A. Skin Tests

Skin Prick Tests 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 (“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 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 or bee/wasp sting allergy in some hypersensitive individuals – in this case a blood test (immunoglobulin 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.

Patch Tests 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.

Intra-cutaneous (intra-dermal) Tests involve a painless injection of 0.02-0.05ml in the dermis, 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.
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 desensitization/immunotherapy, known as Neutralisation.
(Read more on Desensitisation…)

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.

B. Dietary tests

Elimination & Challenge. This is the best method to identify food reactivities, especially intolerances and is far more simple than it sounds. As delayed food allergy usually involves a number of foods (not just one as in the case of peanut allergy), a number of suspected foods must be simultaneously avoided for 10-14 days. Many common foods fall in this category. There are several levels of exclusion, depending on the severity of the problem. Should symptoms improve significantly or clear, each food is re-introduced one-by-one to see if it rekindles any symptoms. The process is known as “unmasking” because the re-introduction following avoidance seems to make the symptom more pronounced. After confirmation, if reactive foods can be avoided for a further 6-8 weeks tolerance can improve and these foods can again be used sparingly, without the same ill effects. People who are quite poorly because of this problem are likely to experience withdrawal symptoms during the first few days. This is a clinical test, not a cure!
Accuracy: High. Several clinical trials have shown that changes of diet, based on the findings of an Elimination & Challenge process, benefit a very high proportion of patients by identifying the food-triggers.
Contra-indications: Very under-weight patients and those with severe or extensive food re-activities, if their menu is limited.

C. Blood Tests

Immunoglobulin E (also known as RAST) is useful when immediate allergy to a food or natural inhalant allergen(s) is suspected. Available in some NHS hospitals and GP practices but training to interpret these results and offer appropriate advice could improve. Not helpful in identifying other food reactivities/intolerance. Broadly speaking the IgE results correlate to the findings of skin prick tests.

Allergy/sensitivity to environmental chemicals is usually investigated with blood tests which scrutinize effects of various types of hydrocarbons, organophosphates and chlorinated compounds on lymphocytes (blood cells known to be involved in chemical sensitivity). A large number of toxic substances can be tested this way including mercury, nickel, benzene, toluene, lindane and so on.


Other Allergy Tests available in the UK

Immunoglobulin G is fashionable and has been marketed as “the best test to diagnose food intolerance” on the assumption that food intolerance always involves IgG allergy. Whilst it is reproducible, it tends to identify only reactions to more common foods and its accuracy tends to help more simple problems.

Leuco-cytotoxic Test is based on the observation that the white blood cells of a person with intolerance might dramatically alter in size (getting smaller or larger), once in contact with a reactive food. Degrees of change reflect different degrees of reactivity. Such tests currently available in the UK seem to screen all blood cells, not just white cells. Accuracy and reproducibility varies.

Interleukin Test assesses levels of hormones secreted by a person’s immune cells, T-lymphocytes, when in contact with various foods. Raised levels correspond to reactivity. Whilst it makes good theoretical sense, clinical trials so far show variable results.

D. Other tests used in Complementary and Alternative Medicine

Vega testing. This measures changes of the bodily low level frequencies, when in contact with a suspected allergen.

The available clinical research of some of these methods to date has not substantiated claims made by the practitioners using them. Periodically, we do meet people who have found them helpful and others who have not. The existence of different methods to test food intolerance, reflects the fact that there are various mechanisms of immune reactivity and there is no perfect test to assess the entire process. In the absence of a highly accurate test, it is not surprising that food intolerance is not a generally accepted diagnosis and it is not found in medical textbooks. However, when supported by a detailed history, some of the above methods are very helpful to identify most causes of allergy.

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