What is a blood test?
A blood test is when a 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 specifically trained to take blood samples) will carry out the test. You may have your blood test at hospital or at your local GP clinic.
How is the blood taken?
For most blood tests, your blood will be taken through a needle which is inserted through your skin into a vein. The arm is usually used as it’s easily accessible, and the common places to take blood are from inside your elbow or the top of your wrist, as your veins are closer to the surface there.
A tight band (tourniquet) is usually put around your upper arm. This causes the vein to swell and fill with blood making it easier for your blood sample to be taken. The area may be cleaned with an antiseptic wipe or swab before the test is taken.
The needle is usually attached to either a syringe or a special blood collecting container, so once the blood sample has been taken it can easily be sent to a laboratory for analysis.
A blood test usually only takes a few minutes.
Does a blood test hurt?
You may feel a slight pricking sensation as the needle goes in, but it should not be painful. You might also feel a slight discomfort when the needle is taken out.
If you don’t like needles and injections, let the person who is taking the sample know so they can make you more comfortable. If you feel anxious about your blood test, ask a family member or friend to go along with you.
Rarely, some people feel faint during a blood test. Tell the person doing the test if you do feel faint as you should immediately lie down.
What about afterwards?
Once the sample has been taken, you or your health professional will apply pressure to the area with a dressing to stop any bleeding and to prevent swelling and bruising.
You may still get a small bruise or swelling where the needle went into your skin. If this feels painful you should rest your arm as much as possible and try putting ice on the area. If you are worried or the pain is severe, speak to your doctor or nurse for more advice.
As only a small amount of your blood is taken in a blood test, you should not feel ill from loss of blood.
Different types of blood tests
The most common types of blood tests used to assess heart conditions are:
- Cardiac enzyme tests (including troponin tests) – which help show if your heart muscle has been damaged
- Full blood count (FBC) – which measures the different types of blood cells in your blood and can show, for example, if there is an infection in your bloodstream
- Thyroid function tests – which show if you have an underactive or overactive thyroid, which can be linked to heart palpitations or a very slow or fast heart rate
- BNP (B-type natriuretic peptides) tests – which show the level of hormones in your blood which can be a sign of heart failure
- U and Es test – this stands for 'urea and electrolytes' and shows if there is too much or too little sodium or potassium in your blood. These are important for the overall function of your heart and also help asses 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.
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