How does my doctor know if dialysis is working?

Part of the series “All about dialysis”

When you begin dialysis treatments, whether hemodialysis or peritoneal dialysis, you should begin to feel better as your blood is getting cleaned. To make sure you are getting adequate dialysis, your nephrologist will order lab work to see how well dialysis is working for you. If your doctor sees that your blood is not being adequately cleaned, changes can be made to help you get better treatments. Ask your doctor about your lab results.

How does my doctor know if dialysis is working?

What is the URR?

The urea reduction ratio (URR) is one way of measuring dialysis adequacy — how much waste is removed by hemodialysis. If you receive hemodialysis three times a week, each treatment should reduce your urea level (also called BUN or blood urea nitrogen) by at least 65%.

For example, if your pre-treatment BUN is 80 mg/dL, then a blood sample taken immediately after your treatment should show a BUN level of 28 mg/dL or less.

What is the Kt/V?

Kt/V (pronounced "kay tee over vee"), like URR, is a measure of dialysis adequacy. For hemodialysis three times a week, K/DOQI (Kidney Disease Outcomes Quality Initiative) national guidelines recommend a delivered Kt/V of at least 1.2.

Kt/V is a mathematical formula: K (clearance) multiplied by t (time) divided by V (volume), where:

  • K = clearance—the amount of urea your dialyzer can remove (liters/minute)
  • t = time—the duration of treatment (minutes)
  • V = volume—the amount of body fluid (liters)

For example, if you have 50 L of body fluid and a dialyzer with a clearance of 0.25 L/min, to get a Kt/V of 1.2, you would need 240 minutes (4 hours) of dialysis per treatment. To clean your blood well, the dialyzer clearance rate (K) multiplied by the time on dialysis (t) should equal a little more than the total volume of fluid in your body (V) that needs to be cleaned.

In continuous ambulatory peritoneal dialysis (CAPD), the guidelines recommend a weekly Kt/V of at least 2.0, taking into account any remaining kidney function along with the CAPD treatment. In peritoneal dialysis, Kt/V is measured by collecting drained dialysate along with any urine you produce in a 24-hour period.

When do I get URR and Kt/V tests?

These lab tests are drawn monthly. In peritoneal dialysis, the Kt/V is usually done quarterly. Ask your dialysis nurse what your values are each time. If your values do not reach adequate numbers, work with your care team to raise your dialysis dose.

What is a good blood flow rate for hemodialysis?

During hemodialysis, a blood pump is set to a constant speed to push your blood through the dialyzer and back to your body. Your doctor prescribes the blood flow rate. It is usually between 300 and 500 mL/min (milliliters per minute). You can see the blood flow rate by looking at your machine. Ask your technician to show you. With many dialyzers, blood flow rates greater than 400 mL/min can increase the removal of toxins. Blood flow rate is limited by the size of your access, the tubing, and the needles. In the case of access types, fistulas and grafts have better blood flow rates than catheters. To make up for a slower blood flow rate, your doctor may prescribe a longer treatment or a dialyzer that removes wastes more efficiently.

What is arterial pressure on the hemodialysis machine?

A hemodialysis machine monitors the pressure of your blood inside the tubing and dialyzer. Depending on the machine, arterial pressure is measured in one of two places. One is between your access and the blood pump (pre-pump arterial pressure). The other is between the blood pump and the dialyzer (post-pump arterial pressure).

An alarm will go off if the pressure is too high or too low, due to:

  • Infiltration of the access (bleeding inside the access arm)
  • Clotting in the access or needle
  • A needle against the access wall
  • A kink in the blood tubing
  • Low blood pressure
  • Too-fast or too-slow blood pump speed
  • Separation of the blood tubing from the access or dialyzer

Ask your dialysis team what your arterial pressure reading should be. Pre-pump arterial pressure is a negative number, because the blood pump pulls blood into the tubing. Post-pump pressure is a positive number, because the blood pump pushes blood through the arterial chamber.

What is venous pressure on the hemodialysis machine?

In the hemodialysis machine, venous pressure is measured between the dialyzer and your access. If your venous pressure rises from week to week, it could mean a narrowing of the blood vessels in your access. Caught early, this can be fixed.

Other problems that can cause a venous pressure alarm include:

  • A needle against the blood vessel wall
  • A kink in the blood tubing
  • Separation of the blood tubing from the dialyzer
  • A blood clot in the dialyzer or tubing
  • Narrowing of the blood vessel in the access
  • Too-fast or too-slow blood pump speed
  • An infiltration
  • Needle dislodgement

Ask your dialysis team what your venous pressure numbers should be.

Summary

The best judge of how you feel is you, but your doctor will also review your lab tests to see if you are receiving the most effective hemodialysis or peritoneal dialysis treatments. Talk with your doctor about how you feel and about your lab results. If you’re not feeling well or your lab results are not on target, changes can sometimes be made to your treatments to help you feel your best on dialysis.

External articles
  • Hemodialysis Dose and Adequacy — National Kidney and Urologic Diseases Information Clearinghouse, A service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDKD)
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