About glomerular filtration rate (GFR)
The glomerular filtration rate (GFR) is the amount of fluid (liquid) that the kidneys filter each minute. Your doctor may measure your child’s GFR. This will give more information about his or her kidney function, or how well his or her kidneys are working.
The GFR is measured by finding out how quickly the kidneys remove a substance from the blood into urine. There are two main methods of measuring GFR in children: blood test and a radioactive tracer.
About the urinary system and kidneys and what the kidney does
Blood test to estimate GFR
Most of the time doctors can use a simple test to estimate the GFR. It is not an accurate measurement, but it is very close and is fine for many children.
In children, the substance that is usually measured is creatinine. This is a natural chemical that is made by the body as we use our muscles. Creatinine goes into the blood at a constant rate, and the kidneys remove most of it into the urine. If there is too much creatinine in the blood, this may be a sign that the kidneys are not working as well as they should.
What happens
- A small amount of your child’s blood is taken.
- Your child’s blood sample is sent to a laboratory to measure the amount of creatinine.
- A formula is used to compare the amount of creatinine in your child’s blood sample to their age, sex, height and ethnicity – these affect the normal amount of creatinine. This gives the estimated GFR (eGFR).
Finding out results
Your doctor will usually be able to get the result on the same day as the blood sample is collected.
Radioactive tracer to measure GFR
Some children need a more accurate way to have their GFR measured.
One way is to use a radioactive tracer. A small amount of the tracer is injected into the blood and the test finds out how quickly it is removed from the body.
Why is this test needed?
Your doctor will explain why your child needs a more accurate test to measure the kidney function. Some common reasons are:
- to help your doctor decide how often your child needs other tests
- to help your doctor give you more accurate information about the outlook for your child
- to allow your doctor to adjust the amount (dose) of medicines that your child may need – if their kidneys are not working normally, they may need a smaller dose
- if your child has other problems that make the estimating method less accurate, for example abnormal muscles
Are there any risks?
If your child’s doctor recommends this test, he or she will have considered that the benefits far outweigh any risks. A small amount of the radioactive substance is used and it will not be in your child’s body for long. The radiation exposure from this test is similar to the natural background radiation that your child receives in one week.
Are there any other options?
The alternative to this test is to continue use the estimating method – the simple blood test to measure creatinine.
Preparing for the test
You and your child do not need any special preparation for this test. He or she can eat and drink as usual, unless your doctor tells you otherwise.
You can be with your child during the whole test.
- A play specialist may be able to meet with your child. They will use dolls and other toys to help your child prepare.
- A local anaesthetic spray or cream can be put on the skin before to help stop them feeling any pain. This is put on about 30 – 60 minutes before the test to give time for it to work.
What happens?
- A small amount of the liquid radioactive tracer is injected into a vein in your child's arm.
- Your child will then have several small samples of his or her blood taken – this is often at 2, 3 and 4 hours after the injection.
After the test
There may be a small amount of radioactive substance in your child’s urine for 24 hours after the test.
Your child should sit on the toilet to urinate (wee) for the day after the test.
If your child uses nappies, put used nappies in a separate bag for 24 hours, then throw them away in your normal rubbish.
If your child accidentally wets their clothes or bedding, put these in a separate bag for 24 hours and then wash them as normal.
Finding out results
You will not get the results on the day of the test. They will be given to your doctor, usually within one to two weeks, and your doctor will explain to you what the results mean..
About GFR results
The GFR tells your doctor the kidney function. Your doctor will tell you and your child what the results mean. He or she may also tell you what your child’s GFR is as a percentage of normal – comparing to healthy kidneys.
Measurements
The GFR measures the volume in millilitres (mL) that the kidneys filter each minute (min). This is then adjusted for your child against a standard body size (body surface area of 1.73m2).
- The GFR for kidneys that are working at 100% (healthy kidneys) is 90 mL/min/1.73m2 or higher.
- The GFR for kidneys that are working at 50% (half as well as healthy kidneys) is 45 mL/min/1.73m2.