What are Blood Tests?

Blood tests are one of the most versatile and useful tools available in diagnostic medicine. If you have ever given blood, then your blood has been tested for various diseases, to ensure patients receiving transfusions do not catch any diseases or infections. A sample of blood is taken from the patient, usually from a vein located on the inside of the elbow, and subjected to various tests to try and determine what, if anything is wrong with the patient. If only a small amount of blood is needed (e.g. the test for diabetes) then instead of drawing blood from a vein, a small prick is made on a finger or earlobe, and a few drops are collected. If the test requires blood from an artery (e.g. to test oxygen levels) then an artery on the inside of the wrist is usually used, as it is close to the surface of the skin. This is slightly more painful than drawing blood from a vein, as the artery wall is thicker, and has more nerves.

What do Doctors look for in Blood Tests?

Lots. Blood travels throughout your body, and is the primary system for transporting the things you need to live around your body. (Its medical name is the Circulatory system - other transport systems include the Respiratory system and the Urinary system) As it travels to every part of the body, it often picks up any infections that are affecting the patient. Blood tests are also used to detect any diseases or abnormalities affecting the blood itself.

Infections - Blood does not normally contain any bacteria or fungi, so if a sample of blood is incubated and a bacteria or fungus is detected (Either by microscope inspection, or by chemical tests) then it is a signal that the patient has a bacterial / fungal infection. Bacterial infections are usually detected after two or three days, but fungal cultures can take a month to be confirmed. When testing for infections, samples of blood are usually taken from two different places, to reduce the risk of skin contamination

Cell Examination - By spinning a sample of blood in a centrifuge, the different types of blood cells can be separated. Once separated, a small sample of cells can be examined under a microscope for abnormalities and diseases(a blood smear).

Red Blood Cells - These cells transport oxygen around the body, and make up the vast majority of the blood. Blood tests can show if a person has a lower than usual proportion of red blood cells in their blood, which can be a sign of anaemia. A red blood cell smear can be used to detect parasites (e.g. malaria)
White Blood Cells - These cells are part of the body's defense system, and help to combat infection. The number of white blood cells signals various different problems. If there is an abnormally high amount of white blood cells, then the patient may have an infection, or be bleeding. A high white blood cell count can also be caused by leukaemia or other cancer. A patient with a very low amount of white blood cells may be suffering from autoimmune problems, or a viral infection.
Platelets - These cells are very small, and help to form blood clots if a blood vessel is cut. A low level of platelets can make a patient vulnerable to bleeding, and may be caused by autoimmune diseases, chemotherapy or leukaemia. A high level of platelets can lead to dangerous blood clots, and can be the result of conditions affecting bone marrow

Blood tests have many other uses, including determining whether a patient's organs are functioning properly, (e.g. the kidneys and liver), testing hormone levels and checking for specific antibodies

sources: webmd.com, patient.co.uk, netdoctor.co.uk, My Dad