Help and support Helpline Information and leaflets Calculating kidney function Download the PDF leaflet What is the GFR? The kidneys have lots of functions, but the most important is clearing waste products from the bloodstream. This is sometimes called “excretory kidney function”. Excretory kidney function is measured as the “glomerular filtration rate”, or “GFR” The GFR is a measure of the rate at which blood is filtered by the kidneys, and is measured in millilitres per minute (ml/min). What is the most accurate method for measuring GFR? If it is important to measure GFR accurately, a tiny amount of a radioactive ‘tracer’ can be injected. This is cleared from the blood by filtration in the kidneys. Blood samples are then taken over the next few hours measure how quickly the radioactivity disappears. The amount of radioactivity given is very small, equivalent to the extra radiation from the sun that someone is exposed to on a two hour aircraft flight. The tracer most commonly used is called “51-chromium EDTA”. This measurement is not suitable for everyday use. How is creatinine used to measure GFR For day to day use, excretory kidney function is measured by measuring the concentration of a substance called creatinine in the blood. This test is called the ‘serum creatinine’ or ‘plasma creatinine’. High creatinine levels indicate poor excretory kidney function. What is eGFR? The modern way to assess GFR is to use a simple formula, called the ‘4-variable MDRD formula’. This estimates the GFR, and is called the estimated Glomerula Filtration Rate (eGFR). This formula takes into account the factors that affect creatinine production, and expresses GFR relative to the size of the person. It is complicated, and requires a computer or powerful calculator to get the answer. What is normal eGFR? Simple question, complicated answer! – because it depends what is meant by ‘normal’. In healthy young adults, a normal GFR is around 100 ml/min/1.73 m2. This is very convenient, because it means that estimates of GFR are the same as percentages of normal – for instance, a person with a GFR of 30 ml/min/1.73 m2 has 30% of normal kidney function for a person of their size. The problem is that reduced GFR is common in older people – because ageing can affect the kidneys, just like it affects the skin, the bones, and other organs, even if they haven’t been damaged by one of the recognised causes of kidney damage. This means that some doctors think that reduced GFR in elderly people is ‘normal’ and can safely be ignored. However, reduced GFR may cause just the same problems in older people as it does in younger people. What is Chronic Kidney Disease (CKD)? “Chronic kidney disease” means any type of kidney disease that is long-lasting. Chronic kidney disease is often called “CKD” for short. More details on Chronic kidney disease The National Kidney Federation cannot accept responsibility for information provided. The above is for guidance only. Patients are advised to seek further information from their own doctor.