Allergy GPSI Dr Adrian Morris rounds up what you need to know about common methods of allergy testing as well as a new recombinant method
A positive allergy test indicates sensitisation to an allergen but does not necessarily predict that a clinically relevant allergic reaction will occur. That is why allergy testing otherwise healthy individuals should be discouraged, as results are often misleading.
A face-to-face allergy consultation to tease out details of the allergic reaction – plus take a full medical and medication history in conjunction with specific allergy testing – is essential to make an accurate allergy diagnosis.
Allergen skin prick testing
Skin prick testing (SPT) is one of the oldest allergy tests available and is still the cornerstone of primary care allergy diagnosis.
It was first performed in 1865 by Dr Charles Blackley, a Manchester GP, to identify grass pollen as the cause for hay fever and remains one of the most highly sensitive allergy tests available.
It measures specific immunoglobulin E (IgE) antibodies to environmental allergens such as house dust mite, pollen, mould spores, cat and dog dander as well as food, insect venom and drug allergies. A positive result is a typically raised wheal and red flare reaction on the skin.
Skin prick tests are cheap, safe and simple to perform if the tester has been properly trained and the results are immediately available.
These tests are useful to demonstrate the acute inflammatory nature of allergy to the sufferer. They are particularly accurate in confirming the cause of allergic asthma and allergic rhinitis.
A variation for food allergy is the ‘prick and prick’ test, so called because a lancet was used to prick the food, then the skin. Today it is performed using a droplet of the fresh food extract. The skin is scratched or pricked with a lancet through the drop of fresh food extract – for example fresh apple, melon or peach.
Standardised glycerinated allergen extracts such as house dust mite, cat and dog dander, tree, grass and weed pollen and mould spores are used to identify inhalant allergies. There is always a negative saline and positive histamine control – used as a quality control reference.
A droplet of each extract is placed on the inner forearm about 3cm apart. A specially modified lancet is used to penetrate the skin through the droplet. The reactions are read after 15 to 20 minutes and a positive reaction should have at least 3mm of raised wheal. Oral antihistamines must be stopped for two to three days beforehand, as these suppress skin reactivity (1).
Atopy patch tests
Atopy patch tests (APT) on the skin can detect delayed hypersensitivity reactions
to foods and uses the same principle as contact dermatitis patch testing to identify sensitivity to nickel, rubber and cosmetics.
Similarly, the APT may demonstrate delayed (non-IgE) food hypersensitivity reactions in children with GORD and in atopic eczema. The allergen is applied to the skin under an occlusive cover and the skin is assessed after 48 and 72 hours for a positive reaction (2).
Measuring IgE in serum
Total serum immunoglobulin E (IgE) was the original blood test for allergy but has been superseded by newer more specific tests. But a total IgE level exceeding 100kU/l is still highly suggestive of atopy and allergy.
Total IgE levels also depend on the size of the organ affected by allergies and may also be raised in parasitic infections and certain immune diseases. It is relatively low with nasal allergies, but very elevated in extensive skin allergy – for example with atopic eczema. Levels naturally increase from infancy through to adolescence when they tend to plateau and slowly decrease with age (3).
Specific multi-allergen screening tests available include an inhalant allergy screen, called an ImmunoCAP Phadiatop, which tests house dust mite, cat and dog dander, tree and grass pollen and mould spores. There are various ImmunoCAP food allergy screening panels such as the fx5 panel for common paediatric food allergens (cow’s milk, hen egg, wheat, soy, codfish and peanut), fx1 for nuts, fx2 for seafood and fx3 for cereals.
There are now more than 450 specific IgE ImmunoCAP or RAST tests available for anything from apple to zucchini – which can be performed on a multi-channel analyser available at NHS immunology laboratories.
New recombinant allergy testing
Some foods such as cow’s milk and many tropical fruits and nuts contain a large number of different potential allergens. One has to be aware that a specific RAST test may only test for a finite number of the potential allergens in that food. Individuals may be sensitised to rarer minor allergens in a particular food.
For this reason, a new range of recombinant allergens have been developed to measure specific protein antibodies to foods. These individual proteins include profilin, tropomyosins and lipid transfer proteins (LTPs) and occur in seemingly unrelated plant families such as in silver birch pollen and apple, or latex and avocado. As a result, an unsuspecting latex allergy sufferer may have an acute anaphylactic reaction when eating avocado, banana, kiwi or chestnut (4).
However, the double-blind placebo controlled food challenge test remains the gold standard for food allergy testing and allergy diagnosis, and open food challenges may also be performed. Food challenges are impractical in the GP setting as they can be dangerous, having the potential to trigger anaphylaxis. For this reason only specialised allergy units with full resuscitation equipment are prepared to do this form of allergy testing (5).
Dr Adrian Morris is a GP with special interest in allergy and clinical assistant at the Royal Brompton Hospital, the Guildford Clinic and London Medical Centre. Further details at www.allergy-clinic.co.uk