A blood test uses a sample of blood to analyze various substances in the blood. It is important for physicians to measure these substances because abnormally high or low levels may affect normal body function.

Blood tests serve a number of purposes related to the diagnosis and monitoring of various conditions and diseases. They may be used to:

• Determine a person’s risk of developing a disease
• Screen for a disease before signs and symptoms appear
• Help determine a diagnosis
• Rule out other diseases during diagnosis
• Monitor the progression of a disease
• Monitor the effectiveness of a patient’s treatment
• Detect complications of a condition
• Detect the spread of the disease to other areas of the body  
• Detect recurrence of a disease

Although blood tests may help detect many diseases, additional tests are usually necessary to verify the diagnosis.

Samples of blood may be taken in one of three ways:

• Vein puncture (venipuncture). A needle is inserted directly into a patient’s vein to draw blood into one or more tubes for laboratory analysis. Blood is usually drawn from a vein in the inner arm, opposite the elbow.
  
  
• Skin puncture. Used when only a small amount of blood is needed to perform the desired blood test. The skin of the finger, earlobe or heel (in infants) is pricked with a needle to produce a drop of blood that is then collected for analysis.
  
  
• Artery puncture (arterial puncture). Used only when the blood test needs to be performed on a sample of oxygen–rich blood traveling from the heart to the body via the arteries. Blood is usually drawn from an artery in the side of a wrist.

Blood tests can be performed on different parts of the blood, including:

• Whole blood (blood that has not been separated into its many components)
  
  
• Blood plasma (the liquid part of the blood)
  
  
• Blood serum (plasma that has had the clotting agent removed)
  
  
• Blood cells (the individual red blood cells, white blood cells and platelets)

Certain blood tests are performed routinely to monitor people with chronic diseases, such as glycohemoglobin tests for people with diabetes. In addition, blood tests such as a cholesterol profile and glucose test are recommended for everyone periodically.