Blood Test Information

How is a blood test normally done?

  • The vein used for blood sampling is usually on the inside of your elbow or the back of your wrist.

  • A tight band (tourniquet) is usually placed around your upper arm. This makes the vein fill with blood and makes it easier for the blood sample to be taken.

  • The skin over the vein may be cleaned with an antiseptic wipe.

  • A needle is then inserted into the vein through the clean skin causing a sharp pricking sensation. The needle is connected either to a syringe, or directly to blood sample bottles.

  • When the required amount of blood is taken, the needle is removed. The small wound is pressed on with cotton wool for a few minutes to stop the bleeding and prevent bruising. A sticking plaster may be put on. The blood is placed in bottles. There may be a slight ache following a blood test.

What other ways can a blood test be done?

  • Some blood tests require several samples taken over a period of time. For example, they may be done to check how you respond to something. If you require repeated samples fairly close to each other (over the following few hours or so), a doctor may insert a 'butterfly' needle into the vein, which can be taped to the skin. Samples of blood can then be taken without using a needle each time.

  • If only a small amount of blood is needed then a few drops of blood can be squeezed out from a small prick in the tip of the finger or earlobe. For example, only a small amount of blood is needed for checking the blood sugar (glucose) level, using a test strip of paper.

  • Some blood tests are taken from an artery in the wrist. For example, to measure the level of oxygen in the artery. This is usually only done in hospital in certain circumstances

  • You may be told not to eat for a time before certain tests. For example, a test of blood glucose is commonly done first thing in the morning before you have anything to eat.

  • Children who need a blood test may be given cream to put on at home before the test to numb the skin.

Are there any complications or health risks from taking a blood test?

  • Sometimes a bruise develops where the needle was inserted. This is much less likely to happen if you press over the site with cotton wool for several minutes with your arm left straight (not bent).

  • As with any wound, an infection may develop where the needle was inserted. See your doctor if the wound site becomes red and inflamed.

  • Rarely, some people feel faint during a blood test. Tell the person doing the test if you feel faint, as you should immediately lie down to prevent fainting.

Why are there different tubes used to collect my blood sample?

Blood can be tested for many different things. Your doctor who requests the blood test will write on the form which test or tests they want our laboratory to do. Different blood bottles are used for different tests which may be run on different machines.

Blood tests are taken for many different reasons - for example, to:

  • Help diagnose certain conditions, or to rule them out if symptoms suggest possible conditions.

  • Monitor the activity and severity of certain conditions. For example, a blood test may help to see if a condition is responding to treatment.

  • Check the body's functions such as liver function and kidney function when you are taking certain medicines which may affect the liver or the kidneys.

  • Check your blood group before receiving a blood transfusion