What is it used for? of Blood tests

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Blood tests are used to check that you are healthy and to find out if you have certain medical conditions.

The most common tests are:

Full blood count. This test looks at all the different cells in the your blood (red cells, white cells, and platelets), to look for conditions like infections, anaemia, and leukaemia.
U and Es (urea and electrolytes). This test looks at your blood chemistry, to find out the level of certain substances like urea (a waste product of the body), potassium, sodium and chloride. These substances are needed to help the body work, but certain diseases will increase or decrease particular substances in the body (see topic on Glomerulonephritis for an example).

So if you are ill, blood tests can be used to confirm a diagnosis. For example, if you have outward physical symptoms of a disease such as anaemia (iron-deficiency), a blood test will be used to check the blood and find out if you have abnormal red blood cells. Blood tests can also confirm a diagnosis by looking for the presence or absence of substances in the blood, for example potassium and sodium. If your kidneys are not working properly to filter the blood, for example, the tests will find high levels of substances such as urea in your blood.

Blood tests can also be used if you appear healthy, to give an early warning to doctors that you may develop a condition. For example, if you have unprotected sex or inject illegal drugs, you are at risk of having an infection such as HIV. HIV has few symptoms so the best way of detecting it is to see if your blood contains the infection-fighting particles (antibodies) for the HIV virus.

Pregnant women and those who are planning to get pregnant are offered blood tests if they are not sure whether they have been vaccinated against certain diseases such as rubella, which can be harmful to an unborn baby. HIV testing is also routinely offered during pregnancy, so that if a mother has the virus, treatments can be used to reduce the chances of it being passed to her baby.

Blood tests can also be used when you are recovering from a condition and to see how well your treatment is working. People with haemophilia are missing a part of the blood called a clotting factor (Factor VIII), and are sometimes treated by receiving transfusions. A blood test may be used to check whether the treatment has been successful and whether there are now enough clotting chemicals in the personâ€™s blood. Some people with diabetes also have to have regular blood tests to check how much sugar is in their blood, and they normally carry out their own tests at home. You might also need blood tests to see whether any medicines you are taking are having side effects on the way your body works.

A blood test can also be used to get a sample of your DNA. By checking your genetic structure, doctors can look for inherited diseases such as Huntingdonâ€™s disease. Genetic tests can also be used to check if you are related to another person, such as paternity testing to see if a man is the father of a particular child.

Finally, if you need a blood transfusion (when donated blood is transferred into your body), you may need a blood test to check what blood group you are. See the topic on Blood Groups for further information.