What is a blood test?
A blood test is when a small sample of your blood is taken for testing in a laboratory.
Your doctor or nurse may take your blood sample, or sometimes a phlebotomist (someone who is trained to take blood samples) will carry out the test. You may have your blood test at the hospital or at your local GP clinic.
How is the blood taken?
Your blood will be taken through a needle which is inserted through your skin into a vein. The most common places to take blood are from a vein inside your elbow or a vein on the back of your hand as your veins are closer to the surface there.
A tight band (tourniquet) is placed around your arm just above the area where the blood is to be taken. This causes the vein to come nearer to the surface of your skin, making it easier for your blood sample to be taken. The area will be cleaned with an antiseptic wipe or swab before the test is taken.
The needle is usually attached to either a syringe or a plastic device called a vacutainer and the blood put into a blood collecting container called a vial, so once the blood sample has been taken it can easily be sent to a laboratory for analysis. The vials often contain small amounts of chemical stopping the blood from clotting in transit.
Taking a sample of blood usually only takes a few minutes.
Does a blood test hurt?
You may feel a slight sharp pricking sensation as the needle goes into your skin, but it should not be painful.
If you don’t like needles, let the person who is taking the sample know so they can reassure you. If you feel anxious about your blood test, ask a family member or friend to go along with you. It may be possible for you to apply a cream that numbs the area before you have the test, but this will take a little while to be effective, so it might be an idea to apply it before you go to have the test done.
Rarely, some people feel faint during a blood test. This is more likely to happen if you are dehydrated, so try to drink some water before having your blood test. Let the doctor or healthcare professional know if you are prone to fainting before the test as they may suggest you lie down while having your blood taken.
What happens after a blood test?
Once the sample has been taken, you will be asked to apply pressure to the area for a few minutes. You can remove any dressing as soon as you feel able.
You may still get a small bruise or swelling where the needle went into your skin, but this is nothing to be concerned about and it should go down within a couple of days.
Different types of blood tests
The most common types of blood tests used to assess heart conditions are:
- Cardiac enzyme tests (including troponin tests) – these help diagnose or exclude a heart attack.
- Full blood count (FBC) – this measures different types of blood levels and can show, for example, if there is an infection or if you have anaemia.
- Thyroid function tests – these show if you have an underactive or overactive thyroid gland, which produces a hormone called thyroxine. If the levels are abnormal they can be linked to some heart conditions and symptoms linked to having a slow or fast heart rate or palpitations.
- Lipid profile - measures your cholesterol level and breakdown of good and bad fats found in your blood.
- Liver tests - the liver is a vital organ with many functions. This test gives a breakdown of how well the liver is functioning, which if not working well can have a major impact on the rest of the body.
- Clotting screen - this measures how quickly your blood clots. This is important if you take blood thinning medication such as warfarin.
- BNP (B-type natriuretic peptides) tests – these show the level of a hormone in your blood which if elevated can be a sign of heart failure.
- U and Es test – this stands for urea (a protein found in the blood) and electrolytes which measures the levels of sodium, potassium and other important chemicals in your blood such as magnesium and calcium. These chemicals are important for the overall function of your heart and also help assess kidney function. Imbalances in the blood can be linked to medication that you may be taking.
Want to find out more?
Tests for heart conditions booklet
This booklet describes the special tests that are commonly used to help diagnose heart diseases.
Some of the tests are also used to assess the current condition of people who have already been diagnosed with heart disease.