Do Dialysis Patients Have Urine Output

Dialysis, a procedure that uses a special machine to replace the kidneys in filtering waste from the bloodstream, may reduce the daily urine output that a person normally produces. This happens because as the blood is filtered during dialysis, fluid is removed, thus reducing the kidneys' traditional role. However, it seems more likely that volume expansion (due to initial sodium retention) and a urea osmotic diuresis (as the daily urea load is excreted by fewer functioning nephrons), due in part to solute intake, play a more important role in the persistent urine output.

Some patients continue to produce normal volumes of urine, which helps to manage their fluid balance. These patients can neither dilute nor concentrate the urine normally; the range of urine osmolality that can be achieved may vary from a minimum of 200 mosmol/kg to a maximum of 300 mosmol/kg, compared with 50 to 1200 mosmol/kg in normal subjects.

In comparison, water intake (which usually determines the urine output via changes in the secretion of antidiuretic hormone [ADH]) plays relatively little role in regulating the urine output in advanced kidney disease.  However, dialysis does not prevent someone from urinating normally; it only reduces the total urine output, so that he or she may only need to urinate once a day, which is not dangerous.

As a result many dialysis patients produce very small amounts of urine.  Some dialysis patients produce as little as one cup or less of urine each day, though urine volume is usually dependent on the underlying cause of kidney failure.  You should be able to get more information from your grandmother's doctor, a nephrologist. This medical specialist is experienced in treating kidney disease and can provide insight into the cause of your grandmother's kidney failure.

Although the glomerular filtration rate (GFR) is very low in patients with end-stage kidney disease (ESKD), the urine output is variable, ranging from oliguria to normal or even above normal levels. These findings are related to the fact that the urine output is determined not by the GFR alone, but also by the difference between the GFR and the rate of tubular reabsorption. If, for example, a patient with advanced acute or chronic kidney failure has a GFR of 5 L/day (versus the normal of 140 to 180 L/day), the daily urine output will still be 1.5 L if only 3.5 L of the filtrate is reabsorbed.

It had been thought that tubular damage impaired the ability to reabsorb sodium and water, thereby contributing to the maintenance of an adequate urine output in this setting.

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