Acute kidney injury (AKI): causes, symptoms, tests and treatment

Your kidneys are two bean-shaped organs in the middle of your back, one on each side of the spine. Kidney Care UK explains that they clean the blood, help control blood pressure, make urine and keep the right amount of fluid in the body, and control minerals such as potassium and phosphate.

Acute kidney injury, usually shortened to AKI, is a sudden drop in kidney function. Kidney Care UK describes it as ranging from a small loss of function to complete failure. It normally happens as a complication of another condition, and it does not mean you have physically hurt your kidneys in the way you might hurt another part of the body by falling over.

NICE Clinical Knowledge Summaries (CKS) describe AKI as a sudden decline in the kidneys' ability to excrete waste over hours or days, which can leave the body unable to keep its fluid, electrolyte and acid-base balance steady. AKI is not the same as chronic kidney disease (CKD), which develops over a long period and does not get better, though it can be slowed or managed.

The NHS says lots of things can cause AKI, and that the problem may be in the kidneys themselves or somewhere else in the body. NICE CKS groups the causes into three broad types, which helps doctors work out what to look for.

The NHS says anyone can get AKI but it is more common if you are aged 65 or older, have had AKI before, have recently had surgery, or take illegal drugs. NICE CKS adds a longer list of people at increased risk:

• People aged 65 or over.

• People with a history of AKI or of a blockage in the urinary tract.

• People with chronic kidney disease, heart failure, liver disease or diabetes.

• People who rely on others for their fluid intake.

• People who are acutely unwell, for example with sepsis, dehydration, low blood pressure or reduced fluid intake.

• People taking certain medicines, which the NHS says can include ACE inhibitors, NSAIDs and diuretics.

Kidney Care UK notes that symptoms of AKI may appear suddenly over just a few days, and that people can feel generally ill, a little like having a hangover. The NHS advises asking for an urgent GP appointment or getting help from NHS 111 if you think you may have symptoms of AKI. If someone is severely unwell, for example very confused or struggling to breathe, call 999 or go to A&E.

The NHS stresses that many people with AKI do not notice any symptoms, particularly if it is mild. When symptoms do occur they can include:

• Passing a lot less urine than usual.

• Feeling very thirsty.

• Feeling or being sick.

• Swollen feet or legs.

• Feeling tired, dizzy, confused or sleepy.

• Feeling breathless, especially when lying down.

AKI is diagnosed from a blood test. The NHS explains that the blood is checked for creatinine, and that a higher level than usual means the kidneys are not working as well as they should. Kidney Care UK adds that a urine test may be used and that an ultrasound scan of the kidneys can check for other problems such as a blockage.

NICE CKS describes the diagnostic criteria used by clinicians: a rise in serum creatinine of 26 micromol/L or more within 48 hours, a rise of 50% or more known or presumed to have happened within the past 7 days, or a fall in urine output to less than 0.5 mL/kg/hour for more than 6 hours. Hospital laboratories may flag a result as an AKI warning stage when creatinine has risen compared with a person's usual baseline.

Because the cause matters as much as the diagnosis, the NHS says a doctor may also take a urine sample, ask how often you have been passing urine, and check your pulse and blood pressure. Depending on what they suspect, you might have an ECG or scans such as an X-ray, CT scan or ultrasound. CKS also recommends checking the blood potassium level, because high potassium is one of the complications of AKI.

NICE CKS advises that urgent hospital admission or same-day referral is arranged where there is stage 3 AKI, a severe or life-threatening cause or complication, or no identifiable cause. People with advanced chronic kidney disease or a kidney transplant are discussed with a kidney specialist.

The NHS says treatment depends on how bad the AKI is and what is causing it, and that some people need treatment in hospital. Kidney Care UK points out that AKI is usually treated by the GP and most people do not need to see a specialist kidney doctor, although severe cases may need hospital care. Possible measures include:

• Treating the underlying cause, such as an infection (with antibiotics) or low blood pressure.

• Fluids given into a vein through a drip, particularly when dehydration is the cause. Kidney Care UK notes that if dehydration is mild, drinking more may be enough.

• Stopping or changing medicines that could be harming the kidneys. Kidney Care UK links this to the "sick day rules", and says medicines are reviewed and restarted once kidney function recovers.

• A thin tube (catheter) placed in the bladder to drain urine, or surgery to remove a blockage in the kidneys or bladder.

• Dialysis in severe cases, where a machine does the job of cleaning the blood. The NHS says this is usually only for a few days or weeks until the kidneys recover.

The NHS describes AKI as a serious condition that can be life-threatening if not treated, but says many people recover fully within a few weeks with no long-term effects. More serious AKI can take several months to recover from. Kidney Care UK says most people with AKI make a full recovery and do not need long-term treatment.

Some people are left with permanent damage. The NHS says this can lead to chronic kidney disease, and that you are more likely to develop it if your AKI was severe or you already have other long-term health problems. For that reason you will have regular blood tests and check-ups afterwards. Kidney Care UK says your GP will monitor your kidney function and blood pressure for at least three years after an episode of AKI.

NICE CKS recommends that the episode is documented, that kidney function is monitored regularly, that any medicines which were paused are reviewed, and that people are given advice on preventing future episodes. It is sensible to check with a doctor or pharmacist before starting any new medicine or herbal supplement, as some can harm the kidneys.

If you take medicines such as ACE inhibitors, diuretics or anti-inflammatory painkillers and become unwell with vomiting, diarrhoea or a fever, ask your GP, pharmacist or kidney team whether you have "sick day rules" that tell you when to pause them. Do not stop prescribed medicines on your own without checking.

The NHS suggests that if you have had AKI before, or are at higher risk because of your age or medicines, you can lower your chances by:

• Staying hydrated, drinking enough that your urine is pale and regular, especially in hot weather or when you are being sick or have diarrhoea.

• Speaking to a doctor before taking any new medicines or supplements.

• Knowing the symptoms of AKI and when to seek medical help.

AKI is usually recognised and treated within NHS GP and hospital services, because it often arises during another illness or hospital stay, and because blood monitoring is routine. Private care in the UK can offer faster access to specialist kidney clinics for follow-up, but the emergency treatment of significant AKI happens in hospital whichever route you use. If a result shows your kidney function has changed, ask what the cause is thought to be, what to do about your medicines, and when the next blood test is due.