Convective clearance of a toxin depends firstly on the sieving coefficient. Sieving coefficient (SC) is a measure of the ease with which a solute passes through the membrane relative to solvent, so this depends largely on the relative size of the solute molecule and the membrane pore cross section. It can be measured during pure ultrafiltration as the ratio of solute concentration before and after passing through the membrane. SC takes values between 0 (the molecule is unable to pass through the membrane) and 1 (the molecule passes just as easily as water).
Secondly, convective clearance depends on the ultrafiltration rate.
In pure haemofiltration, clearance is achieved entirely by ultrafiltration. The loss of fluid by ultrafiltration is replaced by infusion into the blood downstream of the filter (“post dilution”). In this case, clearance (K) for any solute is equal to convective clearance (Kc) and can be calculated from:
K = Kc = Quf * SC
where Quf = ultrafiltrate flow rate
In haemodiafiltration (HDF), there is a mixture of diffusion and convection. Diffusion reduces the concentrations in the ultrafiltrate, reducing Kc. Clearance in haemodiafiltration can be predicted by the following equation:
Kc = Quf * SC * ((Qp Kd) / Qp)
K = Kc + Kd
where Qp = plasma flow rate
Kd = diffusive clearance
The effect of this is to diminish the convective component. In post dilution HDF, convection will always increase total clearance, but this increase is negligible for small solutes which diffuse easily (such as urea). For larger molecules (such as beta 2 microglobulin) convection can significantly and usefully increase clearance.
Pre dilution HDF, in which replacement fluid is given upstream of the dialyser, is less commonly practiced, and more complex mathematically. Convection is similarly reduced by diffusion as above, but in addition both convection and diffusion are substantially reduced by the dilutional effect of the replacement fluid. The ultrafiltrate rate and volume are usually adjusted to account for this.
In publications, the convective component of HDF is quantified as the filtration volume. This means the total sessional filtration volume in post dilution HDF, or the equivalent adjusted volume in pre dilution HDF.