Calculating Kidney Function
| What is the GFR? |
What is the most accurate method for measuring GFR?
How is creatinine used to measure GFR?
What is eGFR?
What is normal eGFR?
What is Chronic Kidney Disease (CKD)?
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” (Click here for further information about GFR on our Glossary page). 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).
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.
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.
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. You can estimate your own GFR using this formula using the link below:
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.
“Chronic kidney disease” means any type of kidney disease that is long-lasting. Chronic kidney disease is often called “CKD” for short.