Allergy blood tests measure the level of allergen-specific immunoglobulin E (IgE) present in the body. IgE is a major factor in the chain of events that triggers an allergic reaction. It is an antibody (a protein that attaches to foreign substances and helps destroy them) that recognizes and binds to a particular antigen (a substance the body perceives to be a threat). The IgE is also attached to a mast cell (a type of white blood cell). When the IgE binds to an antigen, it causes the mast cell to explode, releasing histamine and other chemicals that trigger allergy symptoms.

Skin testing is the preferred method of allergy testing. While blood tests are a good alternative to skin testing, they can be more costly. In addition, testing results can vary from lab to lab, and results are usually not available for at least 24 hours. Skin test results, on the other hand, are usually available within 30 minutes. Therefore, allergy blood tests are typically used only when skin testing is not an option.

Candidates for blood testing include patients with:

• Severe eczema (an inflammatory skin disease with lesions that usually appear very dry, thickened or scaly), dermatitis (an inflammation of the skin), psoriasis (a skin disease characterized by raised, inflamed lesions that join together) or another skin disorder which may make the results of skin testing difficult to detect.
  
  
• History of anaphylaxis (a rare, severe allergic reaction) or anaphylactic shock (a life-threatening condition characterized by difficulty breathing and a dangerous drop in blood pressure).
  
  
• Medications (such as antihistamines or tricyclic antidepressants) that may prohibit reactions to skin testing (thereby giving a false signal that an allergy is not present) and cannot be temporarily discontinued for the test
  
  
• Fear of the needles used in skin prick or scratch testing (blood testing only involves a single needle prick) 

Blood tests can be used to test for nearly all the same allergens as skin tests. There are many different kinds of blood tests that can be used in the diagnosis and management of allergies and asthma. These include:

• Radioallergosorbent test (RAST). The blood test most often used to diagnose allergies. During an allergic reaction, the body produces immunoglobulin E (IgE) antibodies to protect against what it mistakenly perceives to be a dangerous substance. The RAST test measures the amount of specific IgE produced to an individual allergen in a sample of blood. The level of IgE correlates to the sensitivity a patient has to the allergen.
  
  
• Multiple antigen simultaneous testing (MAST). A spinoff of the RAST test that proponents say is more comprehensive and sensitive than its predecessor. MAST tests allow physicians to test for 38 allergens at a time (although it can not diagnose food allergies), while RAST tests look at only a single allergen per test.
  
  
• Immunoassay capture test. One of the newest blood tests. Proponents say the process used to make the patient’s serum and the test medium react leads to a result as sensitive and accurate as those obtained from allergy skin testing.
  
  
• Paper radioimmunosorbent test (PRIST). This test measures the total number of immunoglobulin E (IgE) antibodies in the blood.
  
  
• Complete blood count (CBC). This basic blood test counts the number of red and white blood cells per cubic millimeter of blood in a patient sample. It can be used to diagnose many diseases. Physicians also use it to check for allergies. A high count of a white blood cell called an eosinophil (which mediates allergic reactions) may indicate the presence of an allergy.
  
  An excess number of red blood cells (polycythemia) indicates the body is not getting sufficient oxygen from the lungs and is trying to compensate by creating these cells to maximize oxygen distribution. This may be an indicator of asthma.

• Complement testing. Complement is made up of proteins that bind with antibodies, and is involved in immune system functioning. Patients with hives (smooth, raised pink or white bumps that appear on or beneath the skin) or angioedema (a disorder that affects the deeper layers of the skin and triggers significant swelling) often show a deficiency in blood complement. Complement levels can also help a physician determine whether a case of angioedema is related to an allergy or to hereditary factors. This is important, because treatments differ for the two conditions.
  
  
• Alpha₁-antitrypsin test. People diagnosed with severe asthma should have this test, which measures the level of the alpha₁–antitrypsin protein in the blood. A deficiency in this protein can trigger early emphysema, a lung disorder which often masquerades as asthma.
  
  
• Sedimentation rate (sed rate). Indirectly measures the degree of inflammation in the body. It measures the rate of fall of red blood cells known as erythrocytes in a tall, thin tube of blood. Patients with hives or other allergy symptoms may have their sed rate tested to rule out an underlying illness.
  
  
• Serum metabolic analysis (SMA). Measures several different substances used in the process of metabolism (e.g., electrolytes, blood sugar). Blood levels indicate how well the liver, kidneys or other organs are functioning. It may be used to rule out other potential causes of allergy symptoms and is most useful when used in conjunction with other tests.
  
  
• Cytotoxic testing. A blood test that falls under the category of an “alternative” treatment. It supposedly identifies food or inhalant allergies. However, a number of clinic trials have found these tests completely ineffective at diagnosing allergies, according to the American Academy of Allergy, Asthma and Immunology (AAAAI). Such testing is therefore not recommended.