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.

Doctors can take blood samples and grow the infectious organisms that cause an illness to see exactly what they are through a microscope.

How is a blood test carried out?

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. 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.

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.

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 (RBC)

One of the most important red blood cell tests is used to find out how much hemoglobin there is in the blood. Hemoglobin carries oxygen around your body. This is called the hemoglobin 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 anemia their hemoglobin level will always be less than normal. But the size of the red blood cells depends on the type of anemia you have.

A haematocrit test measures the total volume that red blood cells take up in the blood. In practice, spinning a test tube of blood until the red blood cells -the heaviest part of the blood - go to the bottom of the tube does this. Then their volume is calculated.

Almost all types of anemia 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 diarrhea, burns and sometimes surgery.

If the red blood cells are pale, it can be a sign of iron deficiency anemia. If they have a strange shape, it may be because of sickle cell anemia or pernicious anemia.

Doctors also add stains to the blood smear to test the blood for parasites, for example in the case of sleeping sickness or malaria. They may also test for bacteria in the case of blood poisoning.

White blood cells (WBC)

The doctor counts the total number of white blood cells and works out how many different types of white blood cells the patient has. This is called the differential WBC count.

The number of white blood cells may go up and this may be because of a bacterial infection, bleeding or a burn. More rarely the cause of a raised white count is due to leukemia, cancer or malaria.

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

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, leukemia, 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 leukemia and cancer.

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 there 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).

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.