Reference Range for Urine Red Blood Cell-to-Creatinine Ratio in Japanese Children

Background Hematuria is important parameter for diagnosis of kidney disease and assessing treatment efficacy. However, reference values for the urine RBC-to-creatinine ratio (urine RBC/Cre), caluculated by dividing urine creatinine for correcting urine concentration, are not provided. Methods We retrospectively analyzed urine RBC counts and urine Cre values in children aged 18 years or younger from four facilities between February 2016 and December 2024. From these, we calculated the urine RBC/Cre using the formula: urine RBC [/µL] / urine Cre [mg/dL] × 10 ⁵ . The calculated urine RBC/Cre results were expressed in units of [×10 ⁵ /mgCre]. We established detailed reference values (the median, 2.5th percentile, and 97.5th percentile) for this urine RBC/Cre by sex and age group. Results Based on the aggregated urine RBC/Cre of 280 participants, we established detailed reference values by age and sex. The overall median urine RBC/Cre was 3.7 [×10 ⁵ /mgCre]. The median value was 3.8 for boys and 3.5 for girls. Age-specific median values in boys vs. girls were 8.2 vs. 9.2 for ages 0–5 years, 2.7 vs. 4.0 for ages 6–11 years, and 1.1 vs. 1.7 for ages 12–17 years. Age-specific 97.5th percentile values in boys vs. girls were 175.5 vs. 95.0 for ages 0–5 years, 35.5 vs. 19.3 for ages 6–11 years, and 5.8 vs. 20.0 for ages 12–17 years. Conclusion This study provides the first reference values for urine RBC/Cre in children. This may facilitate more reliable interpretation of urinary findings in pediatric clinical practice.