BACKGROUND AND OBJECTIVES

p-Cresyl sulfate and indoxyl sulfate contribute to cardiovascular disease and progression of renal disease. Renal clearance of both solutes mainly depends on tubular secretion, and serum concentrations are widely dispersed for any given stage of CKD. From this information, it is inferred that estimated GFR is not a suitable proxy of the clearance of these solutes. Formal clearance studies have, however, not been performed to date.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: This study analyzed renal clearances of p-cresyl sulfate and indoxyl sulfate in the Leuven CKD cohort (NCT00441623; inclusion between November of 2005 and September of 2006) and explored their relationship with estimated GFR. Multivariate linear regression models were built to evaluate contributions of estimated GFR, demographics, and generation rates to p-cresyl sulfate and indoxyl sulfate serum concentrations.

RESULTS

Renal clearances were analyzed in 203 patients with CKD stages 1-5. Indoxyl sulfate clearances (median=17.7, interquartile range=9.4-33.2 ml/min) exceeded p-cresyl sulfate clearances (median=6.8, interquartile range=3.4-12.0 ml/min) by about threefold. A linear relationship was observed between estimated GFR and clearances of p-cresyl sulfate (R(2)=0.50, P<0.001) and indoxyl sulfate (R(2)=0.55, P<0.001). In multivariate regression, p-cresyl sulfate concentrations were associated (R(2)=0.75) with estimated GFR and generation rate (both P<0.001). Indoxyl sulfate concentrations were associated (R(2)=0.74) with estimated GFR, generation rate (both P<0.001), age (P<0.05), and sex (P<0.05).

CONCLUSIONS

Estimated GFR provides an acceptable estimate of renal clearance of p-cresyl sulfate and indoxyl sulfate. Remarkably, clearances of indoxyl sulfate exceed clearances of p-cresyl sulfate by approximately threefold, suggesting substantial differences between tubular transporter affinities and/or involvement of separate transporter systems for p-cresyl sulfate and indoxyl sulfate.