What is allergy testing?
Holistic care of a patient with allergies requires accurate allergy diagnosis to identify allergens and enable their exclusion.
What is an allergy?
Allergic symptoms result from a damaging immune response by the body to an otherwise harmless substance, typically a protein, to which it has become hypersensitive. These proteins are called allergens. When this reaction affects the respiratory tract or gut, high levels of allergy antibodies, specialised proteins called immunoglobulin E (IgE) in the bloodstream are directed towards the allergen. When an allergen makes contact with the skin, specialised white blood cells called T-lymphocytes become sensitised to the presence of the allergy-inducing substance and these circulate in the blood stream to the site of skin contact. Sufferers may have other allergies or related conditions e.g. asthma, eczema. Other family members may also have similar problems.
Allergy-focused Medical History
The most important tool for the allergy specialist is the detailed allergy-focused history. Allergy skin-prick and/or specific IgE blood tests are frequently required. However, a draw-back is the existence of both false negative as well as false positive responses on both skin prick and specific IgE blood tests. This also applies to food allergy testing and we believe that this should not be undertaken without expert interpretation of results by an allergy specialist.
Common allergy symptoms include:
- Itchy nose, runny nose and itchy eyes ears and palate (allergic rhinitis)
- Wheezing, cough and shortness of breath (allergic asthma)
- Itchy wheals (hives) or red rash (urticaria) anywhere on the body
- Itchy, weepy, dry or scaly skin on any part of the body (eczema or dermatitis)
- Itchy, and sometimes swollen, lips, mouth, tongue or throat when eating certain foods (oral allergy)
- Non-itchy, painless swellings that affecting the lips, face, tongue throat or elsewhere (angioedema)
- Severe attacks with rash and /or swellings and/or breathing difficulty and/or collapse (anaphylaxis)
Allergy testing: Your options
Skin prick test
A skin prick test is usually the first test to be done when trying to identify allergens for a particular patient. It’s quick, painless and is safe overall, and the results can be read after about twenty minutes. A tiny drop of allergen extract that has prepared in an allergen laboratory is placed on the skin and very lightly punctured by a shallow (half millimeter) lancet. If IgE antibodies are present, an itchy wheal with a diameter between 5 and 15 mm will form, rather like a mosquito bite. These wheals will then disappear in 30 to 60 minutes. There is a very small chance that a patient may develop a stronger reaction but they are carefully monitored and treatment will be given should this be the case.
Specific IgE Blood Test
The blood test that identifies if the blood serum contains specific IgE antibodies is called a specific IgE blood test (formally known as RAST). There are two kinds of test. The first (traditional test) uses an extract of the whole substance (e.g. peanut) whilst the second (component resolved test) uses extracts of the individual allergens (e.g. the component allergens of peanut Ara h1, Ara h2, Ara h8 etc.). The second type of test is better able to distinguish true from false positives and predicts if different allergies may co-exist because they cross-react.
A patch test is used to see if a skin reaction, for example eczema, is caused by contact with a specific chemical or substance. Small amounts of the various allergens being tested are suspended in petroleum jelly in tiny aluminium wells contained in strips of tape that are applied to the skin of the back for 48 hours. If this kind of test is needed, allergy specialists will usually refer the patient to a dermatologist colleague who specialises in this form of testing.
When there are conflicts between the medical history and the results of allergy tests a provocation test may be needed. The suspect allergen is then given to the patient under carefully monitored conditions. According to the allergen and the symptoms it is suspected to cause, the allergen might, in tiny but increasing doses, be breathed in, be placed in the mouth or on the lower lip, be sniffed into the nose or be applied to the conjunctiva. Although rarely needed, examples of the situations in which this type of testing might be needed would be firstly to see if a child has outgrown a peanut allergy or secondly to see if a particular antibiotic can safely be used when other tests have been ambiguous.