Glomerular filtration rate (GFR) test

The glomerular filtration rate (GFR) test is used at Great Ormond Street Hospital (GOSH) to show how well your kidneys are working. 

The kidneys filter your blood to keep the good stuff (nutrients and oxygen) and get rid of all the bad stuff (toxins and waste). There are approximately one million tiny filters, known as glomeruli, in each of your kidneys.

A glomerular filtration rate (GFR) test is used to calculate how much blood is passing through these filters every minute.

Why it's needed

The doctors will suggest this test if they think that:

  • there may be a problem with your kidneys 
  • you have been taking medicine that is known to affect the kidneys 
  • they are thinking about giving you medicine that can affect the kidneys 
Knowing how efficiently your kidneys are working will allow them to plan what medicines or treatment will suit you best.

What happens

When you arrive on the ward, a nurse will measure your height, weight and blood pressure. This is because your GFR is related to growth. You may also have a urine test.

A nurse will take a small sample of blood and then give you an injection that contains a harmless dye (called iohexol).

He/she will then take two more blood samples. They will take the first sample three hours after the injection and the second (and final) sample four hours after the injection.

Doctors can look at how much dye is present in these blood samples to calculate how effective your kidneys have been working over the specific time period.

Usually, the nurse will give you the injection in one arm and take blood samples from the other. They will apply some local anaesthetic cream onto your arm or hand half an hour before the test. This helps to numb the area so you won’t feel the injection so much.

You will usually need to arrive on the ward at 9am and you should be free to go home by mid-afternoon. However, you may need to stay for longer (up to six hours after the injection) if your kidney function is lower.


We will give you the results of your test either at your next appointment or, if we need to speak to you sooner, we will ring you.

Going home

As soon as the test has finished, you will be able to start eating and drinking normally and go home.

Things that may affect your results

There are certain foods that can affect the test. These include tea, coffee, fizzy drinks, chocolate, ice cream and meat. We will ask you not to eat or drink any of these during the test.

Between samples, you will be free to leave the ward but make sure you remember to return on time. Otherwise, your results won’t be very accurate.

If you have a high temperature (38°C or higher) on the day of the test, we will reschedule for another time.


The dye is used in lots of other tests and often in larger doses. It should pass out of your body in your urine by the end of the day of the test.

In rare cases, the dye can cause an allergic reaction. The nurse will ask you to stay on the ward for 15 minutes after you have the injection to make sure you are OK.

Please let the nurses know if you are taking any medicines when you arrive for your appointment. This is because the dye can interact with some medicines.

On very rare occasions, the dye can leak out of the vein during the injection (known as infiltration). If this happens, the nurse will stop the test immediately and monitor you closely. The test will be rescheduled for another time.

What happens next

By the end of the day, the dye should have passed out of you and you can return to your normal routine.

If you have any questions, please ring the ward and ask to speak to a doctor or nurse.

Compiled by:
Great Ormond Street Hospital
Last review date:
April 2012