blood donation, with 500 mL per donation (and is allowed 5 times a year for men and 3 times for women).