The Kinetics of Cystatin C: A Marker for Dialysis Adequacy

When 90% or more of native kidney function is lost, renal replacement therapy must be initiated to sustain life. Renal transplantation is the preferred method, but availability is limited. The ideal dialysis prescription remains elusive. Small molecular weight molecules (such as urea and creatinine) have been used as markers of both kidney (native and transplant) and dialysis toxin clearance (function), but there are pitfalls in using these markers to assess total ‘renal’ dose (kidney plus dialysis). Body weight, gender and other factors also affect the concentrations of these small molecules, but not cystatin C. Furthermore, cystatin C has been shown to be a better marker for estimating kidney function than creatinine, and is associated with cardiovascular morbidity and mortality. Studies have shown that it is removed by dialysis. Therefore, we investigated the use of cystatin C, a naturally occurring endogenous protein, as a marker for estimating dialysis adequacy and renal clearance. This investigation was comprised of four studies to understand the kinetics of cystatin C in patients with advanced kidney disease with or without dialysis. We found that the amount of cystatin C reduction was influenced positively by hemodialysis blood flow rate and treatment time, and negatively by ultrafiltration rate. We further demonstrated that renal hyperfiltration significantly influenced the error of creatinine-based glomerular filtrate rate equation, but not for the cystatin C equation. Therefore, cystatin C appears to be a useful marker for the assessment of kidney function in patients with advanced kidney disease but not yet on dialysis. This was taken further in our third study where we developed an equation, which gave a better estimate of residual renal function than previously published equations in patients on dialysis but who have some remaining kidney function. Finally, we confirmed our hypothesis that cystatin C is cleared during dialysis by both diffusion and convection. It is distributed mainly in the extracellular space but equilibrates slowly between the extravascular and intravascular spaces. Furthermore, we have shown that cystatin C while cleared by dialysis is stable between dialysis treatments rather than being influenced by a single dialysis treatment. It is a marker for both dialysis and renal clearances and, thus, gives a stable index of total renal clearance. The long term goal will be to define the cystatin C threshold level that influences patient morbidity and mortality and to allow better dialysis prescriptions for patients with varying (and changing) residual renal function