Kidney function is not mature in young children. The renal blood flow and filtration rate of the glomeruli is still low. The renal blood flow matures between the age of 5 to 12 months. The glomerular filtration rate reaches normal levels after 5 months. The resorption in the tubules is mature around 7 months of age.
Due to this difference in timing of maturity of the different components of renal function, some drugs show a normal excretion pattern earlier than others.
Digoxin and gentamicin (glomerular secretion) show a normal excretion pattern around 5 months, while penicillins (tubular excretion) are excreted normally after 7 months of age.
Since most drugs undergo glomerular secretion, neonates have a lower rate of renal excretion.
A normal glomerular filtration rate is reached after 5 months of age and is