An effective way of assesing how well your kidneys are working is to calculate your glomerular filtration rate. Glomerular filtration rate is a measurement of how many mililitres (ml) of fluid your kidneys can filter from your blood in one minute (measured in ml/min). A healthy pair of kidneys should be able to filter more than 90 ml/min.
It is difficult to measure the glomerular filtration rate directly, so it is estimated using a formula. The result is called the estimated glomerular filtration rate. Calculating your estimated glomerular filtration rate involves taking a blood sample and measuring the levels of a waste product called creatinine, which is filtered by your kidney. The formula takes into account your age, gender and ethnic group.
The result is similar to the percentage of normal kidney function. For example, an estimated glomerular filtration rate of 50 ml/min equates to 50% kidney function. The normal rate in young fit people is 90-120 ml/min. It is much lower in older people (50-80 ml/min)