Continuous age-dependent centile curves and preliminary reference intervals for renal biomarkers and electrolytes in healthy Kenyan children under five years

Renal biomarkers change rapidly in early childhood, but pediatric reference intervals used in African laboratories are often extrapolated from non-African populations and discrete age partitions. This study derived continuous age-dependent centile curves and preliminary reference intervals for body mass index (BMI), serum creatinine, urea, and electrolytes in healthy Kenyan children aged 0 to 60 months. In this cross-sectional study, 176 children (90 males, 86 females) were systematically recruited at Kitui County Referral Hospital between June 2024 and February 2025. Children underwent clinical screening, World Health Organization–standard anthropometry, and morning fasting venous blood sampling. Creatinine and urea were measured using isotope-dilution mass spectrometry-traceable enzymatic assays, and electrolytes by direct ion-selective electrode. Centiles (2.5th, 50th, 97.5th) were estimated non-parametrically with 90% bootstrap confidence intervals, and continuous age trends modeled using generalized additive models for location, scale, and shape or quantile regression. BMI showed modest age-related variation, with median values rising from 15.8 kg/m 2 (2.5th–97.5th: 11.4–20.2) at 0 to 11 months to 16.5 kg/m 2 (10.1–23.0) at 48 to 60 months, with dispersion narrowing after age 2. Median creatinine rose steeply in infancy and more gradually thereafter, from 17.4 to 31.2 µmol/L. Urea increased gradually from a median of 2.59 to 3.63 mmol/L, with overlapping centile limits across adjacent strata. Electrolytes were tightly regulated and largely age-invariant: sodium ranged from 135 to 147 mmol/L (median ~141), potassium 3.6 to 5.7 mmol/L (median declining from 4.9 to 4.5), and chloride 100 to 111 mmol/L (median ~105–106). Sex-specific partitions were supported only in early life (BMI in infancy, creatinine in toddlers); correlations between anthropometric z -scores and biomarkers were weak (|ρ| < 0.15; P > .10). These continuous centile curves provide the first preliminary age-dependent reference estimates for renal biomarkers in Kenyan children under 5, showing rapid early renal maturation alongside stable electrolyte homeostasis. As this cross-sectional design cannot capture within-subject variation, larger multicenter and longitudinal studies are needed to confirm these trajectories, refine intervals, and develop age-dependent z -score calculators for clinical use.