Blood samples are small volumes of blood that can be collected and used to screen for or diagnose a wide range of disorders.
Blood tests can help the doctor to assess your state of health and look for risk factors associated with certain diseases. For example, a blood sample may be taken to check your blood cholesterol level, which is an indicator of your risk of coronary artery disease.
If you visit your doctor with specific symptoms, a blood test may help to diagnose the cause of your illness. A blood sample is routinely taken when you are admitted to hospital. If you have a long-term illness, you may have blood tests to monitor treatment or the progress of your condition.
How are blood samples obtained?
Blood can be taken from various blood vessels, including the veins, the capillaries (small blood vessels), and, under some circumstances, the arteries. The choice of site depends on how much blood is needed and the purpose of the test. Blood samples are usually taken by a doctor, a nurse, or a phlebotomist, a technician who specializes in collecting blood. In some circumstances, you may need to collect small amounts of blood yourself. For example, people who have diabetes mellitus learn to take blood samples at home in order to monitor their blood sugar levels.
Venous blood samples
The vast majority of blood samples are taken from a vein using a hollow needle attached to a vacuum bottle or syringe. The vein on the inside of the elbow is the most commonly used site, although in children blood is often taken from a vein on the back of the hand or the foot. Most tests require only a few millilitres of blood; even less for children and babies.
Capillary blood samples
If only a drop or two of blood is needed, a sterile instrument called a lancet is used to pierce the skin on a fingertip or, in a newborn or infant, on the heel. A drop of blood can then be squeezed out and collected.
Arterial blood samples
If the blood levels of oxygen and carbon dioxide need to be measured, usually to assess the severity of lung disease, a sample is taken from an artery in the wrist or groin (see Measuring blood gases). This procedure is usually performed by a doctor. A local anaesthetic may be used.
What are blood samples used for?
Blood samples may be used for a variety of tests. Simple tests, such as for blood glucose levels, may be done in the doctor's surgery. Complex tests performed in a laboratory include tests for microorganisms, chemical tests, genetic tests, and tests to look at blood cells and functions (see Tests on blood structure and function).
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