RESEARCH ARTICLE Plasma Levels of Middle Molecules to Estimate Residual Kidney Function in Haemodialysis without Urine Collection

Background Residual Kidney Function (RKF) is associated with survival benefits in haemodialysis (HD) but is difficult to measure without urine collection. Middle molecules such as Cystatin C and β2-microglobulin accumulate in renal disease and plasma levels have been used to esti-mate kidney function early in this condition. We investigated their use to estimate RKF in patients on HD. Design Cystatin C, β2-microglobulin, urea and creatinine levels were studied in patients on incre-mental high-flux HD or hemodiafiltration(HDF). Over sequential HD sessions, blood was sampled pre- and post-session 1 and pre-session 2, for estimation of these parameters. Urine was collected during the whole interdialytic interval, for estimation of residual GFR (GFRResidual = mean of urea and creatinine clearance). The relationships of plasma Cystatin C and β2-microglobulin levels to GFRResidual and urea clearance were determined. Results Of the 341 patients studied, 64 % had urine output>100ml/day, 32.6 % were on high-flux HD and 67.4 % on HDF. Parameters most closely correlated with GFRResidual were 1/β2-mico-globulin (r2 0.67) and 1/Cystatin C (r2 0.50). Both these relationships were weaker at low GFRResidual. The best regression model for GFRResidual, explaining 67 % of the variation, was