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Overview - Blood tests

Blood tests have a wide range of uses and therefore are one of the very commonly ordered medical tests.

A blood test is useful for:

  • checking your general health
  • checking if you have an infection
  • seeing how well certain organs are working - such as the liver and kidneys
  • screening for certain genetic conditions

A blood sample will be needed for your blood tests.

This will usually be taken by a :

  • doctor
  • nurse
  • phlebotomist (a specialist in taking blood samples)

Find out more about common types of blood tests

Preparing for a blood test

The healthcare professional who arranges for your blood sample to be taken will tell you if you need to do anything before your test.

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

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

Follow the instructions you're given

If you do not, it may affect the result of your blood test. It could also mean that your blood test is delayed or needs to be repeated.

During a blood test

A blood test usually involves you getting a blood sample taken from a blood vessel in your arm.

The usual place your doctor or nurse will take a sample from is the blood vessel inside of the elbow or wrist. This is because the veins here are close to the surface.

A tourniquet (tight band) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood. It causes the vein to swell. This makes it easier for your health professional to take a sample.

A needle attached to a syringe or special container is inserted into the vein. The syringe draws 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.

When they take the sample, they will remove the needle. They will apply some pressure to your skin for a few minutes using cotton-wool. They may put a plaster on the small wound to keep it clean.

If you feel nervous about needles and blood, tell the person who is taking the sample. They can make you feel more comfortable. You can lie down for the test.

Blood tests for children

Blood samples from children are often taken from the back of the hand. Their skin may be numbed with a special spray or cream before the sample is taken.

After the test

Only a small amount of blood is taken during the blood test. You should not feel any significant after-effects.

But 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. It's important they know so that they can look after you.

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.

Blood test results

After you give a blood sample, it will be put into a bottle and labelled with your name and details.

It will then be sent to a laboratory for testing. A specialist will examine the sample under a microscope. They may also test it with chemicals. It depends on what they are looking for.

They will send the results to the hospital or to your GP. Some test results will be ready the same day or a few days later. Others may not be available for a few weeks.  Your doctor or nurse will tell you when your results will be ready and how you'll get them.

Sometimes, receiving results can be stressful and upsetting. If you're worried about the outcome of a test, take a trusted friend or relative with you. For some tests, such as HIV or genetic testing, you will be offered specialist counselling to help you deal with your results.

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 22 December 2020
Next review due: 22 December 2023

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.