Effect of age, sex, and chronic kidney disease on urinary creatinine excretion in Japanese patients.

Background: Urinary creatinine levels are used to estimate the excretion rates of certain analytes from the respective analyte-to-creatinine ratios. We clarified the influence of age and sex on estimated daily urinary creatinine excretion (eUCrE) based on the urinary creatinine level and daily urine volume.

Methods: All inpatients aged ≥ 18 years who attended the Kochi Medical School Hospital with serum and urinary creatinine measurement results were enrolled. Serum and urinary creatinine concentrations were extracted from the database and fluctuations with sex and age were investigated. The eUCrE was calculated for patients with early morning spot urine protein excretion (UPE), and daily urine volume was measured on the same day.

Results: Overall, 643 participants (322 men, 321 women) were enrolled. The eUCrE levels of men and women aged 18 - 64 and 18 - 44 years, respectively, significantly exceeded 1 g/day. Those of women aged 65-74 and ≥ 75 years were significantly lower than 1 g/day. Each age group was further categorised into Groups A (patients with eGFR ≥ 30 mL/min/1.73 m² and UPE < 0.5 g/gCr), B (eGFR ≥ 30 mL/min/1.73 m² and UPE ≥ 0.5 g/gCr), and C (eGFR < 30 mL/min/1.73 m² and UPE ≥ 0.5 g/gCr). The eUCrE levels were the highest in Group A, followed by Groups B and C.

Conclusion: This study revealed age-, sex-, and renal function-related biases in adjusted values using urinary biomarkers, including proteinuria and creatinine ratio.