Once it is established that the patient has asthma and the asthma is allergic in nature, the next question that arises is – allergic to what?
The substances that cause allergic asthma (called allergens) are pollens of weeds, grasses, trees, fungal spores, dusts of various types, certain food items, certain medicines etc.
The allergy tests are required to pinpoint which of these are causing the allergic reaction in the particular patient.
Broadly speaking, allergy test are of two types:
- Those which check allergy from drawn blood.
- Those which check allergy directly in the skin of the patient.
Skin Prick tests:
In the skin prick test, a tiny quantity of the dilute form of the allergen is pricked into the skin. 40 or more common allergens are simultaneously checked. The test area is the forearm or back. It is only mildly painful; a skilled doctor can easily do it in children above age 5 or 6 years of age. If the patient is allergic to the tested allergens, a local skin reaction occurs in the form of a tiny, itchy, red wheal which appears in about 10 to 15 minutes and usually subsides on its own in 45 minutes or so.
The reaction can be properly grades and interpretations drawn.
A positive skin reaction in conjunction with clinical suspicion is very strong evidence that the said allergen is the culprit.
A negative test practically rules out the allergen as the cause of asthma.
The patient should not be taking anti-histamine tablets prior to the allergy test.
Blood tests for allergy:
These are not as reliable as the skin tests. In these kinds of tests the blood is drawn and serum levels of the specific IgE are measured for the different allergens. These sometimes over diagnose allergy ie. the blood test says that the patient is allergic to such-and-such allergen whereas there is no true allergy to that allergen.
So these tests should always be viewed with caution. This is particularly true with testing for food items. Asthma being truly caused by allergy to a particular food item is rare. But many a times I see patients carrying a blood report saying they are allergic to a multitude of food items. They avoid these food items- they don’t get any relief in their asthma. They do, however, develop serious nutritional deficiency!
Blood tests for allergy do have a small place in the diagnosis of allergy in the following situations:
- Very small children where prick test will be difficult.
- Extensive skin disease with no normal looking skin available to do the tests.
- The patient has taken anti-histaminic which would suppress the skin test reactions.
- The patient is mortally or irrationally afraid of needles