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xx%20Hemodialysis%20Dose%20and%20Adequacy - Hemodialysis...

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Hemodialysis Dose and Adequacy W hen kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. By itself, urea is not very toxic, but its level represents the levels of many other waste products that build up in the blood when the kidneys fail. To see whether dialysis is removing enough urea, the clinic should periodi- cally—normally once a month—test a patient’s blood to measure dialysis ade- quacy. Blood is sampled at the start of dialysis and at the end. The levels of urea in the two blood samples are then compared. Two methods are generally used to assess dialysis adequacy, URR and Kt/V. What Is the URR? The reduction in urea as a result of dial- ysis, or the URR, is one measure of how effectively a dialysis treatment removed waste products from the body. URR stands for urea reduction ratio, but it is commonly expressed as a percentage. 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 Although no fixed number can be said to represent 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, which is once a month. It may vary considerably from treatment to treatment. For this reason, a single value 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 ade- quacy is the Kt/V. In this measurement, K stands for the dialyzer clearance, expressed in milliliters per minute (mL/min), and the lowercase t stands for time. Kt, then, is clearance multi- plied 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 min- utes (3 hours), Kt will be 300 mL/min × 180 min. This equals 54,000 mL, 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 100 × (initial level – postdialysis level) = (initial level) 100 × (50 – 15) = 70% 50 National Institute of Diabetes and Digestive and Kidney Diseases NATIONAL INSTITUTES OF HEALTH U.S. Department of Health and Human Services
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water by weight. If a patient weighs 70 kilo- grams (154 lbs), V will be 42 liters. So the ratio (K × t) to V, or Kt/V, compares the amount of fluid that passes through the dialyz- er with the amount of fluid in the patient’s body. The Kt/V for this patient would be 54/42, or 1.3.
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