A blood test is when a sample of blood is taken for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test. For example, a blood test can be used to:
- assess your general state of health
- confirm the presence of a bacterial or viral infection
- see how well certain organs, such as the liver and kidneys, are functioning
Blood
Blood is pumped around the body by the heart. It supplies oxygen to the body’s organs, muscles and tissues, and removes carbon dioxide.
The blood that circulates around the body contains many different substances. About 40% of the blood’s volume is made up of blood cells. There are three types of blood cell:
- red blood cells carry oxygen from the lungs and transport it around the body
- white blood cells form part of the body’s immune system and help defend the body against infection
- platelets are cells that help the blood to clot (thicken) when you cut yourself
The remaining 60% of blood is made up of plasma. Plasma mainly consists of water, but also contains proteins and chemicals, such as hormones, glucose and salt.
Uses of blood tests
Blood tests are very useful for a number of reasons, which are described below.
Assessing organs
As blood circulates through your body’s organs, it is very sensitive to any changes or damage to the organs. For example, if your liver becomes damaged, it will release certain enzymes into the blood, which can be detected using a blood test.
Checking for infection
As blood plays an important part in the immune system’s defence against infection, changes in the make-up of blood can provide important clues about possible infections.
For example, certain viruses, such as HIV, will cause your immune system to produce special proteins called antibodies. By checking the blood for certain types of antibody, it is possible to confirm (or rule out) whether an infection has occurred.
Measuring oxygen and carbon dioxide
As blood is the body’s oxygen supply system, testing can provide important information about possible respiratory conditions (conditions that affect your lungs).
For example, the volume of oxygen and carbon dioxide in your blood can be measured. This is one method of determining how well your lungs are working.
Genetic testing
Blood tests are a convenient way to obtain a DNA sample for genetic testing and screening. For example, blood tests can be used to diagnose genetic conditions, such as cystic fibrosis (a genetic disorder that causes internal bodily secretions to become thick and sticky, interfering with the function of certain organs, such as the lungs).
Glossary
Blood test
During a blood test, a sample of blood is taken from a vein using a needle so that it can be examined in a laboratory.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Oxygen
Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.
Lungs
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.
Immune system
The immune system is the body's defence system, which helps protect it from disease, bacteria and viruses.
Plasma
Plasma is the liquid part of blood, which holds other blood cells together.
Glucose
Glucose (or dextrose) is a type of sugar that is used by the body to produce energy.
Last updated: 30 January 2012
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