Your kidneys do the important work of cleaning your blood as it moves through your system. Hundreds of thousands of tiny units in your kidneys (called nephrons) filter waste and toxins out of the blood to produce urine, which then flows into the bladder.

Sometimes, though, there is a potentially serious problem. Acute kidney injury (AKI) is an episode of sudden kidney damage or failure. It causes waste products, like creatinine and urea, to build up in your blood and can cause significant damage to the rest of your body. There are multiple levels of kidney injury, varying from mild to severe. In severe cases, your brain, heart, and lungs can be affected, and you can die.

Multiple conditions can cause AKI. Researchers have shown that infection, conditions that cause lack of blood flow to the kidneys, or conditions and medications that damage the kidneys themselves can all cause a sudden kidney function decline. To be sure, AKI is most common in hospital settings, especially in critical care patients.

Acute kidney injury can be difficult to detect unless the doctor is trained to look for it, says Yale Medicine nephrologist F. Perry Wilson, MD. This is because there are rarely any symptoms until kidney function is severely damaged.

If left untreated, AKI has a very high mortality rate. If the underlying cause is diagnosed and treated, your prognosis will depend on how much damage has been done to the kidneys. 

There are rarely any symptoms until the condition has progressed to a late stage.

However, if there are symptoms, the most common one is a decrease in urine output. Doctors diagnose acute kidney injury by measuring the level of creatinine in the blood. (Creatinine is a chemical waste product removed by the body entirely by the kidneys. If the kidneys are not working properly, there will be an increase in levels in the blood.)

Decreased urine output can be observed by a doctor. There are three stages of acute kidney injury, ranging from mild to moderate to severe. In mild acute kidney injury, patients experience less than 0.5 ml/kg/hr of urine output for 6 to 12 hours or have an increase in creatinine by 0.3 mg/dl over 48 hours or by 50% over seven days. As acute kidney injury progresses to the second stage, the urine output may be lower, and the creatinine level doubles. In the late stages of acute kidney injury, urine output is lower still and the creatinine has tripled.  Most patients never experience worse than Stage I AKI.

Common risk factors include diseases that affects blood flow to the kidneys or causes kidney damage such as chronic kidney disease, impaired left ventricular systolic function, advanced age (over 75 years old), diabetes, and dehydration.

Other risk factors include time spent in a heart-lung machine, use of an intra-aortic balloon pump, the need for blood transfusions and hemodilution. 

There are three major reasons why your kidneys might be injured: lack of blood flow to the kidneys, blockage in urine flow that causes infections, or direct kidney damage by infections, medications, toxins, or autoimmune conditions.

Reduced blood flow to the kidneys can interfere with the kidney’s ability to filter blood. The most common cause is hypovolemia (a condition of low blood volume usually caused by dehydration or excessive bleeding). Other common causes of low blood flow to the kidneys include heart failure or medication side effects. However, these causes of AKI usually don’t result in permanent kidney damage, and the damage is potentially reversible.

In some cases, urine might be blocked up in the bladder or ureters, resulting in a patient's inability to urinate. In these cases, urine may back up into the kidneys, causing them to swell and leading to kidney damage.

There are some diseases and conditions that can damage your kidneys directly and lead to AKI.

These conditions include:

• Sepsis. This is a potentially life-threatening complication of an infection in the blood that triggers inflammatory responses throughout the body. The response can cause your blood pressure to drop, affecting blood flow, and leading to damage to multiple organ systems, including the kidneys.
• Multiple myeloma. This is a cancer that forms in a type of white blood cell called a plasma cell, which can help you fight infections by making antibodies that recognize and attack germs. Multiple myeloma can affect the filter, the tubules, or the tissue of the kidney itself. Acute kidney injury is a common complication of multiple myeloma. When first diagnosed, as many as 20 to 40% of patients with multiple myeloma will have some amount of kidney failure. 
• Vasculitis. This is a rare condition that causes inflammation and scarring to your blood vessels, making them stiff, weak, and narrow.
• Interstitial nephritis. This is an allergic reaction to certain types of drugs that causes swelling between the kidney tubules that filter blood.

Other kidney diseases including tubular necrosis, glomerulonephritis, vasculitis, or thrombotic microangiopathy.

Overuse of medications (such as ibuprofen, ketoprofen, and naproxen) in critical care situations to control other conditions can overtax the kidneys’ abilities to filter, which can also lead to AKI.

When a patient shows sudden high levels of creatinine in the blood, it is likely that the patient is experiencing acute kidney injury. Doctors will use clinical context to rule out other similar conditions, such as chronic kidney disease. Usually, if the patient has been showing abnormal creatinine levels for a while and has co-occurring conditions such as hypertension and diabetes, they are more likely have chronic kidney disease. 

Once AKI is diagnosed, doctors will try to find the underlying cause in order to treat it. Sometimes, the underlying cause is easily identifiable because it correlates with another major condition that may have led to blood loss or infection (such as pneumonia with septic shock, cardiac surgery, trauma with hemorrhagic shock, and diarrhea).

If your doctor suspects that you might have acute kidney injury, he or she might perform the following tests:

• Urinalysis. Your doctor will take a urine sample to detect and manage a wide range of disorders, such as urinary tract infections or kidney disease, to determine the underlying cause of AKI.
  
• Blood tests. Your doctor might take blood tests to find levels of creatinine or urea nitrogen, among other things. High levels of these in the blood usually point to failing kidneys. From there, doctors can determine whether those failing kidneys are an acute condition or a chronic one. 
• Imaging tests. Imaging tests such as ultrasounds can help your doctor see your kidneys and to look for anything abnormal that might be causing their dysfunction. X-rays of arteries or veins can help determine if an obstruction in the blood vessels is causing kidney function to fail.
• Kidney biopsy: In some cases, a biopsy is necessary to determine what is causing the kidney injury. Kidney biopsies are generally performed with only local numbing medication and use a thin biopsy needle to take 1-3 small pieces of kidney for analysis under the microscope.

Treating AKI depends on the underlying cause of the condition. If it is caused by medications, your doctor may ask you to stop taking that specific medication. If it’s caused by an infection, your doctor may look for ways to treat the infection.

In more serious cases, when there are life-threatening changes in fluids, electrolytes, or acid-base balance in your body, your doctor may put you on dialysis—a machine that does the kidneys’ job for you while your kidneys recover.

Yale Medicine is at the forefront of detecting acute kidney injury before it happens. Our researchers are looking for reliable biomarkers for the condition and testing real-time electronic alerts for kidney damage. The goal is to determine who’s at risk beforehand and prevent it from becoming a problem. “We’re taking an active role in acute kidney injury,” Dr. Wilson says. “We believe that by intervening early right when the kidney function starts to decrease that we can mitigate the injury, which could improve outcomes in the long run for patients.”