Formula for dialyzer solute clearance

Trauma Columnist · Nov 25, 2008

Am not sure what you are wanting. What we do to determine clearance is to obtain a BUN pre and post-hemodialysis and as long as the post-BUN is less than half of the pre-HD BUN, then you are achieving adequate clearance during that one particular dialysis treatment. Other things we look at are the kt/V which needs to be >1.40 and the URR which needs to be >70. Found this on Up To Date:

SOLUTE CLEARANCE — Solute is cleared from the intravascular compartment by either diffusive or convective transport. Such transport depends upon multiple factors, including the concentration gradient between the blood and dialysate for a particular solute, the type and amount of blood and dialysate flow, the properties of the dialysis membrane, and the size and physicochemical property of the solute being removed.

Diffusive transport — Diffusive transport is the primary means of metabolic waste removal in patients undergoing hemodialysis. Such transport depends upon blood from the patient flowing within the dialysis apparatus interfacing with dialysate fluid via pores located within each fiber of the dialysis membrane. The diffusion of solutes proceeds down a concentration gradient from blood to dialysate or from dialysate to blood (bidirectional).

The degree of diffusive transport is a function of the concentration difference of the solute with respect to blood and dialysate, membrane surface area, porosity and thickness of the membrane, molecular size of the solute, and flow rate of blood and dialysate. The mass transfer coefficient (KoA) of a dialyzer (see below), which defines its capacity, varies with the depth and porosity of the membrane, the molecular size of a given solute, and the flow rates of blood and dialysate [1].

Blood and dialysate flow in opposite directions through the dialyzer at rates of 300 to 500 mL/min and 500 to 800 mL/min, respectively. Such countercurrent flow is integral to maximizing the diffusive clearance of metabolic waste solutes (show figure 1). If blood and dialysate flow in the same direction across the dialysis membrane (concurrent flow), the diffusion of a solute will lower the concentration of the substance in the blood and raise it in the dialysate, thereby gradually decreasing and perhaps even abolishing the concentration gradient favoring further diffusion. This effect is minimized with countercurrent flow. In this arrangement, the concentration of solute in the dialysate is lowest when it enters the dialyzer at the blood (venous) outflow (where the solute blood concentration is at its minimum) and is highest when it leaves the dialyzer at the blood (arterial) inflow (where the solute blood concentration is at its maximum). As a result, a high concentration gradient is maintained for the length of the fluid paths.

Convective transport — With high rates of fluid transport (ie, ultrafiltration) from blood to dialysate, convective transport of solute also occurs, thereby augmenting diffusive solute transport. In this mechanism, solutes are effectively dragged along with fluid as it moves across the membrane depending upon their size relative to the size of the membrane pores.

The propensity for impedance of any solute is described by the sieving coefficient, a numerical assessment of the potential for convective transport of a given solute. This coefficient equals the ratio of the solute concentration in the filtrate to that in the arterial plasma. A sieving coefficient of one denotes a solute which passes completely unimpeded, whereas a solute which is completely rejected has a coefficient of zero. Very large and small solutes therefore have sieving coefficients approximating zero and one, respectively. For small solutes, such as urea, glucose, and electrolytes, the solute concentration in the fluid removed by convective transport is similar to that in the plasma. Since the capacity for small solute diffusion is generally much greater than that of large solute diffusion (eg, vitamin B12), the relative contribution of convection may be most important in large solute transport [1].

These relationships apply to conventional hemodialysis. A different process of solute removal is used with continuous arteriovenous or venovenous hemofiltration in which very large volumes of water and solutes are removed by convection. (See "Continuous renal replacement therapies: Overview").

Flow rates — The clearance of solutes, either via diffusion or convective transport, is influenced by blood and dialysate flow rates. Increases in clearance occur with increases in flow rates until a plateau is reached; beyond this threshold, no further increases occur with increasing flow. The flow dependence of diffusive transport is greatest for small solutes (eg, urea and electrolytes). These solutes are rapidly cleared, thereby diminishing the concentration gradient for further diffusion. Providing fresh plasma (with a high solute concentration) and fresh dialysate (with a low solute concentration) maximizes solute removal.

Small solute clearances reach a plateau at higher flow rates than large solutes [1]. Since larger solutes diffuse more slowly, the concentration gradient is maintained, and it is the duration of dialysis that is a major determinant of solute removal.