Acute kidney injury needs prompt medical attention, because the kidneys suddenly lose part of their ability to remove waste and balance fluids and salts. Many people recover when the cause is found and treated early. This guide explains how AKI differs from chronic kidney disease, what causes it, its symptoms, how it is diagnosed and treated, and where a urologist is involved.
Medically reviewed by Prof. Dr. Samir Ahmed Al-Samarrai, consultant urologist. Last reviewed: · Last updated:
What is acute kidney injury?
Acute kidney injury is a sudden loss of the kidneys’ ability to filter waste from the blood. According to the National Kidney Foundation, the term replaces the older name “acute renal failure”, and for most people AKI develops within 48 hours, though it can sometimes take as long as 7 days. MedlinePlus describes it as a rapid loss of kidney function over less than 2 days.
Acute kidney injury vs chronic kidney disease
AKI is different from chronic kidney disease in how quickly it happens and how likely recovery is:
| Feature | Acute kidney injury | Chronic kidney disease |
|---|---|---|
| Onset | Sudden, over hours or days | Gradual, over months or years |
| Usual trigger | An acute event such as dehydration, severe infection or blockage | Long-term conditions affecting the kidneys |
| Recovery | Kidneys often recover once the cause is treated | Long-term condition needing ongoing follow-up |
| Where it is treated | Often in hospital | Regular specialist clinic follow-up |
The NHS notes that some people who have had AKI go on to develop chronic kidney disease, which is more likely if the AKI was severe.
What causes acute kidney injury?
Doctors usually group the causes into three types:
- Pre-renal (reduced blood flow to the kidneys): severe dehydration, diarrhoea and vomiting, blood loss, low blood pressure, sepsis or major surgery.
- Renal (within the kidney): damage to the kidney tubules, autoimmune kidney disease, infection including kidney infection, or the effects of some medicines and iodine-based contrast dye.
- Post-renal (urinary obstruction): when urine cannot leave the body. Mayo Clinic lists kidney stones, an enlarged prostate, blood clots in the urinary tract and some bladder and prostate cancers as examples. A blockage can cause urinary retention or hydronephrosis, and this is where a urologist is involved.
The National Kidney Foundation explains that AKI usually results from a combination of factors.
Symptoms of acute kidney injury
According to the NHS, many people have no symptoms, especially when AKI is mild. Possible symptoms include:
- Passing much less urine than usual, or none.
- Feeling very thirsty, nausea or vomiting.
- Swollen feet or legs.
- Tiredness, dizziness, confusion or sleepiness.
- Breathlessness, especially when lying down.
- Flank or chest pain and, in severe cases, seizures, as the National Kidney Foundation describes.
Who is at risk?
Anyone can get AKI, but some people are at higher risk. According to the NHS, it is more common in people aged 65 or older, those who have had AKI before and those who have recently had surgery. NICE guidance and Mayo Clinic add other risk factors, including:
- Chronic kidney disease, diabetes and high blood pressure.
- Heart failure and liver disease.
- Symptoms or a history of urinary tract obstruction.
- Serious illness or intensive care, and recent use of medicines that can affect the kidneys or of contrast dye.
How is acute kidney injury diagnosed?
Your doctor will decide which tests are needed. They commonly include:
- Serum creatinine blood test: a key part of kidney function tests. According to NICE, a rise of 26 micromol/litre or more within 48 hours, or a rise of 50% or more within 7 days, meets the criteria for AKI.
- Urine output: NICE states that urine output below 0.5 ml/kg/hour for more than 6 hours in adults is also a sign of AKI.
- Urine tests and blood salts such as potassium.
- Ultrasound: MedlinePlus describes kidney ultrasound as the preferred test for a urinary tract blockage. NICE recommends urinary tract ultrasound within 24 hours when no cause is found or obstruction is a risk, and within 6 hours if an infected, blocked kidney (pyonephrosis) is suspected.
- Less commonly, a kidney biopsy.
Treatment and recovery
Treatment targets the cause, and the National Kidney Foundation notes that most people need monitoring in hospital. Depending on the situation, treatment may include:
- Fluids for dehydration, or limiting fluids if they are building up.
- Stopping or changing medicines that may harm the kidneys, as decided by the doctor.
- Treating infection and raising low blood pressure.
- Relieving a blockage: a catheter to drain urine from the bladder, or a procedure to remove the obstruction. NICE recommends referring upper urinary tract obstruction to a urologist, with drainage of the blocked kidney within 12 hours of diagnosis.
- Dialysis in severe cases, usually for a few days or weeks until the kidneys recover, according to the NHS.
A kidney specialist (nephrologist) manages much of the medical care and dialysis, while a urologist assesses and treats causes of obstruction and refers to nephrology when needed.
According to the NHS, many people recover fully within a few weeks, while more serious cases can take several months. The National Kidney Foundation advises checking kidney health within 3 months of finishing treatment, because a previous AKI raises the risk of another episode and of chronic kidney disease. For people at higher risk, the NHS advises staying hydrated, especially in hot weather or with vomiting or diarrhoea, and speaking to a doctor before starting new medicines or supplements.
When is it an emergency?
Get urgent medical help or go to the emergency department if you notice:
- Passing very little or no urine, or being unable to empty your bladder with lower tummy pain and swelling.
- Breathlessness or chest pain.
- Confusion, severe drowsiness or seizures.
- Increasing leg swelling with reduced urine.
- Fever with flank pain, especially if you have known kidney stones.
Mayo Clinic advises seeing a healthcare professional right away or seeking emergency care if you have symptoms of acute kidney injury.
Frequently asked questions
Can the kidneys recover after acute kidney injury?
Often, yes. The NHS says many people recover fully within a few weeks with no long-term effects, while serious cases can take months and some people go on to develop chronic kidney disease. Follow-up blood tests after recovery are therefore important.
Does everyone with AKI need dialysis?
No. Dialysis is used only in severe cases, for example dangerously high potassium or fluid build-up that does not respond to treatment. It is usually temporary until the kidneys recover, and a kidney specialist decides whether it is needed and for how long.
How can an enlarged prostate cause acute kidney injury?
An enlarged prostate can narrow the urine channel and stop the bladder emptying. Pressure then builds back up to the kidneys and reduces how well they work. Draining the bladder with a catheter and treating the cause of the blockage is a key step, led by a urologist.
Can you have acute kidney injury without symptoms?
Yes. According to the NHS, many people have no symptoms, especially when AKI is mild, and it is often picked up through blood tests during another illness or a hospital stay. This is why doctors check creatinine in people at higher risk.
Is a slightly raised creatinine the same as AKI?
Not always. A doctor compares the current result with earlier results and looks at how quickly it has changed, urine output and your general condition. AKI is diagnosed using defined medical criteria rather than a single number.
How can I lower my risk if I am in a higher-risk group?
The NHS advises staying well hydrated, especially in hot weather or with vomiting or diarrhoea, speaking to a doctor before starting any new medicine or supplement, and knowing the warning signs. Getting difficulty passing urine checked early also helps catch blockages.
Sources
- Acute kidney injury (AKI) – NHS
- Acute Kidney Injury (AKI): Causes, Symptoms, and Treatment | National Kidney Foundation
- Acute kidney injury: prevention, detection and management (NICE guideline NG148)
- Acute kidney failure – MedlinePlus Medical Encyclopedia
- Acute kidney injury – Symptoms and causes – Mayo Clinic
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