GFR test

A GFR (glomerular filtration rate) test gives an accurate measurement of your overall kidney function.

The results of this test give your doctors more information about your condition, and help them to plan your treatment.

GFR tests are done in our nuclear medicine department.

Preparing for a GFR test

There is no special preparation for this test. You can eat, drink and take any medicines as normal. We will ask you to keep well hydrated by drinking more than usual between the injection and the blood samples. A staff member will give you more details when you come in for your test.

You should be prepared to be at the hospital for 4 to 5 hours.

Please tell our team before your appointment if you are due to have, or have had, a scan with dye (contrast) in the 7 days before your GFR test. This includes CT scans, or radiotherapy planning scans. If you are unsure, please ask us as it could interfere with the GFR test and make it less accurate.

If you are breastfeeding, contact the department before your appointment so we can give you more information before you come for your test.

During a GFR test

You will meet our team of technologists, radiographers or nurses and they will explain the procedure to you. You can ask any questions about the test.

For this test, you will be injected with a tracer. This is a radioactive fluid that travels through your blood, and is filtered by your kidneys.

After the injection we need to take 3 blood samples. These will be after 2, 3 and 4 hours. The exact times of these blood samples will be given to you on the day of your test. Between these times you can leave the department, but it is very important you return at the times you are given.

When you return for your first blood sample we will place a thin plastic tube (cannula) into your arm (the opposite one from where the injection was given), and use this to take the blood samples. Using a cannula means we need to use fewer needles. The cannula stays in for the rest of the tests, and is removed after the last blood sample is taken.

Pregnancy and breastfeeding

As the test involves an injection of a radioactive tracer, you must tell us if you are, or think that you might be, pregnant, or if you are breastfeeding. If possible, please tell us as soon as you receive your appointment.

For all nuclear medicine studies we will check your pregnancy status with you before using any radioactive tracer.

Pain during the test

The injection is similar to a blood test and there is no pain associated with the tracer

After the GFR test

There are no side effects from this test and you can carry on with your normal activities.

After the test you will still have some radioactivity left in your body but this doesn’t present any risk to you, or to anyone around you.

Risks of a GFR test

We feel that the potential benefits of the test outweigh the very small risks. We make sure the amount of radiation you receive is as small as possible.

You can read government information about radiation exposure from medical imaging.

It is perfectly safe for you to travel abroad after your test, but many airports and sea ports are now equipped with very sensitive radiation detectors. It is possible that the very small amount of radioactivity left in your body could set off a detector as you pass through security. If you plan to travel abroad in the week after your test, it could be helpful to take your appointment letter with you.

Follow-up appointments

If you already have an appointment booked in clinic to discuss the results of the test, please let our team know.

Your results will be reviewed by our team and a report will be sent to the doctor that referred you. You will get your test results from them.

Resource number: 5366/VER1
Last reviewed: July 2023
Next review: July 2026

Trusted Information Creator. Patient Information Forum

Contact us

If you have any questions about your test in the nuclear medicine department, please contact us.

Phone 020 7188 4112, Monday to Friday, 9am to 5pm

Email [email protected]

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

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