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Kidney function tests

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Around 10% of people have kidney disease in the UK and Ireland. In most cases, kidney disease is mild and causes no symptoms, so it is possible to have some degree of kidney disease without being aware of it. A kidney function test can detect kidney disease and indicate whether you need to undergo further tests or treatment.

What causes kidney function to decrease?

The kidneys are fist-sized organs found at the bottom of your back, just below the rib cage. Their primary role is to cleanse the blood by filtering out waste products and excess water, salts, hormones and other substances. These are excreted from the body in urine. By controlling the amount of water circulating in the blood, the kidneys also play an important part in regulating blood pressure.

Kidney function naturally declines with age: over the age of 40, you lose 1% of your kidney function each year. This means that by the age of 80, your kidney function falls to around 60%.  Much bigger reductions in kidney function can occur if you get an infection or if you have a health problem that affects the kidneys. The two most common causes of kidney disease are diabetes and high blood pressure. If you have either of these, you should have your kidney function checked regularly to allow problems to be dealt with quickly.

How is kidney function tested?

Reduced kidney function is fairly easy to measure. As one of the main functions of the kidneys is to remove waste products from the blood, an effective way to test kidney function is to do a blood test to check the levels of these waste products to see if they are building up.

One waste product that is measured routinely to test kidney function is creatinine, which is a by-product of muscle activity. Serum levels of creatinine are between 60 and 120 micromoles per litre in people with healthy kidneys. This increases to 350­–600 for someone with severe kidney disease who may be close to requiring dialysis. Other waste products in the blood can indicate problems with the kidneys including urea, which is produced from breakdown of proteins, and dissolved salts such as sodium, potassium, chloride and bicarbonate.

It is also possible to detect reductions in kidney function by looking for blood or protein in the urine, since neither is present under normal circumstances. You can buy home kidney function testing kits that assess the level of a protein – micro albumin – in your urine. If you get a positive test, you should see your doctor straight away.

If you then undergo routine kidney function testing and are found to have abnormally reduced kidney function, you may need to undergo further tests such as an MRI, CT scan or ultrasound to assess the cause. You may also need a test to see how well your kidneys are filtering your blood.

This measures your glomerular filtration rate, the rate at which blood can pass through your kidneys. It involves having a small amount of a harmless, detectable substance injected into your bloodstream, allowing it to circulate and then measuring how quickly it is removed from the blood. This is done by checking your urine at regular intervals to see how quickly the substance appears.


Do I need a kidney function test?

Kidney function tests are among the most routinely performed blood tests. You may be advised to have one for several reasons, including:

  • As part of a routine health check (such as during pregnancy or if you are feeling generally unwell)
  • If you are suffering suspected dehydration
  • To measure your baseline kidney function before starting on a new medication or treatment. This allows any adverse effects on your kidney function to be detected once you start taking the new drug
  • If you are experiencing symptoms that are known to be associated with reduced kidney function

What are the symptoms of reduced kidney function?

Unfortunately, kidney disease generally does not give rise to symptoms until it is in relatively advanced stages. Symptoms vary from person to person, but may include:

  • Tiredness/weakness
  • Increasing need to urinate, especially during the night
  • Itchy, pale skin that bruises easily
  • Muscle twitching/cramps/pins and needles in hands and feet
  • Nausea
  • Erectile dysfunction
  • Shortness of breath
  • Water retention (leading to swollen ankles, feet or hands)
  • Blood and/or protein in the urine
Acute pain in the kidney area may indicate kidney stones or infection and should be investigated immediately.

If I already have reduced kidney function, can I do anything to stop it getting worse?

Kidney function is something we take for granted but, when the kidneys start to fail, the prospect of dialysis or a kidney transplant can be frightening. If caught earlier, many problems can be treated using medication and by changing your lifestyle.

Adopting a healthy lifestyle including giving up smoking, taking regular exercise and maintaining a healthy diet that is low in salt, as well as controlling long-term conditions such as diabetes and high blood pressure, can all help. Your doctor will be able to give you more specific advice depending on the results of your kidney function tests.


Profile of the author
Dr Kathryn Senior is an acclaimed medical journalist who has written over 500 feature articles for leading international journals within The Lancet group


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