A blood test uses a sample of blood to detect and measure various factors in the blood. Blood tests serve a number of purposes in relation to allergies and asthma, including:

• To screen for allergies. Blood tests may be recommended when a person develops symptoms that indicate an allergy, such as hives (smooth, raised pink or white bumps that appear on or beneath the skin), dermatitis (an inflammation of the skin), rhinitis (inflammation of the mucous membrane which lines the nose) or asthma.
  
  
• To determine the specific allergen (a substance the body mistakenly perceives as a threat, triggering the immune system to produce antibodies) responsible for triggering allergy or asthma symptoms.
  
  
• To identify any underlying conditions that may be contributing to allergy or asthma symptoms.
  
  
• To rule out other potential causes of symptoms in order to confirm a diagnosis of allergies or asthma.
  
  
• To determine if a child has outgrown an allergy.

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

• Vein puncture (venipuncture). The most common method for drawing samples of blood for testing. A needle is inserted directly into a vein to draw the needed amount of blood into one or more tubes. 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 jabbed with a needle to produce a drop of blood that is then collected for testing.
  
  
• 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 (to which an anticoagulant has been added in the test tube to prevent clotting)
  
  
• Blood plasma, the whitish yellow liquid that remains in unclotted blood once the blood cells have settled out to the bottom of the test tube
  
  
• Blood serum, the term for plasma that has had the clotting agents removed  
  
  
• Blood cells, the individual red, white and platelet cells

The precise amount of blood to be drawn is determined by the type and number of tests to be done. It is usually around 7 milliliters – about one fifth of a fluid ounce.