New Equations Can Estimate Residual Kidney Function

Residual kidney function (RKF) in dialysis patients can be estimated by equations based on serum measures of endogenous filtration markers, avoiding the need for prolonged timed urine collections, reports a study in Kidney International.

Closely supervised 24-hour urine clearance values were obtained in a cohort of 44 dialysis patients in Baltimore. The researchers developed dialysis-specific equations to estimate urinary urea clearance, based on serum endogenous filtration markers. They then validated the equations in 826 patients from an external cohort of Dutch dialysis patients.

Median urinary urea clearance values were 2.6 mL/min in the development cohort and 2.4 mL/min in the validation cohort. During 24-hour urine collection, concentrations of most serum filtration markers increased over time, with the exception of β-trace protein (BTP).

The equations developed in the Baltimore cohort showed low bias in the Dutch cohort. Compared to an equation using urea plus creatinine, precision was higher for BTP and β2-microglobulin (B2M) equations, while accuracy was higher for BTP, B2M, and cystatin C equations. For detection of a measured urinary urea clearance of 2 mL/min or greater, area under the receiver operator characteristic curve was 0.821 for the BTP equation, 0.850 for the B2M equation, and 0.796 for the cystatin C equation (compared to 0.663 for the urea plus creatinine equation).

Residual kidney function is strongly associated with survival in dialysis patients, but currently must be measured in timed urine collections. The new equations, based on serum filtration markers, can estimate RKF with good performance and diagnostic accuracy.

Further study is needed to evaluate the use of these equations for modifying dialysis dose. An online calculator can be found at www.kidneymodels.org/rkf [Shafi T, et al. Estimating residual kidney function in dialysis without urine collection. Kidney Int 2016; 89:1099–1110].