There are several methods to evaluate renal function during childhood. The use of serum creatinine, either alone or in combination with the Schwartz formula, is reliable and quick, but requires knowledge of conceptual age. A plasma creatinine concentration of 88.4 µmol/L (1.0 mg/dL), for example, represents normal renal function in an adolescent but more than 50% loss of renal function in a 5-year-old child. A timed urine collection for creatinine clearance is another evaluative method, but the adequacy of the urine collection always should be determined first. Urea clearance rarely is used to measure GFR because of the complex factors that influence urea excreation. Measurement of the disappearance of radioactive-labeled substances in plasma can be used to determine GFR. Radionuclide renal scans also can be used and offer the advantage of estimating the GFR of each kidney. Although infants and newborns have an intact urine diluting ability, their concentrating ability is impaired. The maximal urinary concentration in the neonatal period is less than 700 mOsm/kg, but reaches adult values of 1200 mOsm/kg by 6 to 12 months of life. Similarly, the infant kidney has a limited capacity for salt regulation, predisposing the infant to salt disturbances.