Blood tests can find out how well your child’s kidneys are working – their kidney function. This is done by estimating or measuring the glomerular filtration rate (GFR), which is the amount of fluid (liquid) 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 is like a percentage of kidney function. The GFR for healthy kidneys is 90 mL/min/1.73m2 or higher.
Measuring GFR
There are two main methods of measuring GFR in children: a single blood tests or a series of blood tests after injection of a radioactive tracer.
Blood test to estimate GFR
Most of the time doctors can use a single blood 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 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 calculate 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. They will tell you and your child what the results mean.
Some children need a more accurate way to have their GFR measured.
One way is to use a radioactive tracer.