Allergy tests
Avoidance of the provoking allergens is the best therapy to overcome allergies. Hence, it is important to accurately identify the offending allergens causing the symptoms. Since there are over 500 allergens that may cause allergies it is impracticable to do allergy tests randomly. Thus, a careful assessment of the clinical history linked to the development of the symptoms may provide useful clues to the causative allergen. Subsequently appropriate allergy tests can be done to accurately identify the suspected allergens triggering the symptoms. There are two types of allergy tests available that are internationally accepted: (1) Skin Prick Test has been clinically used for many years but there are some limitations to its routine use, and (2) the CAP RAST test developed in 1996 determines the IgE antibody with high accuracy in a small sample of blood and is the preferred option. Not all blood allergy tests are equal in sensitivity and accuracy. The CAP RAST system has high sensitivity and specificity compared to other systems used (Allergy 2003, 58:38). Therefore, it is essential to only use the CAP RAST when testing for allergy.

Unfortunately many private laboratories and also hospital-based laboratories market a variety of unproven tests that have by various scientific studies shown to be no better than placebo. The use of unproven allergy tests sometimes by clinicians to identify allergy triggers has created confusion among allergy sufferers.

Worldwide the tests (skin prick test and the blood Cap RAST for IgE antibody) that are accepted as suitable for the diagnosis of allergy are the following.

In the skin prick test, a drop of solution containing the suspected allergen in appropriate concentration is placed on the skin of the volar region (the inner side) of the forearm. A small prick is made through the solution in the skin using a small needle or lancet without drawing any blood. The development of a raised area (wheal) similar to a mosquito bite and surrounding reddened area (flare) after 15 minutes confirms a positive response. Alongside the challenge allergen test, appropriate positive and negative controls are conducted. The advantage of the skin prick tests is that the results are available immediately and management can be instituted quickly. However, skin tests have some limitations. Patients must stop anti-histamine medication for at least 7 to 28 days (depending on the histamine) because the drug suppresses the skin reaction giving false negative results. Skin testing is dangerous in extremely sensitive patients because of development of life-threatening anaphylactic reactions. Skin tests in general are not useful for food allergies. Moreover, these tests are not suitable for babies and the elderly because of poor skin reaction in such patients. The skin prick test cannot differentiate between allergy reaction and non-allergy reactions often caused by non-immune-mediated de-granulation of the mast cells.

 

On the other hand blood allergy tests are simpler to perform and have no inbuilt limitations since they can be performed on patients undergoing drug treatment. The results are available the next working day in an efficient laboratory. The CAP RAST uses a state-of-the-art technique that is accepted worldwide as the in-vitro gold standard. This technique is widely used in most hospitals around the world. It is the technique of choice because it is sensitive, accurate, safe, convenient and unaffected by medication taken by the patient. Blood tests are suitable for babies and patients with severe eczema in whom the skin test is not advisable.

The total IgE of the serum is an independent marker which indicates the allergy status of the patient. This marker is particularly useful in recognizing IgE-mediated allergy versus non-allergic hypersensitivity reactions. An elevated total IgE levels in asthma relates directly to the severity of the disease. Obviously elevated IgE suggests the need to identify the trigger factors associated with the disease. It is advisable to do the total IgE along side the specific allergen to get a better understanding of the allergy problem so that appropriate treatment can be started.