How blood tests are carried out and what to expect from a routine blood test.
Medically reviewed by Dr Louise Wiseman MBBS, BSc (Hons), DRCOG, MRCGP and based on a text by Dr Carl J. Brandt
Preparing for a blood test and not sure what to expect? Blood tests can be a simple way of getting more information about what’s going on inside your body. They can reveal valuable information about the cause of a disease and its symptoms (diagnosis) and are also useful for monitoring the effects of treatment.
But what’s involved? We explain how blood tests are carried out, what to expect on the day and what your doctor will examine in your blood:
What is a blood test?
Blood tests are a very useful diagnostic tool. Blood is made up of several different kinds of cells and other compounds, including various salts and certain proteins.
The liquid portion of the blood is called plasma. When blood clots outside the body, the blood cells and some of the proteins become solid. The remaining liquid is called serum, which can be used in chemical tests and in tests to find out how the immune system fights diseases.
Blood is made up of several different kinds of cells and other compounds, including various salts and certain proteins.
Doctors can take blood samples and grow the infectious organisms that cause an illness to see exactly what they are through a microscope. This can mean the correct treatment can be used to treat an infection (such as a specific antibiotic).
How is a blood test carried out?
During a blood test the following usually occurs:
✔️ Blood samples for testing can be taken either from a vein (which carries blood to the heart) or from an artery (which takes blood away from the heart).
✔️ If only a few drops of blood are needed (for monitoring blood sugar in diabetes, for example) it is enough to make a small prick in the tip of the finger and then squeeze the blood out.
✔️ Most blood tests are taken from a vein, commonly from those around the elbow. First a cord (tourniquet) is tied around the upper arm to make the vein prominent. It may be a bit tight, but this makes it much easier to take the test.
✔️ In certain situations (such as in blood tests for children) local anaesthetic cream is applied to the area a little while before a blood test.
✔️ The site of the injection is then cleaned with spirit and then a needle is put into the vein. The needle will be attached either to a low pressure blood test bottle, or to a syringe where the plunger is pulled back to create low pressure. When the necessary amount of blood has been extracted, the needle is removed and a little ball of cotton wool is held over the wound. This should be pressed for one to two minutes before applying a sticking plaster.
✔️ If blood is taken from an artery, it is usually extracted from the wrist where there is an artery that is very close to the skin. This may be slightly uncomfortable, as the artery wall has more pain nerves in it than the vein wall.
✔️ After taking blood from an artery it may be necessary to hold a ball of cotton wool on the place where the injection was made for about five minutes to stop any bleeding.
Nervous about having a blood test?
Some people are very sensitive to needles and the sight of their own blood and may feel faint when a blood sample is taken. This is not uncommon and can be reduced by sitting or lying down while the sample is taken.
If you feel faint or think that you might feel faint, immediately tell the person who is taking blood. Medical professionals are trained in dealing with anxious patients and will look after you and ensure you feel well.
What do doctors examine in the blood?
Blood contains two main elements: the fluid that is called plasma and cells. There are three kinds of cells: red blood cells, white blood cells and platelets.
To get the information they need from the blood, doctors actually do several tests with the blood sample. These include measurements of the levels of the cells and a blood smear. A blood smear is a film of blood placed on a slide to allow doctors to look at the individual cells under a microscope. These tests are listed below:
• Red blood cells
One of the most important red blood cell tests is used to find out how much haemoglobin there is in the blood. Haemoglobin carries oxygen around your body. This is called the haemoglobin concentration or level.
Another important test, the mean corpuscular volume or MCV test, measures the size of the red blood cells.
If a person suffers from anaemia their haemoglobin level will always be less than normal. But the size of the red blood cells depends on the type of anaemia you have.
A haematocrit test measures the total volume that red blood cells take up in the blood. In practice, this is done by spinning a test tube of blood until the red blood cells – the heaviest part of the blood – go to the bottom of the tube. Then their volume is calculated.
Almost all types of anaemia will cause a low haematocrit (a low red blood cell volume), as will very severe bleeding. A high haematocrit can occur if a person is dehydrated from not drinking enough fluid or because they are losing fluid as happens with diarrhoea, burns and sometimes surgery.
If the red blood cells are pale, it can be a sign of iron deficiency anaemia. If they have a strange shape, it may be because of sickle cell anaemia or pernicious anaemia.
• White blood cells
The doctor counts the total number of white blood cells (WBC) and works out how many different types of white blood cells the patient has. This is called the differential WBC count.
A person may lose white blood cells because they have autoimmune problems – this is where the antibodies that should fight diseases attack the body instead. Other reasons for loss of white blood cells include viral infections. More rarely, this can be a side effect of certain kinds of medication.
Doctors keep an eye on white blood cells to work out how a disease is changing. By monitoring the blood count in this way they can alter the patient’s treatment as necessary.
Platelets are very small cells in the blood that clump together at sites of injury to blood vessels. They form the basis of the blood clot that would form if you cut yourself.
Low numbers of platelets can make a person vulnerable to bleeding, sometimes even without injury occurring. Causes of low platelet counts include autoimmune diseases where you produce an antibody to your own platelets, chemotherapy, leukaemia, viral infections and some medicines.
High numbers of platelets make a person more vulnerable to blood clots. High platelet counts are found in conditions involving the bone marrow such as leukaemia and cancer.
Blood coagulation examinations
What are blood coagulation examinations? More tests will be needed if a patient is found to be suffering from a blood coagulation disorder so that either their blood doesn’t clot properly, or if it clots too well. When a vein is damaged, usually a little blood clot will form on the inside. This clot is made of blood platelets and proteins from the blood plasma (called the coagulation factors).
More tests will be needed if a patient is found to be suffering from a blood coagulation disorder.
A person will bleed more than normal if they have a low number of blood platelets, if there is a lack of coagulation factors, or if they don’t work. If the bleeding disorder is caused by problems with the coagulation factors more tests will be needed. Sometimes a coagulation disorder is passed on in the family, but it could also be due to a liver problem, as the liver makes many of the blood clotting factors.
Coagulation tests will be performed regularly for people who are on blood thinning medicines such as warfarin. Doctors will change the dose of these medicines depending on the test results.