Haemodialysis UF Volume and UF Rate are NOT the Same

This blog post was made by Dr. John Agar on June 9th, 2016.
Haemodialysis UF Volume and UF Rate are NOT the Same

I have detected some confusion between ultrafiltration (UF) volume and UF rate. These are NOT synonymous—they mean entirely different things.

While the UF volume has been with us for as long as there has been dialysis, recent interest in the UF rate appears to have introduced some uncertainties about exactly what the difference is.

I have been told (correctly or incorrectly) that Fresenius North America has recently introduced a recommendation that the UF rate should be kept below 13 ml/kg/hour. Unfortunately, this half-step in the right direction may have simply added to this confusion.

Dialysis removes two main things:

  1. Solutes – dissolved electrolytes (salts) and molecules of waste (toxins) made by the body’s daily functioning
  2. Fluid (water). Convective positive or negative pressures that are applied to either side of the dialysis membrane largely determine the amount of water that is removed. By adjusting/altering these pressures, more (or less) water can be removed.

The purpose of this blog is to set the ledger straight! So, here goes…

The UF volume is the amount of water that must be removed in a single treatment to return a patient to his or her target (or base) weight. The ultrafiltration volume is commonly expressed in terms of weight (where 1 litre = 1 kilogram).

The UF rate is the speed at which that volume is removed. (Here is a handy, free calculator we built to help you calculate the UF rate).

But, wait a moment: how do we know how much water to “ultrafilter”?

As all who are involved in dialysis know, most patients rapidly gain weight (read water; remember 1 kilogram = 1 litre) between the end of one dialysis and the start of the next. In those lucky few who retain a urine output as a result of residual renal function, this gain can be minimal. However, in time most patients will lose all native kidney function and pass little or no urine. As a result, all fluid consumed between treatments —the inter-dialytic period—will need to be removed at the next treatment to return the patient to their base (NB: I personally prefer the term ‘target’) weight.

While we would like this target end-dialysis weight to be what is often described as “dry weight,” in reality, it rarely is. True dry weight is what a body would weigh IF the volume of water in each of the body’s three fluid primary compartments (cellular, extracellular, and intravascular) were to be ideal. Unfortunately, the exact measurement of dry weight remains a holy grail. Although there are all sorts of methods to try and determine it, dry weight is a notional goal rather than an actual, definable value.

So, we do our best—and, more often than not, our best is a very bad best —to guess at what we think dry weight should be… and as it really isn’t dry weight, we call it “base” or “target weight,” as our best approximation of true dry weight. Maybe, one day, bioimpedance, biochemistry or other biological solutions to the “dry weight” conundrum will be found, but for now, the aimed-for post dialysis weight (the “target weight”) remains just a best guess.

So, why all that talk about weight?

Well, as every dialysis patient knows, they will be weighed at the start of dialysis. The difference between this weight and the post dialysis target weight will then be calculated. Most commonly, the predialysis weight is greater than the target weight. This difference (in kilograms) equals the volume (in litres) that must be removed during the dialysis run by the process called ultrafiltration.

The amount to be removed [gain 2 kg = remove 2 litres; gain 3 kg = remove 3 litres; ‘gain’ 4 kg = remove 4 litres … etc.] is the UF volume. Although small adjustments may be made to this volume to account for saline flushed back at the end of dialysis, or fluid consumed during dialysis (e.g. a cup of tea), in general the UF volume equates the litres (or kg) gained in the interdialytic period.

But…and here’s the crux of this blog…the UF volume is NOT the same as the UF rate! The UF rate is a speed, not a volume, and refers to the volume of water that must be removed in any given time!

This means that:

  • If there are 2 litres of water to remove (UF volume) and the dialysis run is 2 hours, the speed of removal—UF rate—will be 1 litre per hour.
  • If there are 4 litres of water to remove (UF volume) and the dialysis run is 2 hours, the speed of removal (UF rate) will be 2 litres per hour.

The UF rate is governed by two factors:

  1. The volume that must be removed (the UF volume).
  2. The time (or sessional duration) allowed for that removal.

So, the UF volume = litres, but the UF rate = litres per hour.

To add one piece of complexity to this simple distinction, strong data has shown that if water is removed too fast and the circulating blood volume is contacted too quickly, organ perfusion pressures drop. In turn, this risks organ ischaemia and compromises organ oxygenation.

This has led to the concept, advanced by Jennifer Flythe et al1, that there is a maximum rate at which water can be removed. If this rate is exceeded, organ “stun” and cardiovascular morbidity and mortality are at heightened risk.

The UF rate is dependent on a third factor: it is not just the volume that has to be removed and the time allowed for its removal, but the size of the person being water-depleted… i.e. the persons’ body weight…or, from data that Emily See from our service has generated and reported at various meetings in the last year or so, body surface area.

Thus…the UF rate is better expressed in mL/Kg/hour.

Note that the “Kg” in this equation is the patient’s target post dialysis weight. Ideally it should be the patient’s “dry weight,” but dry weight is a notional number, while the target weight is real.

I have argued—especially with my American colleagues—over what a safe UF rate might be. I argue that a maximum UF rate should be no greater than 10 ml/kg/hr. I note that Fresenius (USA) has recently advised a maximum rate of 13 ml/k/hr.

I disagree! Just take a look at Flythe’s graph—data drawn from the HEMO study data.1 What point on that graph says “safe”? I rest my case. I know, if I were a patient, the ultrafiltration rate that I would want!

Associations between UFR and CV and all-cause mortality

As one single example of how this works, imagine a patient whose target weight is 100 kg and who has gained 5 kilograms. S/he must lose 5 litres to return to target weight:

  • A 3-hour dialysis would mean removing 5 litres (= 5000 mL) in 3 hours = 1,666 ml/hour = 1666 ÷ 100 kg or 16.6 mL/Kg/hr. That would do irreparable damage to the heart!
  • If the same patient had 4 hours of dialysis: 5000 mL to remove ÷ 4 hrs ÷ 100 kg target weight ---> 12.5 mL/Kg/hr.
  • Do a 5-hour dialysis and the ultrafiltration rate drops to 5000 ÷ 5 ÷100 = 10 mL/Kg/hr (and only just “safe”).
  • Better would be 6 hours with an ultrafiltration rate of 8.3 mL/Kg/hr.

For comparison, our Geelong data calculates our mean unit UF rate across 150 centre-based patients to be 7.95 ± 3.11 mL/Kg/hr, while our median unit-wide UF rate is 7.73 ml/kg/hr. NB: these data exclude our 50 patients at home on extended hour and high frequency nocturnal dialysis. What, for interest, is your unit’s mean and median UF rate?

So, finally, we come to “what to do”

There are only two ways to alter (i.e. lower) the UF rate!

  1. Have less water to remove in any given time
  2. Take more time to remove the same amount of water

We all now how hard it is for dialysis patients to restrict their water intake. We encourage them, cajole them, some even bully them, but, at the end of the day, limiting water intake is just not possible for some (or most) patients. Before you scold them, try it yourself! Try to limit, day after day after day, to the sorts of limits you seek to impose on your patients.

So, if water intake limitation has its limits, only one course remains: Dialysis sessional duration must be longer. And, that durational extension must be sufficient to ensure that the UF rate is no greater than (in my view) 10 ml/kg/hour.

The issue is how to achieve an ultrafiltration rate ≤ 10 ml/kg/hr?

In Geelong, for almost all patients, we do. How? Well, we do longer dialysis.

End of story.


  1. Flythe JE et al. Kidney Int. 2011 Jan; 79(2):250-7

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Comments

  • Leong Seng Chen

    Jun 22, 4:31 PM

    Appreciate with much thanks that I am lucky to be able to connect on this site for further personal lifelong learning in terms of edcuational culture & value about healthcare matters & business. I am from South East Asia, a little red dot called Singapore with Asian culture & value on haemodialysis treatment/therapy. Normally we have 4 hours solid session of HD 3 times weekly. However, recently we have some improvement to extend the hours of dialysis treatment but 90% are still a one-size-for-all both in theory & practice. I always have cramp of stress & strain on my 3rd hour onward or sometime toward the last half hour session as the Blood Pressure tends to drop quickly during that period. Thank you very much!

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    • John agar

      Jul 2, 6:55 PM

      As you will know, I have always advocated for 'longer is better' and those who get symptoms towards the end of dialysis - be those cramp, low blood pressure, or other signs of 'stress and strain' as you put it - are only reaping towards the end of the run the unwanted effects of rapid fluid removal in the earlier hours of their treatment session. Longer treatment allows the rate of fluid removal in the earlier part of the run to be slowed down. This then prevents and avoids circulatory instability, cramp etc. occurring later in the run. While it is a hard message for people to understand, longer - not shorter- treatment sessions ARE the answer.

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  • Amanda Wilson

    Jun 14, 5:39 PM

    SMH. I was hospitalized last week. The nephrologist (not my usual one, I hasten to add!) asked me why I do eight to ten hours of nocturnal dialysis when I could do 3.5 instead. When you are dealing with such a low level of knowledge about basic physiology from not just a doctor in general, but a nephrologist, it is hardly surprising that many people get such poorly prescribed dialysis. I left it at telling her how much improved all of my parameters were (adequacy, potassium and other electrolytes, normal diet and fluid intake), butI truly felt like clocking her over her head, given her condescending attitude to me, and complete disregard for my knowledge!

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  • Amanda Wilson

    Jun 10, 7:31 PM

    I have just come out of hospital after developing sepsis at the beginning of the week. My blood pressure was very low. They dialyzed me once and only took a liter, so I am now sitting here doing six hours to get rid of some of the excess fluid. The nephrologist asked me why I do so many hours of dialysis (I usually do nocturnal). Really? She would only prescribe me 3.5 hours of dialysis, despite me asking if I could do a longer session. Fortunately I do not drink much, so it could have been worse!!

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  • David Rosenbloom

    Jun 10, 2:54 PM

    Bravo Dr. Agar! Another straight-forward, highly understandable blog on a critical subject -UF rate. You could not have made this critical issue clearer.
    I haven't been on dialysis for almost 8 years, thanks to a successful transplant, but I vividly remember my earliest days when I was first diagnosed in 2002 and immediately placed in-center, and exposed to the torture of the one-size-fits-all approach to fluid removal - a lot of fluid over a short time period. No consideration was taken for residual kidney function, which I maintain throughout 6 1/2 years of dialysis. Nor my small body size and weight. If every nephrologist was made to experience cramping and dangerous drop in blood pressure during dialysis, the UF rate would be as you implore, much reduced to avoid organ stunning and ultimate damage to the heart.

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    • amanda

      Jun 12, 6:31 AM

      That's exactly what happened to me too... small size, residual kidney function, and a one-size-fits-all dialysis that almost killed me every other day.
      Thank you Dr. Agar once again! You are my hero!

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    • John Agar

      Jun 10, 6:47 PM

      Thanks, David ... It is SUCH an important distinction. Volume counts for nothing unless vessel size (body) and alteration rate (speed) are both factored in.

      Why doesn't anyone seem to 'get' this?

      It rots my socks!

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  • Nieltje Gedney

    Jun 10, 9:41 AM

    Thanks John, I know so much has been written about this, but it somehow still remains a mystery to most dialyzors. I see questions every day, especially from incenter dialyzors, about cramping et al... and they still don't understand the process. This explanation is so clear, cut, step by step that it needs to be posted everywhere!!!

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    • John Agar

      Jun 10, 6:44 PM

      Thanks, Nieltje ... Your comments are both much appreciated - and true! Hence the blog.

      I mentioned at the start 'confusion' ... but that was to be kind. Indeed, it is more the lack of understanding for this basic and vital difference that inspired me to try, once again, to get the message across.

      Just like comparing traffic volume and traffic speed on the roads ... it is speed that kills.

      Anything you - or others - can do to draw attention to this blog and the VITAL message it carries, the better. So ... yell it out from the rooftops if you can ... anything to get the facility 'masses' to hear!

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  • John Agar

    Jun 10, 2:40 AM

    Molly .. that is exactly what I hoped you would do.

    Thank you.

    It seems - at least from what I have been told - that at some of the confusion between UF volume and UF rate may actually be occurring within the professional ranks. It is the rate that really matters, not the total volume, for volume does not take into account body weight, or size, and UF volume says nothing about the size of the individual from whom it is being extracted.

    UF rate, on the other hand, incorporates size, and is thus immediately relevant to the individual patient. And, of the two choices, extending sessional time to lower the removal rate for any given UF volume is by far. the best (and better) management choice and option.

    Unfortunately the dialysis population have been fed one central, profit-driven, and mortal lie ... and have been fed it for so long that its alternative has been utterly submerged ... that lie being that dialysis can somehow be magically made short.

    The unpalatable truth is that it can't be. Unpalatable for patients? .. yes - but unpalatable for the profit industry? .. even more so.

    Mel Hodge wrote a blog, here, 2-3 weeks ago, describing a simple way to change the paradigm. If only your policy makers had the eyes to read, the brains to comprehend and the fortitude to act, dialysis would become far more tolerable and way less mortal, at a stroke.

    So .. please Molly .. why stop at your nurse? Why not everyone's nurse? Professionals must grasp this concept - and not - for pity's sake - in-building a cut-off at 13 ml/kg/hr (just LOOK at the graph) when it should clearly be no more than 10!

    Unless and until they do .. dialysis will continue to kill people, and not extend life, health, and happiness, as our dialysis founding fathers - like Belding Scribner- once hoped and intended it should.

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  • greg francis

    Jun 9, 8:23 PM

    I have been a dialysis patient for nearly 5 years, during that time I have had a number of conversations with various Drs in regard to the number of hours I spend on dialysis. On starting I was on 3 days a week doing 4 hours each section. After having a machine at home I wondered if I could reduce the number of days having treatment as I still had a full kidney function. So after a month at home I went on twice a week with no great difference in my blood results.
    This was 4 and a half years ago and my blood tests, as of yesterday are still about the same as when I first was put on dialysis. My action has caused a great deal of discussion with my Drs. I feel fine all the time, play table tennis twice weekly and walk on hills twice a week. Went so far as going on a sea cruise last year and had no choice but not to have dialysis for 7 days, still felt fine. Can anyone give me an answer as why my dialysis is so different as to normal. I am 76 years old and weight, dry weight 65KG. I only take off 350 mls with a pump speed of 280. I dont know of another patient in a similar situation.

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    • John Agar

      Jun 10, 9:11 PM

      Greg ... that is excellent news - at least for you, as an individual - but it also rather proves my point that dialysis must be delivered to, and tailored for, each individual, how and as it best provides quality of life, minimisation of symptoms, and - as a very distant third - laboratory data.

      It is common, though, to read posts that ask ...'my labs are good, but then why don't I feel so good?' My answer is that while some seem to think all that is needed to achieve - or be delivering - good dialysis is a good set of numbers. Not so! Good dialysis is so much more than a set of numbers on a page. Good dialysis is 'personal'.

      You illustrate how different options fit different people best. I don't know your full story, but reading between the lines, I suspect you may possibly have had a significant slice of residual renal function (RRF) at the time you started - even possibly with an element of recovering AKI superimposed on your CKD at the time you started dialysis - factors that have later allowed you to manage with less dialysis support. You may well be a true 'poster boy' for the concept of incremental dialysis. This can last quite some time - even years - depending on the degree of RRF.

      Whatever the case, while you are a lucky man, others don't (or can't) get away with as little dialysis as you do. And, back to my point ... everyone is different, and to treat them with a one-size-fits-all program is nonsensical.

      And ... while your post and my reply have drifted a little away from the initial point of this blog, it does fit the underlying theme that ...dialysis is an individual and not a group therapy, and many seem to have lost all sight of that!

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  • Molly Kelly

    Jun 9, 6:36 PM

    This is a very interesting article. I am going to share it with my dialysis nurse!
    Thank you!

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