Dialysis Urban Legends: The Minimum Ultrafiltration Rate

By Peter Laird, MD

Two days ago, I went in for my iron infusion which I do in conjunction with dialysis and not in an infusion center. This time, techs did not want to place me on my usual low ultrafiltration volume because they have been taught that there is a minimum level of ultrafiltration that will prevent backfiltration of dialysate.  This is not the first time I have heard dialysis techs and nurses bring up this issue. However, it is simply a dialysis urban legend. Increasing ultrafiltration rates to a "minimum" of 300 ml/hour does not prevent backfiltration.

Measurement of back clearance

With improvement in dialysis membranes, back filtration becomes the focus of study. Because new membranes that remove low molecular weight proteins, such as beta 2-microglobulin by diffusion are now available for clinical use, both back filtration and back diffusion become problems. This paper gives the mass transfer rate of solutes from the dialysate to blood compartment. As markers of toxic or antigenic substances, inulin (MW 5,200), lysozyme (MW 14,300), and alpha-lactoalbumin (MW 14,400) were used. The dialyzers tested were the CLSU-12W, AM-FP10, BK-1.0P, and FB-110U. The UFR controller set the flow rate on the blood side at 200, dialysate at 500, and ultrafiltration (QF) as 0 to 50 ml/min. Under these conditions, the inulin back clearance is 16 (CLSU) to 26 ml/min (FB), even if the QF is 50 ml/min, and is 10 to 34 ml/min for lysozyme at QF = 0 ml/min. Although a substantial amount of solute moves into the blood compartment, back clearance does not drastically decrease with increase in QF. To avoid contamination of the blood side, clean-up techniques for the dialysis line or online protein adsorber should be followed.

This is a pervasive dialysis urban legend that can impact those dialysis patients who still have enough residual renal function that they are able to control fluids without ultrafiltration. For most dialysis patients, they take off a minimum of 1 liter of fluids with dialysis and are not affected by any alleged minimum ultrafiltration rates. Backfiltration occurs at all normal ultrafiltration levels with high flux dialyzors and it is not prevented by a minimum level. For the select few who still control fluids with their own residual renal function, placing the ultrafiltration level on 500 ml covering only the priming fluids and some of the rinse back places you at no more risk than dialyzing with the so called "minimum" ultrafiltration rates. It is time to put this urban legend to rest once and for all.

Comments

[cattlekid]

Good morning! I have a question on this - I am on NxStage and notice that if I set my machine to 300 UF (just to cover the rinseback), it takes off more fluid than that and I end up with low BP. Therefore, if I have no fluid to remove (I do have residual function) I set my machine to 0 UF. Am I doing myself any harm with this? It wouldn't seem so but wanted to check. I figure the worst that could happen is that I might end a treatment .5 kilo over dry weight, which doesn't seem like the end of the world.

Thank you!

[Peter Laird, MD]

Dear Cattlekid, I can't really comment on your individual settings, but NxStage takes off 500 ml automatically for the prime and rinseback. Taking off 0.3 is equal to 0.8 in other words. At least, that is what I was taught in my training. You may want to discuss and confirm that with your medical team.

[Thomas Birthistle]

You could always add a periodic saline bolus to replace volume during your treatment.

[Peter Laird, MD]

Thank you Tom, but what would the point of saline infusions be when the theoretical "minimum" UFR is nothing but an urban legend? Yes, you could add saline, but that really doesn't get to the heart of the issue that the so called minimum UFR is not true.

Preventing backflltration at ordinary ultrafiltration rates is impossible. Some estimate you would have to run a UFR of over 4500 ml/hour to accomplish that which of course is an absurd ultrafiltration rate.

In addition, with ultrapure dialysate, it is already a moot issue. It is just one more urban legend that needs to be placed at rest. There is no reason you cannot run a UFR less than 300 ml/hour. It makes no difference whatsoever in "preventing" backfiltration. That teaching is simply in error as the study listed above states quite well.

[Melville Hodge]

Peter,

The prime should not be added to the desired ultrafiltration to set the ultrafiltration goal, only the rinseback volume - or about 300, not 500. The tubing set is filled with saline at start of dialysis - and at the end - so the only net saline the patient receives is the amount set for rinseback.

This misconception results in excessive ultrafiltration - and is not uncommon.

Mel Hodge

[Lora Winchester, MS, RN]

I'm excited to find your blog, Dr. Laird. Your conclusion in this edition goes against everything I was ever taught in my 14 years of hemodialysis nursing. I still have a problem with turning the UFR to zero and am more comfortable doing saline replacement for patients who do not require fluid removal. In regard to "ultrapure" dialysate, I know this holds true for HHD patients using the NxStage system. For in center patients, however, I don't think one could consider the dialysate to be ultrapure. Granted, via the RO system, the water is pure, but the acid bath and bicarbonate are not. In my mind, the risk of reverse transmembrane pressure, whether due to a low uf rate or not, is scary. It makes me wonder if this solute movement into the blood compartment is responsible for more harm to patients than we realize. Chronic inflammation, anemia, etc...hmm. By the way, I always add an additional 500ml to my UF goal to account for prime and rinseback. For most patients, this has proven to be effective in reaching their EDW. Having said that, it is important to know each patient. Some patients have the propensity to "give up fluid" more easily. There are several patient's goals to which I do not add the prime/rinseback volume. I have learned to listen to my patients, realizing that they know their body much better than I do! I am really excited to be afforded the opportunity to see dialysis from a patient's perspective and will become a faithful reader of your blog.

[Peter Laird, MD]

Dear Lora,

Thank you for your kind comments and welcome aboard. Ultrapure dialysate is certainly the best medical option for patients and hopefully more new technology will soon be available taking advantage of this lower inflammatory stimulus by removing endotoxin contamination.

Sadly, you really can't prevent backflush at usual ultrafiltration rates used clinically. Yes, it is a concern, but simply turning up the UF and giving saline is most likely treating our anxiety about the issue more so than an actual advantage to the patient. That is one of the reasons I wrote this short little post since it is a dialysis urban legend taught widely in dialysis units.

Lastly, yes, thank you for listening to your patients. Many of us know exactly how much we have to take off or not despite all of the units calculations. An amazing fact is that dialysis patients can gain weight from food going to the fat cells just as well as from excessive salt and fluid intake. The danger of high ultrafiltration rates bothers me much more than the theoretical danger of a minimum UF. Longer and slower and more frequent dialysis would save lives immediately in the US.

[Anne Diroll, RN CNN]

I am into this a little late - UF Rates aside, giving unnecessary saline boluses increases your sodium burden. The American Heart Association recommends restricting sodium to 1.5 grams a day. There are 9 grams of sodium in a 1000 ml bag of normal saline. Assuming you are dietary compliant, 500 ml of saline added to your diet will result in a sodium burden of 6 grams for a day on dialysis. Dialysis is supposed to remove sodium, not add to the burden....
Anne DIroll.