Abstract: Objective To explore and preliminarily establish reference intervals for four urinary kidney injury biomarkers in healthy children aged 0~18 years in Beijing. Methods Healthy children aged 0~18 years were enrolled from physical examination program at maternal and child health hospitals, kindergartens, schools and Beijing Children′s Hospital from December 2016 to October 2018. Subjects with familial genetic disorders or acute/chronic diseases were excluded via questionnaires. All participants underwent clinical physical examinations to evaluate health and growth/development status. The laboratory parameters—including whole blood cell count, liver and kidney function, electrolytes, and routine urinalysis—were examined. The level of four urinary kidney injury biomarkers, including immunoglobulin G(IgG), microalbumin(mAlb), transferrin(TRF), and α1-microglobulin(A1M), was quantitatively measured using immuno-turbidimetry. The relationships of these biomarkers with age and gender were analyzed using scatter plots, quadratic regression, Wilcoxon rank-sum test, Spearman correlation and nonparametric tests. Reference intervals were established by the nonparametric method (97.5th percentile), and 90% confidence intervals for the upper reference limits were calculated using the bootstrap method. An additional 135 urine specimens from healthy children undergoing health checkups in Beijing were collected to validate the reference intervals in accordance with the Chinese health industry standard 《WS/T 402—2024 Establishment of Reference Intervals for Quantitative Test Items in Clinical Laboratories》. Results A total of 1 861 healthy children (964 males, 897 females) aged from 5 days to 18 years met the inclusion and exclusion criteria. In this study, IgG level showed an age-dependent inverted U-shaped distribution, necessitating age-specific reference intervals: ＜9 years: ＜9.12 mg/L; 9-14 years: ＜14.08 mg/L; ＞14 years: ＜10.36 mg/L.mAlb exhibited significant gender differences(males: ＜21.68 mg/L, females: ＜41.67 mg/L, P＜0.001) and a weak positive correlation with age within each gender group(P＜0.001), that result suggested the necessity of a modified establishment of gender- and age-stratified reference intervals(e.g., upper limit for males ≥9 years: 31.24 mg/L; females ≥9 years: 52.21 mg/L). TRF was below the lower limit of detection(2.00 mg/L) in the vast majority(98.71%) of healthy children, and thus its reference interval could be defined as “＜2.00 mg/L”. A1M exhibited an interaction between age and gender, the upper reference limit for males ≥9 years(16.70 mg/L) was higher than that for males ＜9 years(12.40 mg/L), and for females ≥9 years(15.98 mg/L) higher than that for females ＜9 years(13.00 mg/L). Validation using the 135 independent samples showed that the proportions of results within the reference intervals for all four biomarkers were ≥95%. Conclusions This study established reference intervals for four urinary kidney injury biomarkers in healthy children in Beijing. IgG level showed a significant age dependency and should be reported with age-specific intervals. TRF was below the detection limit in the vast majority of healthy children. mAlb and A1M displayed unique age- and gender-related patterns. These results may support the assessment of kidney health in children.

Key words: urinary immunoglobulin G, urinary microalbumin, urinary transferrin, urinary alpha1-microglobulin, reference interval