Hemodialysis Dose and Adequacy

Hemodialysis Dose and Adequacy

When kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. Urea is not very toxic by itself, but its level represents the levels of many other waste products that build up in the blood when the kidneys fail.

To see if dialysis treatments are removing enough urea, the clinic should periodically–normally once a month–test a patient’s blood to measure dialysis adequacy. Blood is sampled at the start of dialysis and at the end. The levels of urea in the blood in the two samples are then compared. There are two methods to assess dialysis adequacy in general use, URR and Kt/V.

What Is the URR?The percent reduction in urea as a result of dialysis, or the URR (this stands for urea reduction ratio, although it is commonly expressed as a percent), is one measure of how effectively a dialysis treatment removed waste products from the body.

Example: If the initial (predialysis) urea level was 50 mg/dL, and the postdialysis urea level was 15 mg/dL, the URR is computed as

100 x (initial level – postdialysis level)
(initial level) = 100 x (50 – 15)
50 = 70%Although there is no fixed number that represents an adequate dialysis, it has been shown that patients generally live longer and have fewer hospitalizations if the URR is at least 60 percent. For this reason, some groups advising on national standards have recommended a minimum URR of 65 percent.

The URR is usually measured only once every 12 to 14 treatments (i.e., once a month). It may vary considerably from treatment to treatment. For this reason, a single value that is below 65 percent should not be of great concern, but on average, the URR should exceed 65 percent.

What Is the Kt/V?Another way of measuring dialysis adequacy is the Kt/V. In this measurement, K stands for the dialyzer clearance, expressed in milliliters per minute (mL/min), and the small t stands for time. Kt, then, is clearance multiplied by time. This top part of the fraction represents the volume of fluid completely cleared of urea during a single treatment. If the dialyzer’s clearance is 300 mL/min and a dialysis session lasts for 180 minutes (3 hours), Kt will be 300 mL/min x 180 min. This equals 54,000 milliliters, or 54 liters.

In the bottom part of the fraction, V is the volume of water a patient’s body contains. The body is about 60 percent water by weight. If a patient weighs 70 kilograms (154 lbs), V will be 42 liters. So the ratio (K x t) to V, or Kt/V, compares the amount of fluid that passes through the dialyzer with the amount of fluid in the patient’s body. The Kt/V for this patient would be 54/42, or 1.3.

The Kt/V is mathematically related to the URR and is in fact derived from it, except that the Kt/V also takes into account two additional factors: (1) urea generated by the body during dialysis and (2) the extra urea removed during dialysis along with excess fluid.

The Kt/V is a more accurate way than the URR to measure how much urea is removed during dialysis, primarily because the Kt/V also considers the amount of urea removed with excess fluid. Consider two patients with the same URR and the same postdialysis weight, one with a weight loss of 1 kg during the treatment and the other with a weight loss of 3 kg. The patient from whom 3 kg are removed will have a higher Kt/V, even though both have the same URR.

This does not mean that it is better to gain more water weight between dialysis sessions so that more fluid has to be removed, since this has bad effects on the heart and circulation. However, patients who have higher weight loss during dialysis will have a higher Kt/V for the same level of URR.

How Does the Kt/V Compare with the URR? On average, a Kt/V of 1.2 is roughly equivalent to a URR of about 63 percent. For this reason, another standard of adequate dialysis is a minimum Kt/V of 1.2. This is the new standard adopted by the Dialysis Outcomes Quality Initiative (DOQI) group. Like the URR, the Kt/V may vary considerably from treatment to treatment because of measurement error and other factors. So while a single low value is not always of concern, the average Kt/V should be at least 1.2. In some patients with large fluid losses during dialysis, the Kt/V can be greater than 1.2 with a URR slightly below 65 percent (in the range of 58 percent to 65 percent). In such cases, the Kt/V is considered to be the primary measure of adequacy by the DOQI guidelines.

Is a Kt/V of 1.2 Good Enough?These numbers, a URR of 65 percent and a Kt/V of 1.2, have been determined to be benchmarks of dialysis adequacy on the basis of studies in large groups of patients. These studies generally showed that patients with lower Kt/V and/or URR numbers had more health problems and a greater risk of death.

One large study funded by the National Institutes of Health is testing whether a Kt/V of about 1.6 or 1.7 (and a URR of about 75 percent) results in even better patient outcomes. We should have the answer within the next 2 to 3 years.

What Should You Do if Your Kt/V Is Below 1.2 or if Your URR Is Below 65 Percent?

If your Kt/V is always above 1.2 and your URR is close to 65 percent (it may be a few points lower if you have large fluid losses during dialysis), then your treatment is meeting adequacy guidelines.

If your average Kt/V (usually the average of three measurements) is consistently below 1.2, then you and your nephrologist need to discuss ways to improve your Kt/V. Since the V value is fixed (it represents your total body water volume), Kt/V can be improved either by increasing K (clearance) or t (session length). To increase t, you need to dialyze for a longer period. For example, if your Kt/V is 0.9 and you want to go up to 1.2, then you need 1.2/0.9 = 1.33 times more Kt. If K is not changed, this means that your session length needs to be increased by 33 percent. If your session time is 3 hours, it should be increased to 4 hours.

Another way to improve the Kt in Kt/V is to increase K, the dialyzer clearance, which depends primarily on the rate of blood flow through the dialyzer. No matter how good a dialyzer you have, how well it works depends primarily on moving blood through it. In many patients, a good rate is difficult to achieve because of access problems.

If your blood flow rate is good (it should be at least 350 mL/min for adult patients, and preferably higher), you can get further improvements in clearance by making sure that you use a big dialyzer or, in some cases, by increasing the dialysate flow rate from the usual 500 mL/min to 600 or 800 mL/min. A few centers are even using two dialyzers at the same time to increase K in large patients.

However, the rate of blood flow through the dialyzer is key, and a good vascular access is very important to make sure that you are getting good clearance.

If during any given month your Kt/V is very low, the measurement should be repeated, unless there was an obvious reason for the low Kt/V (e.g., treatment interruption, problems with blood or dialysate flow, some problem in sampling either the pre- or postdialysis blood). If there is no clear-cut reason for the sudden drop, then a problem with needle placement (inadvertent needle reversal) or with the vascular access (recirculation) should be suspected.

For more information, contact the following organizations:

American Kidney Fund
6110 Executive Boulevard
Suite 1010
Rockville, MD 20852
(800) 638-8299
Home page: www.americankidneyfund.org/

National Kidney Foundation
30 East 33rd Street
New York, NY 10016
(800) 622-9010
Home page: http://www.kidney.org/

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NIH Publication No. 99-4556