Acute Kidney Injury

Learn More About Acute Renal Failure

Blood Tests Image
Blood Tests Can Determine If the Kidneys Have Been Damaged. Image: © Nicholas Everleigh/Getty Images

An acute kidney injury (AKI) occurs when the kidneys show a dramatic and rapid decrease in the amount of work they are able to do.  Previously called acute renal failure, or ARF, this type of injury can be temporary or a sign of a larger problem.

The kidneys are sensitive organs, and are impacted by blood pressure, medications, overall health, medications, an individual’s diet, and even by the amount of water they drink.

 

Diagnosis of Acute Kidney Injury

There is strict criteria for the diagnosis of acute kidney injuries.  For an accurate diagnosis of acute kidney injury the patient must experience one of the following three things in the course of 48 hours:

  • Serum creatinine (a blood test that looks at kidney function) must rise by .3 mg/dl
  • Serum creatinine must rise by 50% or more from normal levels
  • Urine output must fall to .5 milliliters per kilogram of body weight per hour for more than 6 hours.  (A man who weighs 100 kilograms would have less than 300 milliliters of urine output in six hours).

If one of these three conditions are met, the patient can be diagnosed with an acute kidney injury.

Types of Acute Kidney Injury

There are many different ways the kidney can be injured.  The three main types of problems that cause a problem in the kidneys are categorized in the following ways:

Prerenal: The problem is “pre” kidney, meaning the problem starts before blood reaches the kidney.

  This could be low blood pressure, such as individuals experience when their heart is not working efficiently, or when they are severely dehydrated.  In this case, the kidneys themselves are not the main problem, they are being affected by a problem that is decreasing blood pressure, which make it hard for the kidneys to filter the blood.

 

A patient who is in septic shock, which is a severe infection that dramatically decreases blood pressure, may experience an acute kidney injury when the blood pressure is so low that the kidneys cannot work efficiently.

Intrinsic:  The kidneys themselves have been harmed. They have been damaged by a toxin (which could be a prescribed medication), a lack of oxygen in the bloodstream, or are inflamed.  Infection in the kidneys could cause this type of damage, as could any number of kidney diseases.  A very large kidney stone can also cause intrinsic damage to the kidney.

Postrenal: These types of problems are caused by an issue “after” the kidney, meaning that something in the urinary tract is the issue.  This could be an issue with the ureters, which are the tubes that carry urine to the bladder, the bladder itself, or the urethra, which is the tube that carries urine from the bladder out of the body.  

Imagine a large kidney stone has moved into the bladder and lodged in the urethra.  It is large enough that urine cannot pass through the urethra, and urine begins to back up into the bladder.

  The patient doesn’t seek treatment for this new and unexplained abdominal pain, and the bladder becomes so full that urine starts backing up into the kidneys. The kidneys continue to try to make urine, but this just causes more pressure to build. This pressure, and the backward flow of urine, causes an injury to the kidney.

Urine Output and Acute Kidney Injury

An individual with an acute kidney injury may be making ample urine, or very little urine.   A patient can have a significant increase in creatinine level, which indicates an acute kidney injury, but still make a normal level of urine.  Over half of all patients with an acute kidney injury continue to make urine in what appears to be a normal fashion. 

There are three categories of urine output during acute kidney injury:

Oliguric: The patient makes 400 milliliters of urine or less per day.  30 milliliters is approximately one ounce of urine.

Non-oliguric: The patient continues to make an adequate amount of urine, generally more than 400 milliliters per day, but often significantly more depending on health status.  “Normal” output can vary widely among individuals, a child is going to make far less urine than an adult, but, generally speaking, an adult typically makes one to two liters per day.

Anuric: The patient makes less than 100 milliliters of urine per day.

Normal Urine Output

A rule of thumb for urine output: 

for adults: .5 to 1 milliliter per hour per kilogram of body weight 

for infants: 2 milliliters per hour per kilogram of body weight

Urine output in children varies by age and size, but is approximately 1 milliliter per hour per kilogram.

Treatment of Acute Kidney Injury

The treatment of acute kidney injury varies widely depending on the cause of the problem.  For example, a patient who is severely dehydrated after experiencing heat stroke may have an acute kidney injury that resolves when they are given IV fluids and drinking water.  

A kidney function test, a group of blood tests that examines kidney function, can often determine how severe the problem is, what the cause of the problem might be, and follow up tests can show if the kidneys are responding to treatment.

Another patient may have an acute kidney injury due to a reaction to a medication or supplement they took.  For this patient, stopping the medication and ridding the body of the drug as quickly as possible would be the treatment.

Many patients respond well to treatment for acute kidney injury and do not experience long term problems or chronic renal failure.  Identifying the problem, and obtaining treatment as quickly as possible, is the best way to prevent kidney damage that causes a lifetime of kidney issues or requires dialysis treatments.

Source:

Acute Kidney Injury. Medscape.  Accessed January, 2016. http://emedicine.medscape.com/article/243492-overview#a1

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