In neonates, renal perfusion, glomerular filtration rate and urine output strongly depend on the vaso-dilative effects of prostaglandins on the afferent glomerular arterioles. Glomerular filtration rate in neonates is very low (2–4 mL/min or 20 ml/min/1.73 m2) and can only be maintained due to a delicate balance between vasodilatory effects at the afferent and vasoconstrictor effects at the efferent glomerular arterioli. Despite the overall low clearance, interindividual variability is already extensive and can be predicted by covariates like postmenstrual age, postnatal age, co-administration of a non-selective cyclo-oxygenase inhibitor, growth restriction or peripartal asphyxia.
We still commonly used creatinine as a biomarker of renal clearance capacity. However, before creatinaemia values can be used to estimate renal drug elimination capacity, there are some methodological issues that need to be considered. Creatinaemia at birth does not yet reflect neonatal but maternal creatinine clearance and because of passive tubular back leak instead of active secretion, creatinine clearance does not yet fully reflects GFR. Finally, absolute creatinine values also depend on the technique used. The move towards harmonisation through IDMS (isotope dilution mass spectrometry) traceability has helped, but not completely solved this problem. More research is needed to document the potential add on benefit of more advanced biomarkers (e.g. Cystatin C) or renal tubular function markers. In the meanwhile, clinical driven interpretation combined clinical and biochemical indicators seems the most appropriate approach.