Within this thesis, I present the work and collaborations I have been a part of in Geneva over the last ten years. My work concentrates on biomarkers, first of kidney function in the preterm and last on cardiovascular markers in children with nephrotic syndrome. My first project (Results 3.1 renal function in preterm birth), focuses in diagnosing kidney damage in the neonates’ using biomarkers. In this first study, we measured glomerular filtration rate (GFR) in premature newborn using inulin clearance and explore the associations between measured clearance and markers of interest to estimate renal function (eGFR) such as height, gestational age, weight, creatinine, and cystatin C. We developed a model to predict glomerular filtration rate and evaluated the performance of preexisting equations to estimate renal function (eGFR) in neonates. Markers of glomerular filtration rate behave differently in the neonatal population, as is shown in the presented result. This project established my interest in kidney markers in the neonatal period. A second work explored the interest and specificities of urinary NGAL in preterm newborns (Results 3.2 NGAL) and its usefulness to identify kidney injury. Acute kidney injury (AKI) is frequent in the preterm population and increases mortality and morbidity. We started a third project in collaboration with the neonatology units in Geneva and Paris, focusing on the incidence of AKI and the contribution of the urine output marker to improve the diagnosis. This project aimed to assess the association of AKI and mortality and test whether higher urine output may better discriminate mortality (Results 3.3 AKI in the preterm infant). Increasing evidence on the complications of preterm birth and AKI are now available (HTA, increased cardiovascular risk, chronic kidney disease). We designed an online survey sent to ambulatory pediatricians in Geneva to determine their renal follow-up of former prematures (Results 3.4). This survey allows us to identify the gaps of follow-up to improve care. Finally, the fifth and last project presented was made in collaboration with the adult nephrology unit in Geneva. Proteinuria and hyperphosphatemia are cardiovascular risk markers independent of glomerular filtration rate (GFR). In this project, phosphate handling was studied in nephrotic children, adults with chronic kidney disease and animal models. In our focus study in children with relapsing nephrotic syndrome, a transient hyperphosphatemia was linked to the nephrotic proteinuria and independent of renal function (Results 3.5 Nephrotic Syndrome). The cardiovascular impact of this transient hyperphosphatemia is poorly understood and raises the question of defining cardiovascular marker of endothelial health in this population. Altogether, these studies confirm the need to properly diagnose kidney damage. The future is to define early markers that are sensitive and specific for diagnosis and progression of renal disease adapted to the particularities of the population treated.