Blood tests are done for lots of different reasons, and can provide a lot of information. They are often used to help your doctor find out what is wrong if you are not very well, or to keep an eye on a problem you already have. For example, a blood test can help to see if a medicine is helping you to fight an illness.

There are LOADS of different blood tests, and some of the ones done most often include those to check that your kidneys or liver are working properly, or to check your hormone and blood glucose levels. Blood tests for inflammation or to see if you have antibodies to certain viruses and bacteria are done often, too, as is a full blood count. This is used to check for anaemia and other problems that affect your blood cells.

Blood tests can also be used to find out your blood group before you have a blood transfusion, or to get a sample of your DNA if your doctor needs one.

You will usually be able to eat and drink as normal before you have a blood test. But, for some tests, your doctor will tell you not to eat anything for a certain amount of time before it is done. You might also be asked not to drink certain things, like tea, coffee and fruit juice. This is because for these tests, eating or drinking certain foods can interfere with the results so they might not be clear.

Two examples of blood tests that you can’t eat before having are an iron test and a fasting blood glucose level test. Iron tests can tell doctors how much iron is in your blood, and it helps them to see if you have things like anaemia. Iron tests are usually done first thing in the morning before you eat anything. Fasting blood glucose level tests are usually used to see if you have diabetes.

If there is anything else you need to do before your blood test, your doctor will tell you.

A blood sample will probably be taken from one of the veins in your arm. Your arm is a good part of your body to use, because it can easily be uncovered. The place that is usually used is the inside of your elbow or wrist, because your veins are quite close to the surface of your skin in these places.

A tight band will be put around the top of your arm. This slows blood flow out of your arm and makes the vein fill up with blood so that it is easier to take the blood sample. The doctor, nurse or phlebotomist will clean your skin over the vein and then put a needle in. The needle is attached to a syringe or a special blood-collecting container, so you can watch it fill up as the blood goes in!

When enough blood has been taken, the needle will be taken out and you will be given a bit of cotton wool to put over the tiny break in your skin. Press down as hard as you can with the cotton wool while keeping your arm straight - it will stop you from bleeding and getting any bruises. After a few minutes, you can remove the cotton wool and you might be given a plaster to put onto the small wound to keep it clean and to stop you getting an infection.

If only a small amount of blood is needed (like for checking your blood sugar using a test strip of paper) then a few drops of blood can be squeezed out from a tiny prick in the tip of your finger or earlobe.

For some blood tests, you need to have several samples taken over a few hours or so. This might happen if the doctor needs to see how well you respond to something, for instance. You may have a special needle, called a 'butterfly' needle, put into your vein, this can be taped to your skin. The blood samples can be collected through this each time, so it means you don’t have to have a different needle put in every time a sample is taken!

Some blood tests are taken from an artery in your wrist, but this is usually only done in hospital.

You will just feel a tiny pricking feeling as the needle goes in and it should not hurt very much. If you do not like needles and injections, tell the person who is taking the sample, so that they can help you to relax. If you feel like you are going to faint, you should lie down.

You could get a bruise where the needle was put in, but this much less likely to happen if you press over the break in your skin with cotton wool for a few minutes with your arm left straight. If you often get bruises after having blood tests, make sure you tell your doctor.

Usually, only a small amount of blood is taken, so you shouldn’t feel ill from loss of blood .

You could get an infection where the needle was put in, so if this part of your skin gets red or swollen or sore, tell your doctor as soon as you can.

Some kids feel like they are going to faint during a blood test. Tell the person doing the test if you feel faint, so that you can lie down straight away to stop this happening.

The sample of blood is put into a bottle and labelled with your name. It then goes to a lab where it is looked at under a microscope or tested with chemicals, depending on what is being checked. The results will be given to your doctor and he or she will discuss them with you and explain what they show.

If you have any other questions, you can ask your doctor, or chat to other people about blood tests in the Medikidz forums and virtual world, Mediland.

Or you can have a look at the NHS Choices article on blood tests.