Clearance in Dialysis - How important are URR and Kt/V?

During hemodialysis, there are primarily two things happening. Removal of excess fluid and removal of toxins from the blood. These are the two main things healthy kidneys remove and since the kidneys are not working, they build up in the body and cause all sorts of problems. So, they are removed during dialysis to prevent them from reaching dangerous levels.

Fluid removal is easy to measure. Your pre-dialysis weight minus post-dialysis weight gives you fluid removal (assuming you did not have any fluid or food during the session). The toxin removal though, is more complex. There are many, many toxins that get removed. Some, we know. Some we don't even know exist, forget about knowing how much is removed.

Since there are so many toxins being removed, it is impossible to track how much is being removed of every one of them. What we do is to pick one molecule which we think is representative of most molecules and then measure that and based on the removal, decide if the clearance is adequate or not. 

Enter URR and Kt/V

When this problem presented itself during the early days of dialysis, doctors picked Urea as a representative molecule and believed they could use this to gauge the efficacy of a dialysis session.

They measured Urea before and after the session and the % removal was used as a parameter to decide if the dialysis session went off effectively or not. A 65% reduction in the Urea was thought to be adequate. This was called the Urea Reduction Ratio (URR) and even today, a URR of 65% is considered adequate for a session.

Kt/V is a number that is a slightly better way of measuring the efficiency of a dialysis session. I am not going into the specifics of what K, t and V are. Kt/V is measured using the Daugirdas Equation. This equation takes into account the URR, fluid removal, dry weight and dialysis duration to arrive at the Kt/V. A Kt/V greater than 1.2  is considered adequate for one session of dialysis. 

The problem with URR and Kt/V

The catch with URR and Kt/V is Urea is not actually a representative molecule. There are several toxins that are much larger than Urea and that don't get removed as well as Urea during a dialysis session. So, even if the dialysis session may be considered adequate, it is adequate only so far as small molecules are concerned. Larger sized molecules are not removed as well as Urea. So, we should never be satisfied if our Kt/V is just more than 1.2. We should always aim for better, more optimal dialysis. This can be achieved by using a high flux dialyser, getting more hours of dialysis per week, using a higher blood flow rate and dialysate flow rate and so on.

The larger molecules don't cause immediate issues like say fluid or Potassium cause but often build up over the years and cause long term problems like Vascular Calcification, Dialysis-related Amyloidosis and so on.

So, always aim for more. Ensure you get the most out of your dialysis sessions and try to use the best dialyser you can afford and the most hours of dialysis per week you can manage. You will be on your way to a long, happy life one dialysis that way.