When your kidneys suddenly stop functioning properly, it’s called acute renal failure (ARF). ARF is grouped into one of three categories according to the cause of the kidney failure. Most people recover from ARF after receiving treatment.

Acute renal failure (ARF) happens when your kidneys suddenly stop functioning properly. You’ll also see ARF called acute kidney injury.

There are three different categories of ARF. These are classified by what’s causing the kidney failure to occur:

  • pre-renal ARF
  • renal ARF
  • post-renal ARF

This article explores the three categories of ARF, as well as symptoms to look out for and how ARF is staged and treated.

Risk factors for ARF

There are several factors that can put you at a higher risk of ARF. These include:

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Pre-renal ARF is the most common type, accounting for about 60% of ARF diagnoses. In pre-renal ARF, your kidneys themselves are typically healthy, but ARF happens when blood flow to the kidneys is reduced.

Your blood supplies the organs and tissues of your body with vital oxygen. In fact, your kidneys receive about 25% of your heart’s output. Because of this, your kidneys can’t function properly when they’re not getting enough blood.

Pre-renal ARF can have several different causes, including:

Renal ARF happens due to factors associated with your kidneys. It makes up about 35% of all ARF diagnoses.

Many causes of renal ARF involve damage to your nephrons, which are the parts of your kidney that filter wastes from your blood. Each nephron is made up of a glomerulus (a group of small blood vessels where filtering happens) and a tubule (a structure that returns needed substances back to your blood while removing wastes).

According to a study from 2015, about 60% of renal ARF cases happen due to tubular damage in the nephron. The term used for this is acute tubular necrosis (ATN).

Phases of acute tubular necrosis (ATN)

After pre-renal causes, ATN is the next most common cause of ARF. The clinical course of ATN has several distinct phases, including:

  • Initiation: The initiation phase can last for hours or days, according to a study from 2010, and is when the event that will lead to ATN occurs.
  • Maintenance: The maintenance phase is when a decrease in kidney function occurs. Low urine output, called oliguria, is most severe at this stage and may last up to 2 weeks.
  • Recovery: During the recovery phase, repair of your kidney tissue is taking place and markers of your kidney function are starting to normalize. Your urine output may be increased during this phase, and this phase typically lasts 10 to 14 days.
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Outside of the nephron, renal ARF can also happen due to problems with the blood vessels and connective tissue associated with the kidney.

There are many potential causes of renal ARF, including:

It’s important to note that pre-renal ARF can sometimes convert to renal ARF. This can happen if pre-renal ARF is prolonged, and the tissue of your kidneys becomes damaged.

Post-renal ARF happens downstream from the filtering function of the kidneys. This is the rarest type of ARF, accounting for only 5% of diagnoses.

Most causes of post-renal ARF are obstructive. This means that there’s a blockage affecting the drainage of urine through your urinary tract, which also includes the ureters and bladder.

An obstruction in your urinary tract can decrease your kidneys’ ability to filter your blood. It may also cause urine to back up into the kidneys, leading to damage.

Some of the possible causes of post-renal ARF are:

ARF is the presence of any of the following findings:

  • an increase in serum creatinine by 0.3 milligrams per deciliter (mg/dL) or more within a period of 48 hours
  • an increase in serum creatinine to 1.5 times or more than baseline within the previous 7 days
  • a urine volume that’s less than 0.5 milliliters per kilogram per hour (mL/kg/h) for at least 6 hours

Serum creatinine is a measure of your kidney function, while urine output is a measure of how much urine you’re producing. High levels of serum creatinine and low urine output can mean that your kidneys aren’t working as they should.

The KDIGO criteria also stage ARF based on its severity, with higher stages meaning that your ARF is more serious. The chart below shows the different stages and the measurements that define them.

Serum creatinineUrine output
Stage 1an increase by 0.3 mg/dL or more or an increase of 1.5–1.9 times the baselineless than 0.5 mL/kg/h for 6 to 12 hours
Stage 2an increase of 2.0–2.9 times the baselineless than 0.5 mL/kg/h for 12 hours or more
Stage 3an increase of 3.0 times the baseline or an increase to 4.0 mg/dL or more or initiation of dialysisless than 0.3 mL/kg/h for 24 hours or more or not urinating for 12 hours or more

ARF is often caused by other health conditions or medications. Because of this, treatment involves addressing what’s causing the kidney failure to occur.

If your ARF is very severe, it’s possible that you’ll need dialysis. This is a treatment that helps your kidneys to remove waste and excess fluid from your blood.

Many people with ARF recover after treatment, particularly if kidney tissue hasn’t been severely damaged. But the outlook for people with ARF can depend on many factors, such as:

  • what’s causing your ARF
  • your level of kidney function before ARF
  • your age and overall health

People who’ve had ARF are at an increased risk of experiencing it again. They’re also at a higher risk of developing chronic kidney disease later on.

There are three different categories of ARF: pre-renal ARF, renal ARF, and post-renal ARF. Each category is based on what’s causing the ARF to occur.

ARF is staged based on its severity, and treatment addresses the underlying cause of the kidney failure.

If treatment is given promptly, people can recover from ARF without lasting kidney damage.