Acute kidney failure is the sudden loss of your kidneys' ability to perform their main function — eliminate excess fluid and electrolytes as well as waste material from your blood. When your kidneys lose their filtering ability, dangerous levels of fluid, electrolytes and waste accumulate in your body.

Acute kidney failure is most common in people who are already hospitalized, particularly people who need intensive care. Acute kidney failure tends to occur after complicated surgery, after a severe injury or when blood flow to your kidneys is disrupted.

Acute kidney failure can be serious and generally requires intensive treatment. However, acute kidney failure may be reversible. If you're otherwise in good health, you can recover normal kidney function.

Signs and symptoms of acute kidney failure may include:

• Decreased urine output, although occasionally urine output remains normal
• Fluid retention, causing swelling in your legs, ankles or feet
• Drowsiness
• Shortness of breath
• Fatigue
• Confusion
• Seizures or coma in severe cases
• Chest pain related to pericarditis, an inflammation of the sac-like membrane that envelops your heart

Because acute kidney failure is usually a complication of some other serious disorder, initially the signs and symptoms may go unnoticed or be attributed to the underlying disease.

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Kidney cross section

Acute kidney failure has many possible causes, generally grouped according to the part of kidney function they affect. All your blood flows through your kidneys, which are the key organs in the complex system that removes excess fluid and waste material from the blood.

Your kidneys receive blood through your renal arteries, which branch off the main artery (the abdominal aorta) carrying oxygenated blood away from your heart. On entering the kidneys, blood is diffused through an intricate network of filtering structures. These structures consist of nephrons — approximately 1 million of them — each containing a tuft of capillary blood vessels and tiny lobules that lead to larger collecting tubes. The capillary tufts (glomeruli) filter fluid from your blood, extracting both waste products and substances your body needs continuously — sugar, amino acids, calcium and salts. The filtered fluid then enters the tubules, from which the bloodstream reabsorbs these vital materials. What remains is waste, which is excreted in your urine.

Underlying causes
Three types of conditions can cause acute kidney failure:

• Prerenal conditions, which disrupt blood flow on its way to the kidneys
• Renal conditions, which directly damage the structures of the kidneys
• Postrenal conditions, which interfere with the urinary excretion of waste from the filtering process

Prerenal causes

• Extremely low blood pressure. Severe bleeding, infection in the bloodstream (sepsis), dehydration or shock can all lead to a drastic drop in blood pressure that prevents an adequate amount of blood from reaching your kidneys. Dangerously low blood pressure tends to follow traumatic injury.
• Poor heart function. A heart attack or congestive heart failure can severely limit blood flow to your kidneys.
• Low blood volume. Severe dehydration diminishes the volume of blood in your body and the amount available to your kidneys.

Renal causes

• Reduced blood supply within your kidneys. Your kidneys don't just process blood for the rest of your body — they depend on an adequate blood supply of their own. Reduced blood flow to the kidneys, especially to the renal tubules, can result in acute renal failure.

  In atherosclerosis, for example, deposits of cholesterol on the inner walls of arteries (plaques) may break up, releasing solid fragments into the bloodstream. These fragments (emboli) can get into the kidneys' circulation and accumulate in small vessels, severely restricting the blood supply and causing acute kidney failure, also known as atheroembolic kidney disease.

• Hemolytic uremic syndrome. Hemolytic uremic syndrome — associated with certain strains of Escherichia coli (E. coli) bacteria — is a leading cause of acute kidney failure in children. The bacterium causes inflammation of the intestine. It also produces a toxin that causes damage and swelling in the lining of blood vessels, especially the small blood vessels (glomerular capillaries) in the kidneys. As red blood cells travel through the damaged blood vessels, they're often broken apart (hemolysis). This complex condition may result in acute kidney failure.

• Inflammation in the kidneys. Acute kidney failure may result from sudden inflammation of the spaces between the glomeruli and the tubules (acute interstitial nephritis) and inflammation of the glomeruli (acute glomerulonephritis). Acute interstitial nephritis is usually associated with an allergic reaction to a drug. Examples include certain antibiotics — especially streptomycin and gentamicin — and common pain medications, such as aspirin and ibuprofen (Advil, Motrin, others). Antibiotics pose a greater risk of acute kidney failure for people who already have liver or kidney disease or who use diuretics or other drugs that affect the kidneys.

  Acute glomerulonephritis may follow a bacterial or viral infection, such as strep throat or hepatitis. Immune system diseases, such as lupus or IgA nephropathy (Berger's disease), also may trigger acute glomerulonephritis.

• Toxic injury. Your kidneys are particularly vulnerable to toxic injury from alcohol, cocaine, heavy metals, solvents and fuels. Sometimes, these toxins can induce acute kidney failure. Some medications, including certain chemotherapy drugs and contrast dyes used in medical tests, have a similar effect in vulnerable people.

Postrenal causes
Conditions that block the passage of urine out of the body (urinary obstructions) can, without treatment, also lead to acute kidney failure.

• Ureter obstruction. Kidney stones in both of the tubes leading from your kidneys to your bladder (ureters) — or in a single ureter if only one kidney is functioning — can prevent the passage of urine, as can tumors pushing in on the ureters.
• Bladder obstruction. In men, prostate enlargement is a common cause of urinary retention due to obstruction at the bladder outlet. Other obstructive bladder causes, in both men and women, include a bladder stone, blood clot, tumor or a nerve disorder that prevents the bladder from contracting properly.

Conditions that increase your risk of acute kidney failure include:

• Advanced age
• Chronic infection
• Diabetes
• High blood pressure
• Heart failure
• Various blood disorders
• Immune disorders, such as lupus, IgA nephropathy and scleroderma
• Kidney diseases
• Liver diseases
• Prostate gland enlargement
• Bladder outlet obstruction

Acute kidney failure almost always occurs in connection with another medical condition or event. In fact, most people who experience acute kidney failure are already in the hospital for other reasons, such as severe injury, complicated surgery or overwhelming infection.

When signs and symptoms point to acute kidney failure, blood and urine tests pin down the diagnosis. Changes associated with acute kidney failure include:

• Daily urine output usually falls to less than 2 cups (500 milliliters).
• Blood urea and creatinine levels rise rapidly.
• Blood electrolyte concentrations — levels of minerals such as sodium, potassium and calcium that regulate fluid balance and muscle function, plus many other vital processes — become unstable, causing swelling (edema) and lung congestion.
• Blood potassium, in particular, rises rapidly, often to life-threatening levels.

Ultrasound is the imaging test most commonly used in diagnosing kidney failure, but your doctor may also order an abdominal computerized tomography (CT) or magnetic resonance imaging (MRI) scan.

In a few cases, your doctor may remove a small sample (biopsy) of kidney tissue and send it to a laboratory for microscopic examination to identify the cause of acute kidney failure.

The sooner the cause of your acute kidney failure is resolved, the more likely you'll be to recover your kidney function.

Occasionally, acute kidney failure causes permanent loss of kidney function, or end-stage renal disease. People with end-stage renal disease require either permanent dialysis — a mechanical filtration system for removing toxins and waste from your body — or a kidney transplant to survive.

Acute kidney failure may be fatal. Death rates are highest when the kidneys fail after surgery or trauma or in the context of severe, chronic medical problems.

Other factors that can adversely affect the outcome of acute kidney failure include multiorgan failure, multiple blood transfusions, a recent history of stroke or heart attack, or a postoperative stroke, advanced age, infection, gastrointestinal bleeding and pre-existing malnutrition.

The first goal is to treat the illness or injury that originally damaged your kidneys. Once that's under control, the focus will be on preventing the accumulation of excess fluids and wastes in your blood while your kidneys heal. This is best accomplished by limiting your fluid intake and following a high-carbohydrate, low-protein, low-potassium diet.

Your doctor may prescribe calcium, glucose or sodium polystyrene sulfonate (Kayexalate) to prevent the accumulation of high levels of potassium in your blood. Too much potassium in the blood can cause dangerous irregular heartbeats (arrhythmias).

Dialysis
Most of the time, you also need to undergo temporary hemodialysis — often referred to simply as "dialysis" — to help remove toxins and excess fluids from your body while your kidneys heal. Dialysis, which is a mechanical way of filtering waste from your blood, is an imperfect but lifesaving substitute for kidney function.

In acute kidney failure, dialysis is usually done at a hospital or dialysis center, not at home. The treatment relies on an artificial kidney (dialyzer) to take over kidney function. Blood is pumped out of your body to the artificial kidney through one of two routes — a catheter placed surgically in one of your main blood veins, or a surgically created junction between a vein and artery in your arm. Inside the artificial kidney, your blood moves across membranes that filter out waste before being returned to your body. Less than 1 cup (237 milliliters) of blood is outside your body in the dialyzer and tubing at any one time.

Acute kidney failure is often impossible to prevent. But you may reduce your risk by following these suggestions:

• Don't abuse alcohol or drugs, including over-the-counter pain medications such as aspirin, acetaminophen and ibuprofen. Avoid long-term exposure to heavy metals, such as lead, as well as to solvents, fuels and other toxic substances.
• Carefully follow all of your doctor's recommendations for managing any chronic medical condition that increases your risk of kidney failure.

If you're at high risk of kidney damage induced by contrast dye used for certain X-rays — for example, if you have diabetes or multiple myeloma — your doctor may prescribe a dose of acetylcysteine before the procedure. This medicine can help prevent acute kidney failure under these circumstances.