Continuous age-dependent centile curves and preliminary reference intervals for renal biomarkers and electrolytes in healthy Kenyan children under five years: A single-center cross-sectional study.

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/m2 (2.5th-97.5th: 11.4-20.2) at 0 to 11 months to 16.5 kg/m2 (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.