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Blood tests

Blood tests are tests performed on blood samples. There are many different types of blood tests:

  • complete blood count (CBC)
  • blood chemistry tests

Why a blood test is done

Some blood tests are used as a part of a routine checkup. Different blood tests may be done to look at different aspects of the blood.

  • A complete blood count (CBC) measures the levels of components in blood, such as red or white blood cells and platelets.
  • Blood chemistry tests measure the levels of certain chemicals in blood, such as electrolytes, minerals, hormones, metabolic products, proteins or enzymes.
  • Some tests assess how well the blood, bone marrow or certain organs are functioning.
  • Tumour marker tests assess the effect of cancer treatment by measuring certain proteins in blood.
  • Some tests provide a baseline by which to compare future blood test results done during and after treatment.
  • Blood test may also be used for genetic testing (examining a person’s DNA).

How a blood test is done

A blood test is usually done in a private laboratory or hospital laboratory. Preparation depends on the type of blood test being done.

  • Blood is usually taken from a vein in the arm.
  • A tourniquet or elastic band is wrapped around the upper arm to apply pressure to the area and make the vein swell.
  • The person may be asked to open and close the fist to make the veins stand out more.
  • The skin is cleaned and disinfected.
  • A needle is inserted into the vein and a small amount of blood is removed.
    • The person will feel a prick or stinging sensation.
  • The sample is collected in a tube and labelled with the person’s name and other identifying information.
  • The tourniquet is removed and the needle is withdrawn.
    • Mild discomfort may be felt when the needle is withdrawn.
  • Pressure is applied to the area where the needle was inserted until bleeding stops.
  • A band aid may be applied.
  • The sample is sent to a laboratory to be analyzed by special machines, examined under a microscope or both.

Potential side effects

Potential side effects of having a blood test may include:

  • bleeding
  • bruising
  • infection

What the results mean

There are normal ranges for each blood test.

  • Many factors can affect test results.
  • Normal ranges can vary from person to person and laboratory to laboratory.

Results can provide information about:

  • a person’s general health
  • medical conditions or diseases
  • how well a treatment is working
  • areas that need further investigation

Genetic testing can help identify changes or mutations in the DNA of a gene, which may be associated with a certain disease or disorder. Some genetic mutations are linked to an increased risk of developing some cancers.

What happens if a change or abnormality is found

The doctor will decide if more tests, procedures, follow-up care or additional treatment are needed.

Special considerations for children

Capillary sampling may be used to collect blood samples from infants and younger children.

  • The skin on a finger or heel is cleaned and disinfected.
    • This may feel cold or tingly.
  • The finger or heel is pricked with a sharp needle or a lancet.
    • When the needle is inserted to draw blood, some children feel moderate pain, while others feel only a prick or stinging sensation. Afterward, they may be some throbbing.
  • The blood is collected in tubes.
  • Once sampling is complete, the area is cleaned and pressure is applied until bleeding stops.

Many children worry about not having enough blood after some has been taken from their arm. Reassure them that bodies make new blood all the time and that their body will not run out of blood.

Parents or caregivers can help reassure and prepare children by giving them brief but accurate information about what will happen.

  • Before blood is withdrawn from a vein in the arm, a big rubber band that feels like a balloon will be placed around the arm. This will feel tight, like someone is squeezing the arm.
  • When the skin is cleaned it will feel cold.
  • When the needle is placed in the arm, they will feel a prick or pinch. It may sting or hurt a little, or it may not hurt at all.
  • Once the needle is in place, they will see the blood come out into a vial or tube.

Parents or caregivers can help distract children during a blood test to help make it easier for them.

  • toddlers (1–2 years)
    • Small children might like to watch bubbles or toys that move or make sounds, such as magic wands, light-up toys or pinwheels.
  • preschoolers (3–5 years)
    • Some children want to hold a favourite toy.
    • Watching bubbles or light-and-sound toys may help distract them.
  • school age children (6–12 years)
    • Some children like visual items, such as magic wands, light-up toys, video games, picture and “search and find” books.
    • Bring a favourite stuffed animal or toy for them to hold.
    • Blow bubbles, or practise deep breathing while blowing bubbles.
    • Encourage them to imagine a favourite place.
    • Tell them a joke or story or encourage them to tell you a joke or story.
  • teenagers (13–18 years)
    • Encourage them to try deep breathing.
    • Have them imagine a favourite place.
    • Tell them a joke or story.

The preparation you can provide for these tests depends on the age and experience of the child. See the following for more age-specific information on helping children cope with tests and treatment.


A substance in the blood and other body fluids that carries an electric charge. Electrolytes are responsible for the movement of nutrients and wastes into and out of cells to keep body fluids balanced and to allow muscles to function properly.

Examples of electrolytes include calcium, chloride, potassium and sodium.


A protein that speeds up certain chemical reactions in the body.

For example, enzymes in the intestines help to digest food.

deoxyribonucleic acid (DNA)

The molecules inside the cell that program genetic information. DNA determines the structure, function and behaviour of a cell.


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