Creatinine clearance is one of the most important blood tests used to measure kidney function.

The kidneys purify the blood by excreting all sorts of chemicals and waste products - creatinine and urea are two of these. By measuring the levels of creatinine and urea in the blood it's possible to get a rough idea of how well the kidneys are working to get rid of waste products.

If creatinine and urea levels are high, the kidneys may not be doing their job properly. However, levels of these chemicals may go up for other reasons, such as dehydration.

What it's used for

Creatinine clearance is used to measure the glomerular filtration rate - the rate at which the tiny glomeruli (the kidney's filtration units) are filtering fluid.

A huge volume of fluid passes through the glomeruli and 99 per cent of this is absorbed back rather than collecting as urine. Creatinine is one substance that is mostly excreted (only tiny amounts are reabsorbed). So what your kidneys chuck out directly reflects how much fluid the kidneys have filtered.

What the test involves

The test involves comparing the amount of creatinine in the blood with the amount excreted in urine over a given period (usually 24 hours).

You have to collect all your urine for that period and send it to the laboratory. It's important to save every drop. Forgetting to include one urine sample can make the test result inaccurate.

You'll also give a blood sample to test creatinine levels in the blood.

What the results mean

The normal level for creatinine clearance is greater than 100ml/minute. If your kidneys are excreting less than this they may not be working properly. A few things can complicate the picture, such as if you're very obese or oedematous (swollen).

When the kidneys are failing, creatinine clearance tests may be done regularly to monitor the kidneys and help decide if or when to start dialysis.

Other tests

Other tests, such as radioisotopes, may be a more reliable way to measure the glomerular filtration rate.

This article was last medically reviewed by Dr Trisha Macnair in December 2007