For example, a blood test can be used to:

• assess your general state of health
• check if you have an infection
• see how well certain organs, such as the liver and kidneys, are working
• screen for certain genetic conditions

Preparing for a blood test

Depending on the type of blood test, you may be asked to:

• avoid eating or drinking anything, apart from water (fasting) for up to 12 hours
• stop taking a certain medication

It's important to follow the instructions you're given, as it may affect the result of the test and mean it needs to be delayed or carried out again.

What happens during a blood test?

A blood test usually involves taking a blood sample from a blood vessel in your arm. The arm is a convenient part of the body to use because it can be easily uncovered. The usual place for a sample to be taken from is the inside of the elbow or wrist, where the veins are relatively close to the surface. A tight band (tourniquet) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood and causing the vein to swell. This makes it easier for a sample to be taken. A needle attached to a syringe or special container is inserted into the vein. The syringe is used to draw out a sample of your blood. You may feel a slight pricking or scratching sensation as the needle goes in, but it shouldn't be painful. If you don't like needles and blood, tell the pharmacist who is taking the sample so they can make you more comfortable. When the sample has been taken, the needle will be removed. Pressure is applied to the skin for a few minutes using a cotton-wool pad. A plaster may be put on the small wound to keep it clean.

After the test

Only a small amount of blood is taken during the test so you shouldn't feel any significant after-effects. However, some people feel dizzy and faint during and after the test. If this has happened to you in the past, tell the person carrying out the test so they’re aware and can help you feel more comfortable. After the test, you may have a small bruise where the needle went in. Bruises can be painful, but are usually harmless and fade over the next few days.