Know your dialysis: Ultrafiltration & Dry weight

For those of us lucky to be on dialysis, it is important to know some of the basic aspects of dialysis. As you know, the technicians and nurses looking after us during dialysis have so much time for individualized care and are so knowledgeable that we can completely trust our life in their able hands. They are totally devoid of ego and are so humble that they will listen to us if we tell them to set a particular parameter to some level. Right? :-)

So, coming to ultrafiltration. The lucky ones among us don't produce any urine. Yeah, think about it. No waiting in line for the loo during the interval of a film. Who wants to use those terribly unhygienic public loos? No pacing around impatiently on a long flight waiting for someone to come out. No need to take breaks during a long car journey. No bio-breaks during meetings. Yes, being anuric has its advantages!

When you're kidneys are not working as well as they should, they don't remove all the water you drink. Some or all of it stays inside your body. This water is partly in the blood, partly in the body's tissues and partly in between the tissues.

This has a number of undesirable side-effects - your blood volume goes up, your feet and hands may swell and you could also get breathless because the excess fluid could go and accumulate in your lungs. Not everyone will experience all these symptoms. Each one of us has his or her own favorites.

Dry weight is your body weight assuming there is not one drop of excess fluid that has not been removed by your kidneys. It is your weight assuming your kidneys were working. Now it is difficult to tell exactly what your dry weight is because your kidneys aren't working! So, that leaves it to our able and knowledgeable technicians and nurses to determine the dry weight!

Since there is extra fluid in your blood, the amount of blood your heart has to pump is not normal. The average human body has around 5 liters of blood. Even if you are one of the techs' favorite patients and are putting on a measly 2 liters between sessions, that's a whole 40% more volume for the heart to pump! This cannot be good. If you're more like me and want to make sure you're getting your money's worth on dialysis, you're likely putting on about 4-5 liters when you have a one day gap! That's a whopping 100% more! This is much worse!

Now, on dialysis, one of the most important things that happens is the removal of this excess fluid. So, you can drink again! The process of removing this excess fluid from your body by the dialysis machine is called ultrafiltration.

You can read here how the body is capable of letting go of fluid only at rates of 400 ml/hour or less.

So, if you are about 2 kg over your dry weight, you're removing fluid at 500 ml per hour - assuming you do four hours. If you're 4 kg over your dry weight, you would be removing fluid at 1 liter per hour. This is very dangerous for your body especially your heart.

So, it is very important for you to be aware of your dry weight, to know how you feel when your dry weight has gone up (because, maybe you've been eating well) or down (you've been exercising) and have complete control over how much water you are removing every session. Discuss your Ultrafiltration Goal (also called UF) with the person who is setting it and arrive at a number keeping all this in mind.

Remember - remove too much - and chances are that you could get a very low Blood Pressure and cramps; remove too little - and you will have fluid in your body when you get off - which means you cannot drink as much until you get your next session. What could be worse???


I'm struggling with this problem right now. My doctor and I are trying to figure out my dry weight. I start to get cramps towards the end of the dialysis, so I guess I am at the edge.

I am about to try out night dialysis. One question I have is: how many litres of blood do you run through the machine? In a 4.5 hour session I do about 80 liters. Today I dialysed for the first time for 6 hours, and ran through 82 liters (at a slower speed of 250). If I now dialyse for 8 hours, should i be aiming for 82 liters or so? I.e., pump speed should be slower still. What do you do?

Kamal D Shah said…
About your dry weight, if you are getting cramps towards dialysis, it might be worth trying increasing your dry weight goal by about half kg and seeing if it helps.

I run at a blood flow rate (pump speed) of 250 ml/min when I do 7 hours. Running at less than that, my doc says, could clot the blood.

Congratulations on moving to night!
Marie Lacroix said…
I found this post very informative. This process seems very extensive and complicated specially measuring the dry weight. Great comment about the blood pressure dropping if too much fluid is removed.
Anonymous said…
My husband has to have an extra day of dialays this week, in order to get him to his dry weight . They told him that if they cant get him down to his is dry weight that meant dialays is no longer helpin him . Is this true?