Dialysis Adequacy

When kidneys fail, dialysis is necessary to remove waste products such as urea from the blood. By itself, urea is only mildly toxic, but a high urea level means that the levels of many other waste products that are more harmful and not as easily measured are also building up.

To see whether dialysis is removing enough urea, the dialysis 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 two blood samples are then compared. Two methods are generally used to assess dialysis adequacy, URR and Kt/V.

What is the URR?

URR stands for urea reduction ratio, meaning the reduction in urea as a result of dialysis. The URR is one measure of how effectively a dialysis treatment removed waste products from the body and is commonly expressed as a percentage.

Example: If the initial, or predialysis, urea level was 50 milligrams per deciliter (mg/dL) and the postdialysis urea level was 15 mg/dL, the amount of urea removed was 35 mg/dL.

50 mg/dL - 15 mg/dL = 35 mg/dL

The amount of urea removed (35 mg/dL) is expressed as a percentage of the predialysis urea level (50 mg/dL).

35/50 = 70/100 = 70%

Although no fixed percentage can be said to represent an adequate dialysis, patients generally live longer and have fewer hospitalizations if the URR is at least 60 percent. As a result, some experts recommend a minimum URR of 65 percent.

The URR is usually measured only once every 12 to 14 treatments, which is once a month. The URR may vary considerably from treatment to treatment. Therefore, a single value below 65 percent should not be of great concern, but a patient's average URR should exceed 65 percent.

What is the Kt/V?

Kt/V is another way of measuring dialysis adequacy. In this measurement,

• K stands for the dialyzer clearance, the rate at which blood passes through the dialyzer, expressed in milliliters per minute (mL/min)
• t stands for time
• Kt, the top part of the fraction, is clearance multiplied by time, representing the volume of fluid completely cleared of urea during a single treatment
• V, the bottom part of the fraction, is the volume of water a patient's body contains

Example: 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 multiplied by 180 minutes. The result comes to 54,000 mL, or 54 liters.

Kt = 300 mL/min multiplied by 180 minutes
Kt = 54,000 mL = 54 liters

The body is about 60 percent water by weight. If a patient weighs 70 kilograms (kg), or 154 pounds (lbs), V will be 42 liters.

V = 70 kg multiplied by .60 = 42 liters

So the ratio—K multiplied by 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 1.3.

Kt/V = 54/42 = 1.3

How Does the Kt/V Compare with the URR?

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:

• urea generated by the body during dialysis
• extra urea removed during dialysis along with excess fluid

The Kt/V is more accurate than the URR in measuring 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—about 2.2 lbs—during the treatment and the other with a weight loss of 3 kg-about 6.6 lbs. The patient who loses 3 kg will have a higher Kt/V, even though both have the same URR.

The fact that a patient who loses more weight during dialysis will have a higher Kt/V does not mean it is better to gain more water weight between dialysis sessions so more fluid has to be removed, because the extra fluid puts a strain on the heart and circulation. However, patients who lose more weight during dialysis will have a higher Kt/V for the same level of URR.

Peritoneal Dialysis Adequacy

When kidneys fail, waste products such as urea and creatinine build up in the blood. One way to remove these wastes is a process called peritoneal dialysis (PD). The walls of the abdominal cavity are lined with a membrane called the peritoneum. During PD, a mixture of dextrose (sugar), salt, and other minerals dissolved in water, called dialysis solution, is placed in a person's abdominal cavity through a catheter. The body's peritoneal membrane enclosing the digestive organs allows waste products and extra body fluid to pass from the blood into the dialysis solution. These wastes then leave the body when the used solution is drained from the abdomen. Each cycle of draining and refilling is called an exchange. The time the solution remains in the abdomen between exchanges is called the dwell time. During this dwell time, some of the dextrose in the solution crosses the membrane and is absorbed by the body.

Many factors affect how much waste and extra fluid are removed from the blood. Some factors—such as the patient's size and the permeability, or speed of diffusion, of the peritoneum—cannot be controlled. Dialysis solution comes in 1.5-, 2-, 2.5-, or 3-liter bags for manual exchanges and 5- or 6-liter bags for automated exchanges. The dialysis dose can be increased by using a larger fill volume, but only within the limits of the person's abdominal capacity. Everyone's peritoneum filters wastes at a different rate. In some people, the peritoneum does not allow wastes to enter the dialysis solution efficiently enough to make PD feasible.

Other factors that determine how efficiently a person's blood is filtered can be controlled. Controllable factors include the number of daily exchanges and the dwell times. When fresh solution is first placed in the abdomen, it draws in wastes rapidly. As wastes fill the solution, it cleans the blood less efficiently. For example, a patient may perform one exchange with a 6-hour dwell time, during which the solution pulls in nearly as much urea as it can hold. But in the second half of that dwell time, urea is being removed from the blood very slowly. If the patient performed two exchanges with 3-hour dwell times instead, the amount of urea removed would be substantially greater than that removed in one 6-hour dwell time.

Another way to increase the amount of fluid and waste drawn into the peritoneal cavity is to use dialysis solution with a higher concentration of dextrose. Dialysis solution comes in 1.5 percent, 2.5 percent, and 4.25 percent dextrose concentrations. A higher dextrose concentration moves fluid and more wastes into the abdominal cavity, increasing both early and long-dwell exchange efficiency. Eventually, however, the body absorbs dextrose from the solution. As the concentration of dextrose in the body comes closer to that in the solution, dialysis becomes less effective, and fluid is slowly absorbed from the abdominal cavity.

Types of Peritoneal Dialysis

The two types of peritoneal dialysis differ mainly in the schedule of exchanges. In continuous ambulatory peritoneal dialysis (CAPD), the patient empties a fresh bag of dialysis solution into the abdomen. After 4 to 6 hours of dwell time, the patient returns the solution containing wastes to the bag. The patient then repeats the cycle with a fresh bag of solution. CAPD does not require a machine; the process uses gravity to fill and empty the abdomen. A typical prescription for CAPD requires three or four exchanges during the day and one long—usually 8 to 10 hours—overnight dwell time as the patient sleeps. The dialysis solution used for the overnight dwell time may have a higher concentration of dextrose so that it removes wastes and fluid for a longer time.

To remove even more wastes, a mini-cycler machine can be used to exchange the dialysis solution once or several times overnight as the patient sleeps. Such additional exchanges may also help prevent the body from absorbing excessive amounts of dextrose and dialysis solution from the overnight dwell time.

Continuous cycler-assisted peritoneal dialysis (CCPD) uses a machine to fill and empty the abdomen three to five times during the night while the person sleeps. In the morning, the last fill remains in the abdomen with a dwell time that lasts the entire day. Sometimes one additional exchange is done in the mid-afternoon to increase the amount of waste removed and to prevent excessive absorption of fluid. The dialysis solution used for the long daytime dwell may have a higher concentration of dextrose.

Testing for Efficiency

The tests to see whether the exchanges are removing enough urea are especially important during the first weeks of dialysis, when the health care team needs to determine whether the patient is receiving an adequate amount, or dose, of dialysis.

The peritoneal equilibration test—often called the PET—measures how much dextrose has been absorbed from a bag of infused dialysis solution and how much urea and creatinine have entered into the solution during a 4-hour dwell. The peritoneal transport rate varies from person to person. People who have a high rate of transport absorb dextrose from the dialysis solution quickly, and they should be given a dialysis schedule that avoids exchanges with a long dwell time because they tend to absorb too much dextrose and dialysis solution from such exchanges.

In the clearance test, samples of used solution drained over a 24-hour period are collected, and a blood sample is obtained during the day when the solution is collected. The amount of urea in the solution is compared with the amount in the blood to see how effective the current PD schedule is in clearing the blood of urea. If the patient has more than a few ounces of urine output per day, the urine should also be collected during this period to measure its urea concentration.

From the used solution, urine, and blood measurements, one can compute a urea clearance, called Kt/V, and a creatinine clearance rate—normalized to body surface area. The residual clearance of the kidneys is also considered. Based on these measurements, one can determine whether the PD dose is adequate.

If the laboratory results show that the dialysis schedule is not removing enough urea and creatinine, the doctor may change the prescription by

• increasing the number of exchanges per day for patients treated with CAPD or per night for patients treated with CCPD
• increasing the volume—amount of solution in the bag—of each exchange in CAPD
• adding an extra, automated middle-of-the-night exchange to the CAPD schedule
• adding an extra middle-of-the-day exchange to the CCPD schedule
• using a dialysis solution with a higher dextrose concentration

Compliance

One of the big problems with PD is that patients sometimes do not perform all of the exchanges recommended by their medical team. They either skip exchanges or sometimes skip entire treatment days when using CCPD. Skipping PD treatments has been shown to increase the risk of hospitalization and death.

Residual Kidney Function

Normally the PD prescription factors in the amount of residual kidney function. Residual function typically falls, although slowly, over the months or even years of treatment with PD. This means that, more often than not, the number of PD exchanges prescribed, or the volume of exchanges, needs to be increased as residual function falls.

The doctor should determine the patient's dose of PD on the basis of practice guidelines published by the National Kidney Foundation's Kidney Disease Outcomes Quality Initiative (K/DOQI) (see For More Information). Health care providers should work closely with their patients to ensure that the proper PD dose is administered. To maximize health and prolong life, patients should follow instructions carefully to get the most out of their dialysis exchanges.