OBJECTIVES: Measuring the exact glomerular filtration rate (GFR) is difficult. Iohexol can be used instead of inulin or labeled EDTA or DTPA. In recent years, different studies have validated GFR-estimating equations in adults. Validation of these estimations in adolescents and elderly is lacking. With this study, we aim to develop a simplified (only 1-3 blood collections) Iohexol protocol to measure the true GFR for patients of all ages and try to develop GFR-estimating equations for adolescents and the elderly. DESIGN AND SETTING: Participants of different ages will be recruited: 50 adolescent (14-18years) and 30 adults (20-65years), 60 elderly (65-80years) and 60 very elderly (80+ years old) stratified based on their GFR. Biometric data, serum creatinine and cystatin C will be measured. After an injecting 5ml iohexol, 9 blood samples will be taken between 20 and 360minutes. First, the GFR will be calculated using the double exponential decay method and different GFRs based on 1-3 blood samples, which will be compared with the GFR of the abovementioned 9 samples. Second, the GFR will be calculated using new and existing equations and compared to the true GFR. DISCUSSION: The availability of a reliable GFR measurement is important in situations such as screening patients for kidney donation or when taking potentially nephrotoxic treatments. This study will allow us to develop a simplified protocol for measuring the true GFR in all ages and will allow us to validate existing equations and develop new eGFR equations for adolescents and elderly.