Causes include conditions that decrease blood flow to the kidneys, that damage the kidneys themselves, or that block drainage of urine from the kidneys.
Symptoms may include swelling, nausea, fatigue, itching, difficulty breathing, and symptoms of the disorder that caused the acute kidney injury.
Serious complications include heart failure and high levels of potassium in the blood.
Diagnosis is with blood and urine tests and usually imaging studies.
Treatment involves correcting the cause of acute kidney injury and sometimes doing dialysis.
(See also Overview of Kidney Failure Overview of Kidney Failure This chapter includes a new section on COVID-19 and acute kidney injury (AKI). Kidney failure is the inability of the kidneys to adequately filter metabolic waste products from the blood. Kidney... read more .)
Acute kidney injury can result from any condition that decreases the blood supply to the kidneys, any disease or toxic substances (also called toxins) affecting the kidneys themselves, or any condition that obstructs urine flow anywhere along the urinary tract.
In many people, no cause of acute kidney injury can be identified.
If both kidneys function normally, damage to one kidney (for example, due to blockage by a kidney stone Stones in the Urinary Tract Stones (calculi) are hard masses that form in the urinary tract and may cause pain, bleeding, or an infection or block of the flow of urine. Tiny stones may cause no symptoms, but larger stones... read more ) does not usually cause major problems because the remaining good kidney can compensate and usually maintain near-normal laboratory measurements of kidney function. Thus, acute kidney injury may be hard for doctors to detect. For acute kidney injury to cause significant problems, usually both kidneys must be damaged or function abnormally.
Symptoms of Acute Kidney Injury
Symptoms depend on the
Severity of kidney function decline
Rate of kidney function decline
Cause of kidney function decline
Early symptoms include
Water retention, causing weight gain and swelling of the feet and ankles or puffiness of the face and hands
Decreased amount of urine
The amount of urine (which for most healthy adults is between 3 cups [about 750 milliliters] and 2 quarts [about 2 liters] per day) often decreases to less than 2 cups (about 500 milliliters) per day or stops completely. Very little urine production is called oliguria, and no urine production is called anuria. However, some people with acute kidney injury continue to produce normal amounts of urine.
Later, as acute kidney injury persists and waste products accumulate in the body, people may experience
Decreased ability to concentrate on mental tasks
Loss of appetite
Overall itchiness (pruritus)
People with acute kidney injury may develop more serious symptoms of chest pain Acute Coronary Syndromes (Heart Attack; Myocardial Infarction; Unstable Angina) Acute coronary syndromes result from a sudden blockage in a coronary artery. This blockage causes unstable angina or a heart attack (myocardial infarction), depending on the location and amount... read more , muscle twitching, or even seizures. If fluid accumulates in the lungs Heart failure , people may become short of breath.
The passing of cola-colored urine may indicate a number of kidney diseases that damage the glomeruli, the filtering units of the kidney. The brown color results from blood passing through the filtering units and may be the first sign of a condition known as a glomerulonephritis Glomerulonephritis Glomerulonephritis is a disorder of glomeruli (clusters of microscopic blood vessels in the kidneys with small pores through which blood is filtered). It is characterized by body tissue swelling... read more (inflammation of the filtering units). Examples include postinfectious glomerulonephritis, antiglomerular basement membrane disease, and lupus nephritis.
If acute kidney injury is caused by a blockage (an obstruction), the backup of urine within the kidneys causes the drainage system to stretch (a condition called hydronephrosis—see figure Hydronephrosis—A Distended Kidney Hydronephrosis: A Distended Kidney ). Urinary tract obstruction Urinary Tract Obstruction Urinary tract obstruction is a blockage that inhibits the flow of urine through its normal path (the urinary tract), including the kidneys, ureters, bladder, and urethra. Blockage can be complete... read more often causes a constant dull ache under the lower ribs but may cause crampy pain—ranging from mild to excruciating—usually along the sides (flanks) of the body. Some people with hydronephrosis have blood in their urine. If the obstruction is located below the bladder, the bladder will enlarge. If the bladder enlarges rapidly, people are likely to feel severe pain in the pelvis, just above the pubic bone. If the bladder enlarges slowly, pain may be minimal, but the lower part of the abdomen may swell because of the enlarged bladder.
If acute kidney injury develops during hospitalization, the condition often relates to some recent injury, surgery, drug, or illness such as infection. Symptoms due to the cause of the acute kidney injury may predominate. For example, high fever, life-threateningly low blood pressure (shock) Low Blood Pressure and Shock , and symptoms of heart failure Heart Failure (HF) Heart failure is a disorder in which the heart is unable to keep up with the demands of the body, leading to reduced blood flow, back-up (congestion) of blood in the veins and lungs, and/or... read more or liver failure Liver Failure Liver failure is severe deterioration in liver function. Liver failure is caused by a disorder or substance that damages the liver. Most people have jaundice (yellow skin and eyes), feel tired... read more may occur before symptoms of kidney failure and be more obvious and urgent.
Some of the conditions that cause acute kidney injury also affect other parts of the body. For example, granulomatosis with polyangiitis Granulomatosis with Polyangiitis Granulomatosis with polyangiitis often begins with inflammation of small- and medium-sized blood vessels and tissues in the nose, sinuses, throat, lungs, or kidneys. The cause is unknown. The... read more , which damages blood vessels in the kidneys, may also damage blood vessels in the lungs, causing a person to cough up blood. Rashes are typical of some causes of acute kidney injury, including microscopic polyangiitis Microscopic Polyangiitis Microscopic polyangiitis is inflammation of mainly small blood vessels throughout the body. Symptoms vary depending on which organs are affected. People have a fever, lose weight, and have achy... read more , systemic lupus erythematosus Systemic Lupus Erythematosus (SLE) Systemic lupus erythematosus is a chronic autoimmune inflammatory connective tissue disorder that can involve joints, kidneys, skin, mucous membranes, and blood vessel walls. Problems in the... read more (lupus), and some toxic drug exposures.
Diagnosis of Acute Kidney Injury
Blood and urine tests
Did You Know...
The doctor evaluates whether the person has acute kidney injury and, very importantly, its cause.
The physical examination findings may help doctors identify the cause of the acute kidney injury. For example, enlarged or tender kidneys may indicate obstruction of the urinary tract causing hydronephrosis.
Blood tests that measure levels of creatinine and urea nitrogen in the blood are needed to confirm the diagnosis. A progressive daily rise in creatinine indicates acute kidney injury.
The level of creatinine is also the best indicator of the degree or severity of kidney function decline. The higher the level, the more severe the decline in kidney function is likely to be.
Other blood tests detect metabolic imbalances that occur if the decline in kidney function is severe, such as an increase in blood acidity (acidosis Acidosis Acidosis is caused by an overproduction of acid that builds up in the blood or an excessive loss of bicarbonate from the blood (metabolic acidosis) or by a buildup of carbon dioxide in the blood... read more , which means low bicarbonate level), a high potassium level (hyperkalemia Hyperkalemia (High Level of Potassium in the Blood) In hyperkalemia, the level of potassium in blood is too high. A high potassium level has many causes, including kidney disorders, drugs that affect kidney function, and consumption of too much... read more ), a low sodium level (hyponatremia Hyponatremia (Low Level of Sodium in the Blood) In hyponatremia, the level of sodium in blood is too low. A low sodium level has many causes, including consumption of too many fluids, kidney failure, heart failure, cirrhosis, and use of diuretics... read more ), and a high phosphorus level (hyperphosphatemia Hyperphosphatemia (High Level of Phosphate in the Blood) In hyperphosphatemia, the level of phosphate in blood is too high. (See also Overview of Electrolytes and Overview of Phosphate's Role in the Body.) Phosphate is one of the body's electrolytes... read more ).
Urine tests, such as urinalysis and measurement of certain electrolytes (sodium, potassium, calcium, phosphate), may enable doctors to determine whether the cause of kidney injury is insufficient blood flow to the kidneys, damage to the kidneys, or urinary obstruction.
Imaging of the kidneys using ultrasonography Ultrasonography Ultrasonography uses high-frequency sound (ultrasound) waves to produce images of internal organs and other tissues. A device called a transducer converts electrical current into sound waves... read more or computed tomography Computed Tomography (CT) In computed tomography (CT), which used to be called computed axial tomography (CAT), an x-ray source and x-ray detector rotate around a person. In modern scanners, the x-ray detector usually... read more (CT) is helpful, sometimes by identifying hydronephrosis or an enlarged bladder. Imaging can also reveal the size of the kidneys.
X-rays of the arteries or veins that lead to and from the kidneys (angiography Angiography In angiography, x-rays are used to produce detailed images of blood vessels. It is sometimes called conventional angiography to distinguish it from computed tomography (CT) angiography and magnetic... read more ) may be done if obstruction of blood vessels is the suspected cause. However, angiography is done only when other tests do not provide enough information, because angiography uses an intravenous contrast agent that contains iodine, which carries a risk of additional kidney damage.
Magnetic resonance angiography (MRA) can provide information similar to that provided by angiography. However, MRA has traditionally used gadolinium, a substance that, in people who have severely reduced kidney function, rarely causes a disorder that triggers production of scar tissue in the body (nephrogenic fibrosing dermopathy). The risk of this condition decreased with the newer gadolinium formulations, so MRA could be a good option after discussion with the health care team. If other tests do not reveal the cause of kidney injury, a biopsy Kidney biopsy Site-specific biopsies and cell sampling are also used in the evaluation of people with suspected kidney and urinary tract disorders. (See also Overview of the Urinary Tract.) A kidney biopsy... read more may be necessary to determine the diagnosis and the prognosis.
Prognosis for Acute Kidney Injury
Acute kidney injury and its immediate complications, such as water retention, high acid and potassium levels in the blood, and increased urea nitrogen in the blood, can often be treated successfully. Typically, prognosis is favorable for people whose acute kidney injury is due to decreased blood flow because body fluids have been lost through bleeding, vomiting, or diarrhea—conditions that are reversible with treatment. Prognosis is worse for people in whom some other organs (such as the heart, lungs, or liver) are failing at the same time. How long the person requires dialysis therapy Dialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more (for example, from days to life-long) depends on the person's overall health and kidney health before acute kidney injury develops.
Treatment of Acute Kidney Injury
Treating any treatable causes
Restricting or adjusting doses of drugs eliminated through the kidneys
Some complications of acute kidney injury are serious and may even be life-threatening. People may need to be treated in a critical care unit (also called an intensive care unit, or ICU).
Any treatable cause of acute kidney injury is treated as soon as possible. For example, if obstruction is the cause, a catheter (a tube placed into the bladder to remove urine), cystoscopy (procedure to look inside the bladder with a camera to eliminate the blockage), or surgery may be needed to relieve the obstruction.
Often, the kidneys can heal themselves, especially if the kidney injury has existed for only a few days and there are no complicating problems such as infection. During this time, measures are taken to prevent the decreased kidney function from causing serious problems. Such measures may include the following:
Restricting use of certain drugs that may be harsh on the kidneys
Restricting fluids, sodium, phosphorus, and potassium in the diet
Maintaining good nutrition
Giving drugs if blood levels of potassium or phosphate are too high
Salt (sodium) and potassium intake is usually restricted. Fluid intake is restricted to replacing the amount lost from the body, unless fluid is needed because there is too little blood flowing to the kidneys. Weight is measured every day because a change in weight is a good indicator of whether there is too much or too little fluid in the body.
A healthy diet is provided to people whose condition allows them to eat. Moderate amounts of protein are acceptable, typically 0.8 to 1 gram per kilogram of body weight (0.4 to 0.5 gram per pound) per day. Restricting the intake of foods that are high in phosphorus, such as dairy products, liver, legumes, nuts, and most soft drinks, lowers the phosphate concentration in the blood.
Potassium-lowering drugs are sometimes given by mouth or rectally to treat a high level of potassium in the blood. Phosphorus-lowering drugs (also called phosphorus binders) may be given by mouth with meals to prevent or treat a high level of phosphorus in the blood.
Acute kidney injury may be prolonged, necessitating removal of waste products and excess water. Waste removal can be done through dialysis, usually hemodialysis Hemodialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more . If loss of kidney function is predicted to be prolonged or changes in diet and drug therapy are predicted to be ineffective, dialysis is started. Dialysis helps control the complications of acute kidney injury. Dialysis may be needed only temporarily, until the kidneys recover their function, usually in several days to several weeks, sometimes after hospital discharge. If the kidneys do not recover, dialysis (hemodialysis Hemodialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more or peritoneal dialysis Peritoneal dialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more ) is continued indefinitely or kidney transplantation Kidney Transplantation Kidney transplantation is the removal of a healthy kidney from a living or recently deceased person and then its transfer into a person with end-stage kidney failure. (See also Overview of Transplantation... read more is pursued.
Treament of acute kidney injury caused by obstruction
Fluids are not restricted in people who are recovering from acute kidney injury caused by obstruction. During the recovery period, the kidneys are unable to reabsorb sodium and water normally, and a large amount of urine may be produced for a period of time after the obstruction is relieved. During recovery, people may need replacement of fluids and also electrolytes, such as sodium, potassium, and magnesium.