Dialysis is a medical treatment that takes over the function of failing kidneys, primarily by removing waste products and excess fluid from the blood. When the kidneys stop working efficiently, they lose their ability to filter blood and regulate the body’s fluid balance. A common question for patients starting this therapy is whether they will still produce urine, since the body’s natural fluid removal system is compromised. The answer is not a simple yes or no, as it depends entirely on how much remaining function the patient’s own kidneys possess.
The Variability of Urine Output on Dialysis
The ability to pass urine after starting dialysis is highly personal and tied to Residual Renal Function (RRF). RRF refers to the small amount of filtering capability that may still exist in the native kidneys, even after end-stage kidney disease has been diagnosed. Many patients begin dialysis with some RRF, meaning they continue to produce a noticeable amount of urine daily.
This remaining urine output contributes to a better quality of life and is associated with improved survival rates. Urine production helps manage the balance of salt and water, which can make daily fluid restrictions less severe. For patients with preserved RRF, fluid management is easier and blood pressure control is often better.
The amount of RRF, and therefore urine output, varies widely among patients. Some people may still produce a near-normal volume of urine, while others produce very little (oliguria), or none at all (anuria). Unfortunately, RRF tends to decline over time due to the progression of kidney disease.
As RRF diminishes, the patient’s urine output decreases, often leading to total dependence on the dialysis machine for fluid removal. Once a patient becomes anuric, their body retains all consumed fluid between treatments, making strict fluid control imperative. The physical sensation of needing to urinate becomes less frequent or stops entirely.
How Dialysis Replaces Kidney Fluid Removal
When the kidneys fail to produce enough urine, the dialysis machine mechanically removes the excess water that accumulates in the body. This process is called ultrafiltration (UF), which uses the principle of pressure gradients to pull fluid across a semipermeable membrane within the dialyzer.
During a hemodialysis session, the patient’s blood flows on one side of this membrane, and a special fluid called dialysate flows on the other. The machine creates a pressure difference, known as transmembrane pressure, by lowering the pressure on the dialysate side. This difference physically pushes excess water from the blood into the dialysate.
The goal of ultrafiltration is to bring the patient to their “dry weight,” the weight at which they have a normal amount of fluid in their body. The rate of fluid removal is carefully calculated by the dialysis team based on the weight the patient gained since their last treatment. This removal relies on mechanical pressure rather than biological filtration and reabsorption, distinguishing it from the kidney’s natural process.
Practical Management of Daily Fluid Intake
For patients with limited or no urine output, managing daily fluid intake is a constant task to prevent fluid overload between dialysis sessions. The primary measure of fluid management is the interdialytic weight gain (IDWG), the weight difference between the end of one treatment and the start of the next. Excessive IDWG, typically defined as more than 4% of the patient’s dry weight, requires aggressive fluid removal during treatment, which can lead to complications like low blood pressure and cramping.
The daily fluid allowance is generally calculated as a fixed volume, such as 500 milliliters, plus the amount of urine the patient passes in a 24-hour period. This personalized restriction is communicated by the care team and must include all liquids, such as water, coffee, soup, and anything liquid at room temperature.
Signs of fluid overload include swelling (edema) in the ankles, hands, and face, as well as shortness of breath due to fluid backing up into the lungs. A major driver of thirst, which complicates fluid restriction, is high sodium intake, as salt causes the body to retain water. Patients are encouraged to restrict sodium consumption to help manage thirst and make fluid control more tolerable.
To cope with thirst, patients can use several strategies:
- Sucking on sugar-free hard candies.
- Chewing gum.
- Rinsing their mouth with cold water or mouthwash instead of drinking.
- Eating frozen fruit, such as grapes or berries.
Consistent monitoring of daily fluid intake and weight gain, along with open communication with the dialysis team, helps ensure a safe and comfortable fluid balance.