Acute kidney injury (AKI) is sudden damage to the kidneys that causes them to stop working properly.

It can range from minor loss of kidney function to complete kidney failure.

AKI is common and normally happens as a complication of another serious illness. It is not the result of a physical blow to the kidneys, as the name may suggest.

This type of kidney damage is usually seen in older people who are unwell enough to be admitted to hospital. For more information, read about who’s at risk of AKI.

If it’s not picked up in time, the kidneys become overwhelmed and shut down, leading to irreversible injury, which can be fatal. Abnormal levels of salts and chemicals build up in the body, stopping other organs working properly. 

It’s essential that AKI is detected early and treated promptly.

What are the symptoms?

In the early stages of AKI, there may not be any symptoms. The only possible warning sign may be that the person isn’t producing much urine, although this isn’t always the case.

However, someone with AKI can deteriorate quickly and suddenly experience any of the following:

• nausea and vomiting
• dehydration
• confusion
• high blood pressure
• abdominal pain
• slight backache
• a build-up of fluid in the body (oedema)

Even if it doesn’t progress to kidney failure, AKI needs to be taken seriously, as it has an effect on the whole body and can make some existing illnesses more serious.

AKI differs to chronic kidney disease, where the kidneys gradually lose function over a long period of time.

Who’s at risk of AKI?

You’re more likely to get AKI if:

• you are aged 65 or over
• you already have a kidney problem such as chronic kidney disease
• you have a long-term disease such as heart failure, liver disease or diabetes
• you are dehydrated or dependent on a carer for fluids
• you have a blockage in your urinary tract (or are at risk of this)
• you have a severe infection or sepsis (where the body’s immune system goes into overdrive, setting off a series of reactions including widespread inflammation and blood clotting)
• you have taken certain medicines within the past week, including non-steroidal anti-inflammatory drugs (NSAIDS, such as ibuprofen) or certain blood pressure drugs, such as ACE inhibitors or diuretics; these are usually beneficial and are only an issue when a person is dehydrated or suffering from a severe illness)
• aminoglycosides – a type of antibiotic; again, these are only an issue if the patient is dehydrated or ill

AKI is estimated to affect 13-18% of people admitted to hospital. It affects both adults and children.

What’s the cause?

Most cases of AKI are due to reduced blood flow to the kidneys, usually in a patient who is already unwell with another health condition.

This reduced blood flow could be caused by:

• low blood volume after bleeding, excessive vomiting or diarrhoea, or as seen with severe dehydration
• the heart pumping out less blood than normal as a result of heart failure, liver failure or sepsis, for example
• problems with the blood vessels – such as inflammation and blockage in the main blood vessels supplying the kidneys (a rare condition called vasculitis)
• certain medicines (see above), which can affect the blood supply to the kidney or cause a reaction in the kidney itself
• contrast medium – the liquid dye used for some types of scans and X-rays

AKI can also be caused by a problem with the kidney itself, such as glomerulonephritis (damage to the tiny filters inside the kidneys).

It may also be due to a blockage affecting the drainage of the kidneys, such as an enlarged prostate, a bladder tumour or kidney stones.  

When should it be suspected?

Doctors should suspect AKI if their patient:

• suddenly falls ill and is already at risk of AKI
• has been unwell for a while and has:
  – chronic kidney disease, or 
  – a urinary system disease, or 
  – new or worsening urinary symptoms or
  – symptoms or signs of a disease affecting the kidneys and other organs
• has symptoms suggesting they have developed complications of AKI

How is acute kidney injury diagnosed?

AKI can be diagnosed after measuring urine output and doing blood tests.

Blood levels of creatinine – a chemical waste product produced by the muscles – will also be measured. Healthy kidneys filter creatinine and other waste products from the blood, in the form of urine. It is an easy and quick marker of kidney function, with higher levels indicating poorer kidney function.

In adults, a diagnosis of AKI can be made if:

• blood creatinine level has risen from the baseline value for that person (by 26 micromoles per litre or more within 48 hours)
• blood creatinine level has risen over time (by 50% or more within the past 7 days)
• they are passing much less urine (less than 0.5ml per kg per hour for more than 6 hours)

In children and teenagers, doctors should use the plasma creatinine level to calculate “estimated glomerular filtration rate” (eGFR). A diagnosis of AKI is made if they have a 25% or greater fall in eGFR within the past 7 days. Read more about AKI in children.

Investigating the underlying cause

Urine will be tested for protein, blood cells, sugar and waste products, which may give clues as to the underlying cause.

Doctors will need to know if their patient has any other symptoms (such as signs of sepsis or signs of heart failure) or any other medical condition.

They will also want to know about any drugs their patient may have taken in the past week, as some medicines can cause AKI.

An ultrasound scan should reveal if the cause is a blockage in the urinary system, such as an enlarged prostate or bladder tumour.

How is it treated?

Treatment of AKI will depend on the underlying cause and extent of illness. In most cases, treating the underlying problem will cure the AKI.

GPs may be able to manage mild cases, if the patient isn’t in hospital. They may:

• advise stopping any medication that may be the cause
• treat any underlying infections
• advise on fluid intake to prevent dehydration (which could cause or worsen AKI)
• take blood tests to monitor levels of creatinine and salt, to check their patient is recovering 
• refer their patient to a urologist (urinary specialist) or nephrologist (kidney specialist) if the cause isn’t clear or if a more serious cause is suspected 

Admission to hospital is necessary if:

• the underlying cause needs urgently treating – such as a urinary blockage, or if the person is seriously unwell; most people need hospital care to treat the underlying cause of AKI, not the AKI itself 
• there’s a risk of urinary blockage – because of prostate disease, for example
• the patient’s condition has deteriorated and regular blood and urine tests are needed to monitor how well their kidneys are working
• the patient has a complication of AKI

The majority of people who recover from AKI end up with the same level of kidney function as they had before they became ill, or go on to have normal kidney function.  

However, some will go on to develop chronic kidney disease or long-term kidney failure as a result. In severe cases, dialysis – where a machine filters the blood to rid the body of harmful waste, extra salt and water – may be necessary.

How can acute kidney injury be prevented?

Anyone who is at risk of AKI should be monitored if they become unwell or start new medication.

They should have regular blood tests to measure their levels of creatinine. It’s also useful for the patient or carers to check how much urine they are passing.

Any warning signs of AKI, such as vomiting and producing little urine, should prompt immediate investigation for AKI and treatment. Fluids may need to be given via a drip if the patient is dehydrated or at risk of dehydration.

Any medicine that seems to be damaging the kidneys needs to be stopped. 

In 2013, the National Institute for Health and Care Excellence (NICE) produced detailed guidelines on preventing, detecting and managing AKI. Read the NICE guidelines here.

What are the possible complications?

The most serious complications of acute kidney injury are:

• high levels of potassium in the blood – in severe cases this can lead to muscle weakness, paralysis and heart rhythm problems
• fluid in the lungs (pulmonary oedema)
• acidic blood (metabolic acidosis), which can cause nausea, vomiting, drowsiness and breathlessness