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What is meant by dry weight in a hemodialysis patient?

Dear Doctor: My 16-year old son was admitted to the hospital in kidney failure three weeks ago. He just got out two days ago and during his stay started hemodialysis. While in the hospital doctors inserted a peritoneal line so we can transition to peritoneal dialysis (PD) in the next couple weeks. My son had a bilateral nephrectomy (both kidneys removed). He is having trouble adjusting to hemodialysis. In three weeks he has lost 40 pounds. I know a lot was “fluid” weight, but I question what his doctor says should be his dry weight. What is meant by “dry weight” in a hemodialysis patient?

Answer: When applied to a patient, the term “dry weight” means the amount of body mass (weight) without extra fluid (water). There are a number of reasons a person may retain fluid including heart failure signaled by swelling of the legs (edema) and shortness of breath caused by water in the lungs. The liver makes albumin, a protein that holds water in blood. Reduced production of albumin in liver disease results in fluid leaving the blood stream into tissues causing edema, wet lungs and swelling of the abdomen (ascites).

A major function of healthy kidneys is the emptying of water as urine. When extra fluid is added to our diet, we urinate more frequently, passing larger amounts of urine each time. Kidney failure typically limits water leaving our body leading to edema, wet lungs, ascites and a rise in blood pressure (hypertension).

Often tricky to determine, the true amount of total body fluid depends on knowing:

  1. 1.Intracellular fluid – how much water is held in the body’s cells.
  2. 2.Extracellular fluid – water outside of cells in tissues and body spaces such as the chest and abdomen.
  3. 3.Body sodium – may affect fluidbetween compartments, weight gain between dialysis treatments and affect the success of fluid removal during hemodialysis.

Several techniques are used to estimate the amount of water held in these three “spaces.” Each kidney failure patient’s prescription for the frequency and duration of hemodialysis treatments targets the amount of fluid removal to restore what is estimated to be the patient’s dry weight, meaning the lowest weight tolerated without developing low blood pressure. The dry weight stated for a hemodialysis patient is a bit lower than the weight of a healthy person without extra fluid. This is to protect against the impact of the build up of fluid that was not discharged between dialysis sessions. Generally, finding each patient’s dry weight is done by trial and error. The health care team tries to minimize the symptoms of extra fluid volume and high blood pressure between dialysis treatments versus the weakness and low blood pressure that occurs when too much water is removed during hemodialysis.

Answer provided by Eli. A. Friedman, MD. Dr. Friedman is a Distinguish Teaching Professor at SUNY Health Science Center in Brooklyn, NY. Dr. Friedman is also Chairman of the AAKP Medical Advisory Board and an AAKP Life Member.

The Dear Doctor column provides readers with an opportunity to submit kidney-related health questions to healthcare professionals. The answers are not to be construed as a diagnosis and, therefore, alterations in current healthcare should not occur until the patient’s physician is consulted.

This article originally appeared in the November 2009 issue of aakpRENALIFE.