Minimum UF rate

The question came up somewhere else about a minimum UF rate being necessary for the dialysis machine to work. This wasn’t a problem for me during the two months I was on short daily, because with only 2 hours of treatment time, it’s extremely unlikely that the UF rate would be too low. However, with the 6-8 hours of nocturnal treatment, it’s very easy to drink all day when you’re thirsty, and still end up not gaining enough to ensure a high enough UF rate - especially on hot summer days when you lose quite a bit of fluid via perspiration and breathing. It has already happened to me a number of times that I ended up with a UF rate under 300ml/hour at treatment time. We’ve had particular hot weather here over the past month. So far, I’ve been able to add enough liquid intake during the treatment to make up for it.

As it was explained to me, while the dialysis machine itself can handle low UF rates, the high-flux dialyzer can’t. A Fresenius Hemoflow dialyzer requires a minimum UF rate of 300ml/hour. If you can’t drink enough, you have to infuse enough saline just after starting treatment in order to reach a UF rate of 300ml/hr.

This is good to know, because the dialysis machine itself will not prevent you from having a UF rate lower than the minimum for the dialyzer being used.


There is one person in the my area on the PHD that is doing nocturnal,
The PHD back flushes every 15 to 30 minutes (you can set the backflush volume and the interval) it must eliminate the problem of the Dialyzer NeT-UF pulling too much fluid off.
I have never had to infuse fluids to prevent getting too dry.
That’s quite the flip side of the problem for me.


Hi Guillaume

I’m not complaining though. After 3 years of regular hemo in-centre, it’s a nice switch to have to drink more than what I’m thirsty for :slight_smile:


Well, I’m sure you know what you’re doing, Mel. My information comes straight from the experts. Whenever I’ve had a UF rate in the low 200’s, I’ve had numerous TMP alarms during the night - way too frequent to make for a practical overnight treatment. This is why the matter came up in my case. The documentation for the F70NR, F70NRe itself recommends a minimum UF rate of 300ml/hr. Whatever variables come together to make an ultra low UF rate work in your wife’s case, I’m glad it works for you.

My experience with dad is the same as Mel’s. Only about twice in 5 years has dad ever had a UF rate of 300ml it’s always lower and we haven’t seen any problems.

I was one who commented on a too low u/f rate. My calcium and phosphorus went high with no change in diet, we were at our wits end. I decided to think about any changes I had made and I had started running the last two hours at minimal u/f rates 300 or less. When I changed that and ran at a higher rate my numbers came back under control.

My guess is that this is more important in short runs, not nocturnal. I’ve heard that in nocturnal you often have to add phosphorus and calcium to keep them from going too low. Maybe with your ultra low u/f rates it is able to remove enough due to the long hours, but not too much because you are running such low u/fs.

I don’t doubt for a minute that you can, but I’m intrigued as to how you and Mel can do this without any problem. There are a variety of factors which affect transmembrane pressure. I’m wondering if maybe you just happen to have a happy coincidence of factors which make it possible to run a low UF rate without any problems. Whenever I’ve tried running a treatment with a UF rate much less than 300ml/hr, I’ve had multiple strings of “TMP low” followed by “TMP high” alarms throughout the treatment. In my setup, the TMP usually seems to run in the neighbourhood of 100 or so, on average. Is yours lower than this? I’ve seen the same machines run a TMP of 10 or 20 in-centre. I think it has something to do with the length of hose or resistance to the drain, among other things.

You might want to research this question at the technician boards. I have read about it before. I had read that FR 2008H machine can not run at a UF less than 300 as it is preset not to run lower than that rate. I also have observed my tmp and for the longest time it ran at about 100. Then at other times I have seen it run 20-60 (all negative numbers). I have read that this can be due to a calibration problem as well as other factors.

I have the 2008K and it can and does run below 300, obviously you can even turn u/f completely, the default setting is 300 but it can be set above or below that number.

One of the questions I have is regardless of the Machine values,
Does the dialyzer still remove a minimum UF as long as there is blood and dialysate flow through it.


No it does not, when I turn off my u/f no more is taken off, and when I run below 300 only the set amount is taken off.

As I understand it, if there isn’t sufficient pressure from the blood side to the dialysate side of the membranes in the dialyzer (as determined by the UF rate), as recommended by the manufacturer of the dialyzer, you could conceivably get backwash of dialysate into the blood side. In almost all dialysis machines except the Aksys I think, the dialysate is NOT intended to be sterile, and it’s not meant to be mixed with your blood. The TMP indicated on the machine is actually negative TMP, even though there’s no minus side in front of it. If the TMP becomes positive (as shown by a plus sign in front of it), you can’t dialyze. I’m guessing that dialyzing is safe as long as TMP is negative, but, with very low UF rates, you might not be getting as efficient a dialysis - because the UF rate does affect this to some extent. My problem with it is not that, rather, it’s that the machine tries to cope with it by constantly adjusting the TMP, which inevitably generates TMP LOW and TMP HIGH alarms because it drops down below the low limit, and then it rises up above what is now the new upper limit. I know my machine is properly calibrated, because they checked this specifically.

I know another patient who didn’t have much to take off before an overnight treatment and actually had a positive TMP. He had to drink or infuse saline.

At any rate, it’s not a problem, because it’s terrific being able to drink all I can handle. I usually top if off with a coffee while I’m setting up the machine for my nightly treatment, and if necessary, I add some fluid to drink during the first hour of treatment. I would rather do that than go to the trouble of infusing saline into myself and then changing the bag before rinseback.

BTW. I hope nobody interprets this as being argumentative. It’s just a technical matter that does not seem to have a clear, consistent answer.

[b]Pierre, just for comparison purposes, as noted we use F70NR on a 2008K machine with Qb 200, Qd 300, arterial pressure typically 120-150, venous pressure 140-160, UFR typically 50-100 or so, MediSystems tubing set, nominally 8:00 runs, Ash Split catheter connected to blood lines via a TLC/Baxter cap (which raises the pressures 40-50). We essentially never have a TMP alarm (unless I’ve stupidly left a line clamped, etc.). As others noted we also run fine even with UF turned off.


Maybe the lower blood pump speed of 200 is why you’re not having any problems with TMP alarms.

The Fresenius machine will not let the uF rate go to low, when you tell it you have a High Flux dialyzer, it automatically sets the UF rate high enough, you may not see it, but it does it. I worked on 2008 H/s and K’s for 4 years.

The TMP is Venous pressure minus Dialysate pressure.

Interesting Jim. I use a 2008K (not home model) for my dialysis. There have been times that I have had to turn off the u/f for as much as 25% of my treatment (especially in the beginning and when I challenge my dry weight and go too low). When this happens the machine continues to run, total u/f does not increase and my post weight is in line with the lower number shown as removed not the amount originally programmed to remove.

Are you saying it stops dialyzing even though it says it is? FWIW I use an Optiflux 160 and yes it is set for high flux dialyzer.

Maybe you can answer another question. I almost always remove the same amount, my pressures are almost always within 10 points, yet my tmp varies from 20-60, can you explain why??

The Fresenius machine will not let the uF rate go to low, when you tell it you have a High Flux dialyzer, it automatically sets the UF rate high enough, you may not see it, but it does it. I worked on 2008 H/s and K’s for 4 years.

Hi Jim,

My husband is on the 4008 machine. I didn’t know that you set the machine for a high flux or low flux dialyser. Is this on all fresenius machines that you do this?

My husband also runs ufr below 300, usually around 180, bfr-280, tmp is usually around 60/80, arterial pressure 100/120, venous pressure 80/100.

thanks Queenie.

There must be more to this, otherwise, a person doing a treatment with say, a nominal UF rate of 200, would end up removing 300ml/hr instead. That just doesn’t make sense.

All I know is when I’ve ended up not having to remove enough weight, and my UF rate was under 300, the machine removed the programmed amout of fluid correctly, but while it did so, there were frequent TMP alarms to deal with. In my procedures manual, it says that frequent TMP alarms can be generated when the UF rate is lower or higher than the dialyzer can handle.

Since I’ve started ensuring that my UF rate is at least 300, I’ve had no more problems. In fact, most nights, I have zero alarms from beginning to end of my nocturnal treatment, except the one that wakes me up when the treatment is finished.

I imagine there could be different versions of the same machine though. My home dialysis unit was the first to use the home version of the 2008K.