Sex- and age-specific reference intervals for urinary creatinine

Urinary creatinine is commonly used to adjust for variation in fluid intake when spot urine samples are collected for evaluation of urinary biomarkers. However, a uniform creatinine correction that does not account for demographic characteristics may not adjust appropriately, as urinary creatinine varies substantially with age and sex. Therefore, this study aimed to evaluate urinary creatinine concentrations by age and sex and to determine indirect stratified reference intervals (RIs) in adults using retrospective data. We identified spot urine samples requested for urinary creatinine measurement as part of patient management in general practice from routine hospital laboratories in the Northern Denmark Region. Urinary creatinine was measured in all samples using enzymatic assays on automatic analysers. The reference cohort included samples from 157,634 unique individuals, and RIs were established non-parametrically (2.5th-97.5th percentiles). Median urinary creatinine was lower in females (6.58 mmol/L) than in males (9.44 mmol/L) and decreased with increasing age (females < 50 years: 9.06 mmol/L and ≥ 50 years: 6.39 mmol/L; males < 50 years: 11.74 mmol/L and ≥ 50 years: 9.24 mmol/L). Similar trends were found in the established RIs for urinary creatinine (females < 50 years: 1.37-24.72 mmol/L and ≥ 50 years: 1.62-18.77 mmol/L; males < 50 years: 1.95-27.03 mmol/L and ≥ 50 years: 2.44-22.20 mmol/L). In this Danish regional study, urinary creatinine was lower in females than in males and decreased with increasing age. These findings emphasise the importance of considering age and sex when applying urinary creatinine in clinical or scientific practice.