Glomerular Filtration Rate (GFR)

What is glomerular filtration rate?

The glomerular filtration rate (GFR), sometimes known as estimated glomerular filtration rate or eGFR, is a test that is used to determine how well the kidneys are functioning. It is not used to diagnose kidney conditions but can be useful in the diagnostic process.

The estimated GFR is based on the amount of creatinine in the blood. It also takes your age and gender into account (if you are of African-Caribbean heritage this will also be taken into account when the results of the test are analysed). As a waste product creatinine is created when the muscle cells are broken down. The kidneys usually remove creatinine from the bloodstream and into the urine, so that it can be excreted from the body. However, if the kidneys are not functioning normally there will be a high level of creatinine in the blood.

The glomerular filtration rate is so-called because the test determines how well the glomeruli, the tiny filters inside the kidney, are doing their job.

What does the GFR number mean?

The GFR number is determined based on age, gender, ethnicity and the level of creatinine in the blood. The average GFR decreases as you age, but a result of 60 or lower shows that the kidneys are not functioning as well as they should be. The results of the test are classified in stages, as follows:

  • Stage 1: GFR- 90 or above: this indicates that the kidneys are functioning normally; it does not completely rule out kidney disease but is an indication that the kidneys are working well.
  • Stage 2: GFR 60-89: the function of the kidneys is slightly impaired.
  • Stage 3: GFR 30-59: the function of the kidneys is moderately reduced.
  • Stage 4: GFR 15-29: the function of the kidneys is severely impaired.
  • Stage 5: GFR 15 or under: the function of the kidneys is severely reduced and is often classified as kidney failure (sometimes known as renal failure)

It is important to note that it is not possible to assess the GFR accurately when people have an abnormal level of creatinine due to underlying conditions or a large amount of muscle tissue. This may include conditions and circumstances such as:

  • Pregnancy
  • Muscle wasting conditions
  • Kidney failure (acute)
  • Oedema (this occurs as a result of fluid retention)

It is also not possible to carry out the test on children or people who have had a limb amputated.

How significant is my GFR number?

The GFR is not used to diagnose kidney conditions but it does enable doctors to see how well the kidneys are working. The test will also evaluate factors such as:

  • Blood sugar levels and whether you have diabetes
  • Blood pressure to determine if you have high blood pressure

Your doctor may also check for the presence of albumin in your urine.

Tests will determine if chronic kidney disease is present. If you do have it then it is extremely important to keep an eye on blood pressure and blood glucose levels. This will help to prevent further damage to the kidneys and decrease the risk of life-threatening conditions, such as heart disease and strokes.

The function of the kidneys

The kidneys play an important role in the body as they are responsible for filtering the blood and removing waste products and excess water. These are then excreted from the body via the urinary system in the form of urine. The waste products found in the bloodstream come from the breakdown of tissues, cells and the food you take into your body. Once the body has obtained what it requires from the food you eat, waste will be directed to the blood and carried to the kidneys. If the kidneys are not functioning well the waste will build-up and this can cause damage to the body.

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