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WHAT IS GFR?

The kidneys have a lot of functions, but the most important is clearing waste products from the bloodstream. The rate at which these products are cleared is referred to as “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).

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 is a product of day-to-day muscle breakdown, and the amount released into the blood varies depending on race, age and body build. The test is called the ‘serum creatinine’ or ‘plasma creatinine’. High creatinine levels indicate poor excretory kidney function. As you may appreciate, creatinine supplements taken for muscle building can falsely increase your GFR. Hence it is important you disclose to your kidney doctor if you take such supplements.

WHAT IS E-GFR?

This estimates the GFR, and is called the estimated Glomerular Filtration Rate (eGFR) and there are a number of formulae to calculate this. CKD-EPI equation is now more widely used in the UK. The formula takes into account factors that affect creatinine production, like age, sex and race. It is complicated, and requires a computer or powerful calculator to get the answer. The eGFR reported on a blood test is an estimation of the combined function of both kidneys.

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 to 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 “Technetium 99m DTPA”. Not practical for everyday use. Useful for patients at extremes of muscle bulk, of certain race or take creatine supplements.

WHAT IS NORMAL E-GFR?

Simple question, complicated answer! – because it depends what is meant by ‘normal’. In healthy young adults, a normal eGFR is around 80-120 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 mhas 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.

Last reviewed March 2022
Next review March 2025

Reviewed by Dr Oshini Shivakumar 'Specialist Renal Registrar'

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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.