A–Z of prescribing for children

Excretion is the final phase of the drug's journey as it passes through the body, and this elimination – usually by the kidneys – depends on glomerular filtration, tubular excretion and also tubular reabsorption (Van den Anker et al, 2018). However, the processes are clearly influenced by nephrogenesis – or the kidney's development – in an infant (Sage et al, 2014).

Normal nephrogenesis begins at 9 weeks' gestation and is complete by 36 weeks' gestation, which is then followed by postnatal changes in the blood flow in the kidney (Kearns et al, 2003). Therefore, premature and low birthweight babies will have under-developed kidneys: lower birthweight babies will have fewer glomeruli per unit area in the cortex than normal birthweight babies (Manalich et al, 2000). Nevertheless, term babies' kidneys will be nephrogenetically complete, as there are no more nephrons formed after 36 weeks' gestation (Bertram et al, 2011). However, the kidney still does not reach full maturity until the child reaches puberty (Eidelman and Abdel-Rahman, 2016), and structural differences are well described between neonate and adult kidneys (Box 1).