Dialysis Clearance : Drug Removal During Dialysis

Let’s talk about the general use of dialysis. Of course it’s used to remove metabolic wastes or the uremic toxin. It’s also used to remove extra water. Hemodialysis also has a potential application for detoxification in the case of drug overdose. And therefore you can find dialyzability information in the drug package insert for most drugs. Let’s talk about the determination of dialysis clearance. First, by the A-V difference method. Let’s look at this equation. Dialysis clearance is equal to Q times CA minus CV divided by CA. Here the CA is the arterial concentration that is the concentration entering the dialyzer. CV is a venous concentration or the concentration leaving the dialyzer. Q is the blood flow going through the dialyzer.

And CA minus CV divided by CA is the so called extraction ratio. Dialysis clearance by the dialysate method. And here is the equation. Dialysis clearance is equal to Qd times Cd over CA. Here the Qd is the dialysate flow which is about 500 milliliters per minute. Cd is the drug concentration in the dialysate. CA is the arterial line concentration of the drug. And the dialysate could be instantaneous dialysate, could be intermittent dialysate or total dialysate. However, the blood concentration must be a mid point of blood sample. That is, if the dialysate is collected between zero to one hour, then the midpoint blood sample is taken at 0.5 hours. Drug removal during dialysis.

So how much drug is removed by dialysis based on the A-V difference. Here drug removal is equal to dialysis clearance times the integration of the arterial concentration. And as you know, the integration of the arterial concentration is equivalent to area under the curve. Therefore, drug removal is dialysis clearance times area under the curve. And typically you would need three to four blood samples in order to have a good estimate for the area under the curve. In addition to the drug removal by A-V difference, actually the most direct measurement for drug removal is from the dialysate or from the drug recovered in the dialysate. That is to say drug removal is the summation of Cd times Vd.

Here the Cd is the drug concentration in the dialysate and Vd is the dialysate volume. The dialysate could be the total dialysis. However, total dialysate is rarely collected because of the volume, which is about 60 to 90 liters in one hemodialysis session. Instead, intermittent dialysate samples are collected which is more manageable. For example, by collecting them in hourly interval and then pull them together.