Reference intervals of urinary clearance and excretion of uric acid (RICE-U) in Japan

To establish reference intervals for indices of renal uric acid excretion efficiency and amount using 24-h, 60-min, and spot urine measurements, and to assess relationships among these methods in healthy Japanese adults.

This multicenter prospective observational study included 445 reference individuals (214 men and 231 women; age, 20-65 years) recruited from 26 National Hospital Organization institutions. Reference intervals were estimated separately by sex using the non-parametric central 95% range (2.5th-97.5th percentiles). Associations between spot urine-derived and timed-urine indices were assessed using Spearman rank correlations.

Reference intervals for 24-h Cua were 3.1-11.5 mL/min in men and 4.4-14.6 mL/min in women, and those for 60-min Cua were 4.7-15.4 and 5.1-17.9 mL/min, respectively. BSA-standardized Eua reference intervals were 280-827 mg/day/1.73 m² in men and 263-812 mg/day/1.73 m² in women for 24-h collection, and 16.4-42.9 and 12.4-36.3 mg/h/1.73 m², respectively, for 60-min collection. Spot urine reference intervals were 3.0-9.7% in men and 3.2-12.4% in women for FEua, 0.19-0.50 and 0.14-0.43 mg/dL for EuGF, 3.6-12.3 and 4.0-16.1 mL/min/1.73 m² for eCua, and 336-937 and 254-806 mg/day/1.73 m² for eEua. Spot eCua correlated with 24-h Cua (ρ=0.627), spot FEua with 24-h FEua (ρ=0.736), and eEua with 24-h Eua (ρ=0.430); all P<0.001.

This study provides comprehensive reference intervals for renal uric acid handling under standardized preanalytical conditions in healthy Japanese adults. Spot urine-derived indices showed significant associations with timed-urine measurements, particularly for clearance-related indices, and may provide practical complementary measures when timed urine collection is difficult.