Blood tests detect a variety of disorders in the human body. During these tests, a medical professional takes a drop of the patient's blood from the finger, earlobe or heel (in the case of a baby), or a more substantial amount of blood from a vein (venipuncture) or artery (arterial puncture).

The blood may then be tested for many different factors. For example, the physician may need to know the number of red or white blood cells that are present, or the amount of certain antibodies (proteins that attach to foreign substances and help destroy them) circulating in the blood.

Obtaining a blood sample takes only a few minutes, and most patients find them to be virtually painless. For most blood tests, there is very little preparation beforehand, and people can generally go right back to their usual daily activities afterward.

Allergy blood tests measure antibodies to a particular allergen (a substance the body mistakenly perceives as a threat, triggering the immune system to produce antibodies) in the blood. This helps physicians determine whether or not a patient has an allergy. While skin tests are more commonly used to detect allergies, blood tests can be used on patients who are not good candidates for skin testing. This includes those with skin disorders, a history of anaphylaxis (a rare, severe allergic reaction), or fear of the needles used in skin testing. Patients taking certain medications also may find blood testing more useful than skin tests.

There are many different types of blood tests used in allergy diagnosis. The most popular is the radioallergosorbent test (RAST). Blood tests are easy to administer and rarely have side effects.