What do the kidneys do?

The kidneys receive a rich supply of blood from the circulation and use this to make urine. Urine drains from the kidneys out through tubes (called ureters) into the bladder. When the bladder fills we feel the urge to pass urine.

The production of urine allows our bodies to stay in overall balance (a process scientists call 'homeostasis').

The volume and composition of the urine is tightly controlled. Urine removes waste products (mainly from the diet) that may be harmful to the body. These wastes includes urea, which we can measure in the blood, and acids. The kidneys also remove excess amounts of salts (such as sodium, potassium and phosphate). The kidneys have a key role in determining our water balance, by adjusting the amount of water we pass in the urine according to our levels of hydration.

In addition to its 'waste disposal' functions the kidneys have hormonal functions. They activate vitamin D (from diet or sunshine), that in turn regulates the levels of calcium in the body. The kidneys also produce erythropoietin, that determines the amount of red cells our bone marrow produces.

What is acute kidney injury (or AKI)?

AKI is a sudden loss of kidney function. It does not mean that the kidneys have been damaged by trauma. AKI, depending on the cause, is usually temporary. AKI used to be called 'acute renal failure' and can also be called 'acute kidney failure'. The term 'uraemia' has also been used to describe AKI patients who have metabolic complications of this condition (such as an accumulation of urea).

What causes AKI?

AKI may be caused by poor blood flow to the kidneys. This can be a consequence of severe dehydration, severe infections, major surgery, trauma or due to cardiac or liver failure. AKI may also be caused by toxic damage to the kidneys themselves (such as in drug toxicity). AKI may also be caused by specific, acute kidney diseases such as glomerulonephritis (an inflammatory, immunological disease) or haemolytic uraemic syndrome (sometimes caused by food poisoning). Finally AKI may be caused when the drainage system of the kidneys (the ureters or bladder) is blocked. Patients who have chronic kidney disease are particularly at risk of AKI. This is because the kidneys of these patients are more vulnerable to damage and because they have less background 'reserve' of kidney function.

What happens during an AKI episode?

Most AKI is self-limiting and mild. In more severe forms the loss of kidney function may be such that urea, potassium and phosphate levels become elevated and the blood may become more acidic. Salt and water accumulation may lead to accumulation of oedema in the legs or in the lungs (which can lead to breathlessness).

How do medical staff diagnose AKI?

Doctors measure the blood level of a substance called creatinine. This is produced by body muscles at a fairly constant rate and is removed by the kidneys. If there is a reduction in kidney function the creatinine levels will rise. Doctors use this phenomenon to diagnose AKI. Doctors and nurses also monitor kidney function by measuring urine volumes in patients who are particularly at risk. This often requires catheterisation of the bladder. Blood tests will also detect complications of AKI such as increased acidity of the blood.

How is AKI investigated and treated?

The medical team will endeavour to find the cause of AKI. This will involve taking background details of recent symptoms, medical history and medication history. A full examination will be performed. Basic investigations include blood tests and dipstick tests of the urine (to look for signs of inflammation). An ultrasound scan of the kidneys is often performed to exclude obstruction of the kidney's drainage system. Occasionally a kidney biopsy is undertaken to clarify the diagnosis. This is a local anaesthetic procedure performed with ultrasound and allows pathologists to examine kidney tissue, and any abnormalities, in detail.

Treatment is then directed at the underlying cause and this will vary. Hydration is assessed and intravenous fluids are given as appropriate. Drug prescriptions are reviewed. Some drugs may be stopped while others require a dose adjustment to account for the loss of kidney function, as many drugs are eliminated by the kidneys. Patients with AKI are have their physical signs (blood pressure, pulse and temperature), urine volumes and blood tests regularly monitored.

What is kidney dialysis and when is it required?

Kidney dialysis is a technique which removes substances that accumulate in kidney failure from the blood. This may include potassium, phosphate, sodium, water and protein breakdown products such as urea. Dialysis also provides a means to infuse substances (such as bicarbonate) that renal failure patients are lacking (rather like an intravenous drip).

Kidney dialysis in AKI usually takes two to four hours. It may be performed initially on a daily basis but this may be reduced to less frequent treatments (often three times a week). Dialysis in AKI usually requires the insertion of a dialysis catheter, under local anaesthetic, into a large vein in the neck or groin. Blood is then removed and passed through tubes to the 'artificial kidney' and then returned to the patient. Patients who need artificial kidney treatment in critical care units normally have a different form of treatment called haemofiltration. This uses different technology and often runs continuously over twenty-four hours. This allows the critical team to purify the blood in unstable, critically ill patients more slowly and with tighter control.

These treatments are needed for the minority of AKI patients who develop more serious complications. It is usually undertaken on a temporary basis prior to restoration of kidney function through treatment. Some patients do, however, need treatment for several weeks, or even months.

What are the long-term effects?

If you have had AKI you need to be followed up by your medical team to ensure your kidney function has returned to normal. Some patients develop a degree of scarring of the kidneys after AKI and need long term follow up. A very small minority of AKI patients have suffered irrevocable damage to the kidneys and need ongoing dialysis or, longer term, kidney transplantation.