What is a blood test?

Key points about blood tests

  • Blood tests are one of the most common types of medical test.
  • They have many uses, including assessing your general health, checking if you have an infection, seeing how well specific organs are functioning and screening for certain genetic conditions.
  • They involve having a sample of blood taken from a vein in your arm.  


Woman with arm resting on table having blood drawn by healthcare worker
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A blood test is when a sample of blood is taken out of a vein in your arm. This is done by a nurse at your doctor's practice, or in a laboratory by a phlebotomist. A phlebotomist is a medical laboratory technician trained to collect blood and samples from people for testing in a laboratory.

The 2 most common blood tests (especially in hospital) are a full blood count and blood chemistry test.

  • Full blood count – this test provides important information about the types and numbers of cells in your blood. It is used to check your general health and to help diagnose and monitor a variety of conditions and treatments. Read more about the full blood count.
  • Blood chemistry – this test gives information about your heart, muscle, kidney and other organs. It includes checking the number of electrolytes in your blood such as sodium, potassium, calcium and other minerals, blood sugar, proteins, glucose or enzymes. Examples of blood chemistry tests are the kidney function blood test and the liver function test

Your doctor may need to do these blood tests to help diagnose your condition and monitor your progress, and to check whether a treatment is working for you.

Video: Blood Test Procedure

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(Nuffield Health, UK, 2015)

First, a band is put around your upper arm to make the veins below expand. This makes it easier to draw blood from them. Your arm may feel tight for the short time the band is applied.

The injection site is then cleaned with an alcohol swab.

You may feel a brief sting or pricking as the needle is inserted. A tube will be attached to the needle for the blood to be collected in. Depending on how many blood tests you are having, you may need more than one vial of blood to be collected while the needle and tube are in place.

Blood being drawn for a blood test
Image credit: Canva

Some people feel nervous about having blood taken. However, the person taking the sample is trained in this procedure, and it is nearly always straightforward.

If you feel squeamish or nervous, it can be better not to watch while your blood is being taken. If you feel faint or lightheaded before, during or after the blood test, tell the nurse or phlebotomist. There should be the option for you to lie down.

If you have had this sort of reaction in the past, or if you have any other concerns, tell the person taking the blood sample before they start, if they don't ask you about this.

Video: Scared of needles? Here’s how to stay calm during a blood test

This video may take a few moments to load.

(Nuffield Health, UK, 2019)

After the blood sample is taken, a cotton swab is put on the site. You may be told to hold the swab and apply pressure there for at least 3 minutes. This will stop bleeding. It also reduces the chance of a bruise developing. A plaster is applied, which can be removed after about half an hour.

After your blood test, wait for several hours before using the arm the test was done on for any heavy lifting or anything else that uses a lot of effort.

Some people develop a bruise at the injection site. Any small bruise should fade after a few days.

Less often, there may be bleeding or a lump (called a haematoma) may develop at the injection site. If this happens, cover the area with a clean cotton swab or tissue and apply firm pressure to the area for 10–15 minutes. A cold pack (eg, ice cubes in a plastic bag or wrapped in a cloth) can also reduce swelling.

The injection site may be tender and a more extensive bruise may develop from a haematoma. The bruise can take up to 10 days to go away, but there are no long-term side effects from this.

If you have any ongoing concerns after the test, contact your doctor.

Should I have more blood tests every day?

More testing doesn't help if your blood results stay the same after a few days, as the results won't tell your doctor anything new. 

Your doctor will advise when a blood test is necessary again, such as when your treatment changes. 

When do you need a blood test every day?

You may need to have a blood test every day in the following situations:

  • you are in an intensive care unit
  • your doctors are not sure about your condition
  • you are having a new treatment
  • your doctor thinks you may have internal bleeding.

What are the risks of having too many blood tests?

Blood tests are generally very safe. However, like any other procedure a blood test can have risks and complications, including the following:

  • Anaemia – a condition in which you have a low red blood cell count. This can happen when you lose too much blood from a blood test. Read more about anaemia
  • Increased risk of infection – the risk of getting an infection by inserting a needle through your vein is low, but the risk can increase as you have more blood tests. 
  • Sleep disturbance – nurses or phlebotomists (trained medical laboratory technicians who do blood tests) wake you up for a blood test, which can cause poor sleep and affect healing. 

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Gwenda Lawrence, medical laboratory scientist, Auckland

Last reviewed:

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