Types of Allergy Tests

Advantages of skin tests:

• Fast and cheap
• Sensitive at detecting low-level allergies

Disadvantages of skin tests:

• Some medications affect how skin reacts to allergens and need to be discontinued prior to testing, e.g. antibiotics (amoxicillin or penicillin), or painkillers (aspirin or ibuprofen)
• Interpretation of skin reactions requires a good deal of experience and training, therefore the skill of the tester can affect the results
• Mild discomfort from the reactions generated, e.g. itch

Blood Tests

Blood tests may be conducted in addition to, or instead of, skin tests. A skin prick test may not suitable when:

• There is high risk of triggering a known severe allergy
• The patient is taking a medicine that can interfere with skin testing
• The patient has signs of a severe skin condition, such as eczema or psoriasis

A blood test may also be used to confirm results if skin tests are not clear. There are a variety of blood tests available, including: enzyme-linked immunosorbent assay (ELISA), in-vitro basophil histamine release assay and radioallergosorbent test.

Blood is drawn from a vein in the arm to provide a blood sample. An allergen is added to the sample and the number of immunoglobulin E antibodies produced in response to the allergen are measured. Although allergies are likely to increase the number of circulating IgE antibodies, this is only an indication.

High numbers of IgE antibodies can also result from parasite infections and smoking, therefore it is difficult to form definitive conclusions based on blood tests alone. In addition, the number of antibodies cannot be correlated with symptom severity.

Advantages of blood tests:

• Requires only one, simple needle prick
• Medication does not affect results

Disadvantages of blood tests:

• Longer time taken to obtain results
• Less sensitive and more expensive

Challenge Tests

Challenge tests are used to identify a trigger when a person’s allergy does not produce a strong skin reaction. The body is exposed to the allergen in its natural form, via a typical route, and then monitored for signs of a reaction.

For example, to determine the cause of hay fever, a specific type of pollen can be applied to the nasal mucosa with the patient subsequently monitored for sneezing, a stuffy nose and watery eyes. Food allergies can be tested by feeding the patient the suspected trigger. Allergens can also be used to challenge the lungs or eyes in a similar way.

Patients should not have any allergic symptoms before a challenge test and medical supervision is necessary throughout as they can stimulate serious allergic reactions.

Steps to Identifying an Allergy Trigger

In general, to identify an allergy trigger, a skin test is the first step. If this is inconclusive, too risky, or the allergen cannot be made into an appropriate solution, a blood test is performed.

A challenge test is the third option if the former two are not possible, or results are inadequate. Whichever test is used, the results must be interpreted along with the patient’s medical history in order to provide a formal diagnosis of the allergy.