These have been used for nearly 100 years to investigate allergic conditions. After placing a drop of a dilution of an allergic substance on the forearm, a tiny, painless prick causes a seepage into the skin. If there is a reaction, a localised swelling or "weal" is formed within 10-20 minutes; this is the result of fluid being flushed into the site, brought on by immune cells lying in the skin, some of which release histamine.
Accuracy: Skin prick tests are more accurate in the identification of inhalant allergens such as hay fever, dust mites, pollens or animal dander and immediate allergies to foods e.g. peanut, but they are unlikely to identify more mild type of allergy e.g. food intolerance. Accuracy also depends on the expertise of the clinician or clinic staff.
Precautions/contra-indications: There is a small risk of severe reaction if skin prick tests are used for the diagnosis of peanut, bee or wasp sting allergy in some hypersensitive individuals. In these cases a blood test, Iimmunoglobulin E, is preferred and safer.
Uses: Hay fever, rhinitis, asthma, eczema, urticaria, angio-oedema and to determine the allergens to use in patients' vaccines for standard immunotherapy.
Age restrictions: None, selective tests can be carried out even on babies.
These are mainly used for the identification of contact allergy to chemical agents such as latex, hair dyes, detergents and so on.
A multiple patch containing a number of common or suspected chemical allergens is placed on the patient's upper back and is left sealed for 2-3 days. Superficial swelling and redness indicates reaction. Up to 24 chemicals can be tested simultaneously.
Accuracy: High, but it depends on an accurate history to short list possible culprits.
Age restrictions: None.
These involve a painless injection of 0.02-0.05ml in the top layer of the skin and the measurement of the resulting swelling immediately and 10 minutes later. A growth of the swelling reflects a reaction. These tests are more time-consuming than ordinary skin prick tests but several items can be tested within 2 hours. They are particularly used when testing a number of food intolerances.
Accuracy: High. Intra-dermal tests are less liable to show false negative results than skin prick tests and our clinic staff are fully trained to use them to confirm allergies both to inhalants and foods.
Uses: All types of immediate or delayed allergy or intolerance, including foods. They are also used as part of the low-dose method of desensitisation or immunotherapy, known as Neutralisation.
All types of skin tests can show an allergic response, which can sometimes reflect not a current allergy but the person's propensity to develop an allergy later on in life. This is why the findings need to correlate accurately with one's detailed allergy history. Failure to do so may result in inaccurate advice.