I’ve noticed that my TMP is real low, 10 to 30, and wonder if this could cause poor treatments?

For many consecutive txs my tmp has run an averge of 60-100. Now, on a different machine (same model) it runs 20-40. I, too, would like to know why the tmp has dropped. I asked at my unit, but did not get a clear answer. I’m sure it has a simple explanation.

Hi Cathy

I’ll leave any technical explanations to the experts (I’m definitely not an expert, but I have asked the technicians questions about it), but TMP is not a concern as long as its relatively constant and you are coming off tx with more or less the amount of fluid removed that you asked for. With all of the treatment parameters exactly the same, I’ve been treated in centre with a TMP as low as 10. During the first few months at home, my machine showed a TMP in the area of 150-200, but after a while, it started running lower, in the area of 10-60 (most often about 30). I think some of that change may be due to the hoses gradually “relaxing” after the initial installation (this is how it was explained to me). Apparently backpressure in the drain hose can affect it. I don’t think we need to worry about TMP unless we’re coming off tx way off our target weight.

If TMP starts jumping around enough to generate frequent alarms, then you have to suspect problems mainly related to clotting in the venous line (at fistula through the venous air chamber and into the dialyzer), problems with the fistula, or a technical problem with the machine’s ultrafiltration system. TMP itself doesn’t matter as long as it’s not positive. The TMP displayed on the screen is actually a negative number, but the display doesn’t show the minus sign.


The TMP reading on most dialysis machines is an approximation, and is not preceisely accurate. Because the pressures change so rapidly, the machine has to average the readings over a period of time, and there are significant short term pressure changes. As the blood pump roller pushes the blood, the pressure increases, and when it gets to the end of the tubing segment, the pressure drops just as the next roller starts pushing. This pressure is transfered through the membrane into the dialysate, and is picked up by the pressure sensor there as well. With a UF controlled machine, the TMP reading is informational, and does not affect the amount of fluid removed.

In a UF controlled machine (which all of them are), the machine preceisely matches the amount of dialysate going to the dialyzer with what is coming out in what is called a “closed loop”. If the UF pump is not on, no fluid will be removed. When the UF pump is on, it will remove 1 cc of fluid from this closed loop each time it strokes. When 1 cc is removed from the dialysate, it is replaced by 1 cc from the blood stream. If you need to have 500 cc’s removed each hour, the pump will stroke 500 times each hour.

If you know the “KUF” of the dialyzer, you can easily calculate the theoretical TMP. This formula looks complicated, but is isn’t really. The KUF is stated as the amount of fluid removed per millimeter of mercury (mmHg) per hour. For example, if the dialyzer KUF is 40, the dialyzer will remove 40 cc’s per hour at a TMP of 1 mmHg.

So, if you need to remove 4 liters in a four hour treatment, that works out to 1 liter per hour, or 1,000 cc’s. You divide 1,000 by the KUF of 40, and you TMP shoud be 25 mmHg.

The KUF of the dialyzer you are using is most likely listed on the package insert in the box.