People with acute renal failure (ARF) often make low amounts of urine. This is called oliguria. During the oliguric phase of ARF, your urine output is at its lowest. Prolonged oliguria can worsen your outlook.

Your kidneys filter out waste and excess fluid from your blood. Filtered substances and fluids eventually become urine.

ARF is when your kidneys suddenly stop functioning properly. One of the features of ARF is low urine volume. The medical term for this is oliguria.

Some people with ARF will experience an oliguric phase, in which your urine output is at its lowest. This article takes a closer look at the oliguric phase of ARF, including causes, symptoms, and treatment.

Oliguria refers to a urine output that’s lower than normal. It’s a sign that your kidneys aren’t functioning properly.

The oliguric phase of ARF refers to the timeframe when your urine output is lowest.

The phases of ARF

The clinical course of ARF can be broadly divided into several different phases.

  • Initiation: This is the phase during which the injury that leads to ARF occurs. It typically happens over a period of hours or days.
  • Oliguric: The oliguric phase is when your urine output is low.
  • Diuretic: The diuretic phase is characterized by a marked increase in your urine output as your kidney function starts to return.
  • Recovery: The recovery phase is when your kidney function begins to stabilize. While this may happen over a period of weeks, it can take months to fully recover from ARF.
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There are three different causes of ARF. These causes can each contribute to oliguria in different ways.

Pre-renal ARF

Pre-renal ARF happens when there’s a decreased flow of blood to your kidneys. In this situation, your kidneys themselves are working properly, but a lack of blood flow is affecting their function.

Decreased blood flow to your kidneys activates various hormonal pathways in your body that promote water and salt reabsorption. This means that your urine output will become lower.

Renal ARF

Renal ARF happens due to a direct problem with the filtering ability of your kidneys. The filtering units of your kidneys are called nephrons. Each of your kidneys has about one million nephrons, which are made up of two parts:

  • glomerulus: the part of the nephron that actually filters your blood
  • tubule: the part of the nephron that removes waste products and returns necessary substances back to your blood

When either of these parts don’t work properly, it can affect kidney function and urine output. A common cause of renal ARF is called acute tubular necrosis.

Post-renal ARF

Post-renal ARF occurs because of a problem that happens after filtering. This is most often due to something blocking your urinary tract. A few examples include kidney stones, tumors, or blood clots.

In post-renal ARF, urine has a more difficult time moving through the urinary tract so that it can be voided from your body. This means that your urine output is lower.

Risk factors for acute renal failure

Some of the risk factors for ARF include:

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The main finding during the oliguric phase of ARF is low urine output. It’s also possible that the urine you’re producing will be more concentrated, meaning that it will be darker in color.

Oliguria may have other effects on the body as well. For example, oliguria can lead to increased fluid retention in your body. This can cause issues such as:

Additionally, if wastes aren’t being effectively filtered and voided from your body, uremia can happen. This is when waste substances accumulate in the blood. Uremia can cause symptoms such as:

In order to diagnose the oliguric phase of ARF, a doctor will first do a physical exam and take your medical history. They’ll ask you if you have any preexisting health conditions and what medications you’re taking.

A doctor can see if you have oliguria by doing a urine collection. They can measure the amount of urine produced over a period of time to estimate your urine output.

Oliguria is typically defined as producing less than 400 milliliters (mL) of urine in a day. There are also oliguria guidelines that take weight into consideration. One of these is urinating less than 0.5 mL per kilogram (kg) of body weight each hour.

Your doctor will also do other tests. These include:

According to Kidney Disease: Improving Global Outcomes (KDIGO) guidelines, at least one of the following must be true to diagnose ARF:

  • serum creatinine is increased by 0.3 milligrams (mg)/deciliters (dL) or higher within a 48-hour period
  • serum creatinine has increased by 1.5 or higher from baseline within the past 7 days
  • urine volume is less than 0.5 mL/kg per hour for at least 6 hours

The treatment of ARF, including its oliguric phase, depends on its cause. Once the cause is known, treatment is directed at what’s causing your ARF to occur.

Post-renal ARF

Post-renal ARF can be treated by addressing what’s causing the blockage in your urinary tract. Once this is accomplished, the flow of urine can resume as normal.

Pre-renal ARF

Pre-renal ARF also typically improves when its cause is addressed. This often involves supplemental fluids, since one of the causes of pre-renal ARF is low blood volume.

When fluids are given, the amount used is determined individually. Care must be taken not to overload on fluids, because this can have harmful side effects. If fluid overload happens, diuretics can be used to return fluid balance to normal.

Renal ARF

Oliguria caused by renal ARF can take longer to resolve and won’t quickly respond to fluids.

If you have severe ARF, you may need dialysis. This is a therapy that works to help your kidneys filter your blood, helping to maintain fluid and electrolyte balance while you recover.

It’s also important to stop taking medications that are toxic to the kidneys while being treated for ARF. After recovery, your doctor may adjust the dosage of your medication or switch you to another medication that’s not toxic to the kidneys.

Many people with ARF recover completely. Still, the individual outlook for people in the oliguric phase of ARF depends on factors such as:

  • the cause of ARF
  • your age
  • your overall health

Having ARF also increases your risk of other health conditions, such as chronic kidney disease.

Having oliguria is typically associated with having a poorer outlook. For example, a 2021 study looked at how the KDIGO criteria for ARF, which involve serum creatinine and urine output levels, contributed to people’s outlook.

It found that oliguria lasting longer than 12 hours was associated with a higher risk of death over a 90-day period, regardless of changes in serum creatinine levels.

Oliguria is when you produce lower-than-normal levels of urine. It happens in many people with ARF. The oliguric phase of ARF is when your levels of urine output are at their lowest.

The treatment of the oliguric phase depends on what’s causing your ARF. While many people with ARF recover completely, having prolonged oliguria can worsen the outlook for people with ARF.