Blood tests 



Blood is pumped around the body by the heart. It supplies oxygen to the body’s organs, muscles and tissues, and removes carbon dioxide.

Blood is made up of:

  • plasma, which is a mix of water and chemicals such as proteins, glucose and salt
  • red blood cells, which carry oxygen from the lungs and transport it around the body
  • white blood cells, which form part of the body’s immune system and help defend the body against infection
  • platelets, which are cells that help the blood to clot (thicken) when you cut yourself

Donating blood

Doctors rely on blood donations to carry out life-saving treatments every day. Find out how you can help

A blood test is taking a sample of blood for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test.

For example, a blood test can be used to:

  • assess your general state of health
  • confirm the presence of a bacterial or viral infection
  • see how well certain organs, such as the liver and kidneys, are functioning
  • screen for certain genetic conditions such as cystic fibrosis or spinal muscular atrophy

Read about some common types of blood test.

Most blood tests only take a few minutes to complete and are carried out at your GP surgery or local hospital.

Preparing for a blood test

The healthcare professional who arranges your blood test will tell you whether there are any specific instructions you need to follow before your test.

For example, depending on the type of blood test, you may be asked to:

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.

Blood samples from children are usually taken from the back of the hand. The child's hand will be anaesthetised (numbed) with a special cream before the sample is taken.

A tight band (tourniquet) is usually put around your upper arm. This squeezes the arm, temporarily slowing down the flow of blood out of the arm, and causing the vein to swell with blood. This makes it easier for a blood sample to be taken.

Before taking the sample, the doctor or nurse may need to clean the area with an antiseptic wipe.

A needle attached to a syringe or to a special blood-collecting container is pushed into the vein. The syringe is used to draw out a sample of your blood. You may feel a slight pricking sensation as the needle goes in, but it should not be painful. If you do not like needles and injections, tell the person who is taking the sample so they can make you more comfortable. If you feel faint, lie down.

When the sample has been taken, the needle will be removed. Pressure is applied to the tiny break in the skin for a few minutes using a cotton-wool pad to stop the bleeding and to prevent bruising. A plaster may then be put on the small wound to keep it clean and prevent infection.

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 do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so they can help you feel more comfortable.

After a blood test, you may have a small bruised area on your skin where the needle went in. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.

Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.


After the blood sample has been taken, it will be put into a bottle and labelled with your name. It will then be sent to a laboratory where it will be examined under a microscope or tested with chemicals, depending on what's being checked. The results are sent back to the hospital or to your GP, and you will be told when and how you will be given them.

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

Page last reviewed: 25/03/2014

Next review due: 25/03/2016


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The 7 comments posted are personal views. Any information they give has not been checked and may not be accurate.

BLVD said on 04 September 2014

I think I have a B12 deficiency. I am 66, vegan, take h2 blockers twice a day and it got worse 2 1/2 years ago when I was given nitrous oxide for a broken compacted radius break. I have had macrocytosis but not the most recent test, also high folate result and since taking cyanocobalamin I have a blood serum test of 1207. I am tired, depressed and sometimes experience little electric feelings, sore tongue, cracked corners of mouth, feeling shaky and psychiatric problems (panic attacks) my doctor says because my result is so high I can't have a B12 deficiency.

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AJON said on 18 July 2014

For many years I have managed to avoid blood tests, through feeling healthy, then recently, after my GP acted concerned after high blood pressure, a blood test was arranged with Practice Nurse, where I experienced same problem as previously, (by being a blood donor years ago.) The veins in my arm are very difficult to find! Now only a few day later, before my test results, I have been called back for further blood test, I have not been told the reason! But I have to go through same thing again! Why can't NHS take blood from hand, under the circumstances, as in children, where the veins are more prominent! Am I able to request this? Also, I hate needles!

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TimRed said on 06 April 2014

I have three different blood test forms from various ares of the medical profession. Can I use these at one blood test or do I need three different appointments?

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AquaBabe said on 29 March 2014

It's now 24 hours after my blood test and my arm is very painful when I bend my elbow. I have had lots of blood tests and donated blood but never had pain afterwards. It would help on your site if you stated how long the pain could go on for.

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dijohn said on 12 January 2014

I think it would help if on your site you could give

Information on what you need to do for a starving blood test. Patiants dont always take instrutions in when told in surgury
Diana Noble

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ethna ellen said on 13 February 2013

There is no mention of the fact that you may faint because you havent eaten. I have no fear of blood or needles, but faint each time! also, have painful arm, armpit, and feel weak 24hrs later.

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michaelcanon said on 27 April 2012

next to last sentence on this page (blood test) should read 'but are not usually harmful...the text at present says 'harmless !

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