The Urine-to-Plasma Urea Concentration Ratio is associated with eGFR and eGFR decline over time in a population cohort.

BACKGROUND: Evaluation of renal function and of factors associated with its decline are important public health issues. Besides markers of glomerular function (e.g., GFR), those of tubular functions are rarely evaluated. Urea, the most abundant urinary solute, is markedly concentrated in urine when compared to plasma. We explored the urine-to-plasma ratio of urea concentrateions (U/P-urea-ratio) as a marker of tubular functions.

METHODS: We evaluated the relationship of the U/P-urea-ratio with eGFR at baseline in 1043 participants (48±17y) from the SKIPOGH population-based cohort, using mixed regression. In 898 participants, we assessed the relation between U/P-urea-ratio and renal function decline between two study waves 3 years apart. We studied U/P ratios for osmolarity, Na, K, uric acid for comparison.

RESULTS: In a transversal study at baseline, eGFR was positively associated with U/P-urea-ratio (βscaled = 0.08, 95%CI[0.04;0.13]) but not with the U/P ratio of osmolarity. Considering separately participants with renal function > or ≤ 90 ml/minx1.73m2, this association was observed only in those with reduced renal function. In the longitudinal study, eGFR declined at a mean rate of 1.2 ml/min per year. A significant association was observed between baseline U/P-urea-ratio and eGFR decline (βscaled = 0.08, 95%CI[0.01;0.15]). A lower baseline U/P-urea-ratio was associated with a greater eGFR decline.

CONCLUSION: This study provides evidence that the U/P-urea-ratio is an early marker of kidney function decline in the general adult population. Urea is easy to measure with well-standardized techniques and at low cost. Thus, the U/P-urea-ratio could become an easily available tubular marker for evaluating renal function decline.