Assessment of Kidney Function in Children, Adolescents, and Young Adults

Kidney function undergoes developmental changes, including postnatal adaptation of nephron recruitment until approximately 18 months of age and glomerular hypertrophy thereafter. Accurate assessment of kidney function across post-conceptual age is important and needs to incorporate factors associated with the person (e.g., body habitus, growth, muscle mass, health status, illnesses, inflammation, and intra-personal variability), measurement technique (e.g., endogenous biomarkers, exogenous techniques, or imaging studies), and interpretation of measurements (e.g., equation used, correction factors, indexing to body surface area, or extracellular volume). While nephron endowment would be the most important factor for an accurate clearance calculation of more than 60% of drugs processed by the kidney, this cannot be assessed directly. Effective renal plasma flow (ERPF) may be better to assess nephron endowment as it does not undergo autoregulation or hyperfiltration, in contrast to glomerular filtration. However, glomerular filtration rate (GFR) is still the best surrogate tool for kidney function assessment. The gold standard to measure kidney function is inulin clearance; however, this is impractical. Measurement of GFR using either radiolabeled or cold exogeneous substances such as iohexol, iothalamate, etc. now serve as replacement for more accurate GFR determinations. In the clinical routine, endogenous markers are used, especially serum creatinine and cystatin C. Health care providers need to know when to use which endogenous biomarker(s). In this chapter we will discuss historical aspects of kidney function measurement, exogenous and endogenous methods, and the technical aspects of assessment or interpretation in health and sickness, from the neonate to the transition-age periods.