Diagnosing Allergic Diseases Allergies affect around 30% of the adult population and 40% of children. The prevalence, severity and complexity of allergy in the population are rapidly rising. Allergies are a significant cause of morbidity. For some patients the inconvenience of their hay fever, the embarrassment of their eczema and the time off work and school because of asthma is the most significant problem in their life. Allergen identification, avoidance, and environmental control are important steps in the management plan of patients with allergic diseases. The increase in allergy awareness in the western world has created an increased demand for "Allergy Tests,” It is important to be aware of all the tests available, and emphasize the merits of the proven, useful tests, so that patients will not be forced to resort to alternative (Unproven) allergy tests. Allergy tests should be accurate, reliable, and specific for a particular allergen to which the patient reacts on each exposure. Positive results should always correlate with a clinical allergic disease identified in the allergy history. In the following sections we describe the methods available for diagnosing allergies and highlight the pitfalls of interpretation of these results. Detailed case history A careful history is essential and forms the basis for the diagnosis and management of allergic diseases. General points of particular importance are: • Family history of allergies (atopy) • The patient’s perception of triggering factors • The home environment, e.g. pets, carpet in bedroom, clutter • The work environment, e.g. chemicals, irritants • Dietary factors e.g. hives within 2 hour of eating • Timing of symptoms, e.g. day, night, seasonal, at work, at home • Hobbies and interests, e.g. horse allergy in keen rider • Medication. Physical examination A carefully taken history should be followed by an appropriate physical examination. Investigations Skin Prick Test . • Skin test can provide useful confirmatory evidence for a diagnosis made on clinical grounds. • A positive skin prick test merely identifies sensitisation to a particular allergen but it does not predict clinical relevance independent of the history. • It is a simple, quick, and safe method with a high degree of specificity and sensitivity. • Selection of the antigens and performing the tests require experience, knowledge and proper training. • It is not affected by nonspecific hyper-reactivity in the mucous membrane, as are nasal and bronchial provocation tests. Indications for skin prick test. • To demonstrate an increased tendency to react to environmental agents (i.e. atopy): There is a good argument for performing skin prick tests as part of the routine work-up of all patients presenting with asthma, eczema and rhinitis — for proper management of these conditions the identification of allergenic triggers is essential before deciding on a long-term management plan. • Acute urticaria and angioedema. • Anaphylaxis: test should be timed for 6 weeks after reaction • Educational value: provide a clear illustration for patient that may reinforce verbal advice. • Essential when expensive or time-consuming allergen avoidance measures, removal of a family pet, or removing carpets from the bedroom are being contemplated. What Allergen Should We Test For?
Allergens should be selected on an individual basis, as dictated by the history. Skin prick test is generally a safe procedure and not associated with significant side effects, however, testing with food allergens may cause systemic reactions. Thus, skin prick test with peanuts, eggs or latex should only be done in a specialised clinic/hospital setting in patients who have had anaphylactic reaction to these agents. Relevant allergens include the following:
Allergy services provided

Ingested allergens
Nut panel	Seafood Panel	Dairy panel	Fruit Panel	Vegetable Panel	Meat panel	Cereal panel
Walnut	Shrimp, prawns	Egg White	Oranges	Tomato	Beef	Wheat flour
Peanut	Crab	Milk	Peach, strawberries	Celery	Chicken	Rye flour
Cashew nut	Mussels	Cheese	Avocado	Potato	Pork	Barely flour
Pistachio	Cod		Kiwi, Apple	Soya bean	Lamb
	Salmon		Tomato

Inhalant allergens
Epithelia Panel	Mites	Moulds and Fungi	Pollen Panel
Cat	House dust mite	Asp. Fumigatus	Mixed grass
Dog		Alternaria alternata	Spring trees
Rabbit		Cladosprium
Horse
Hamster
Goose feathers
Duck feathers
Chicken feathers

Techniques of skin prick testing

• The inner aspect of forearm is marked off with a skin-marking pen.

• The dots can be numbered to correspond to the number of allergens being tested. Dots are usually at least 2cm apart.

• Allergens are placed alongside the dots using dropper from allergen vial.

• A positive (histamine) and a negative control (normal saline) should be included.

• A sterile prick lancet is used to make a small prick through the drop: a new lancet is used for each allergen.

• Excess allergen is removed by laying a tissue on the arm (not by wiping).

• Test is read at 15 minutes .

Allergens used for skin prick testing.

Applying allergens to the skin.

Pricking the skin with the lancet.

Positive skin tests to different allergens. Positive skin tests for latex.

Positive skin tests

Measuring the wheal after 15 minutes.

Interpretation of skin prick test

• Reactions are assessed by the degree of erythema (redness) and the size of wheal (swelling) produced: A positive test is 2mm > the negative control.

A wheal 6mm or more across is more likely to be clinically relevant

A wheal > 15mm diameter suggests patient is very sensitive

A wheal 10 to 15mm diameter suggests patient is moderately sensitive

A wheal 5 to 10mm diameter suggests patient is mildly sensitive.

Advantage of skin prick test

• Easy to carry out and repeat if necessary.

• Virtually painless so that young children do not object

• Risk of systemic reaction is very low due to a small volume inoculated (0.000003ml is introduced with lancet prick).

• Provocation /Challenge Testing

Skin tests and blood test can determine triggering factors for an allergic state. They can also can identify individuals with a tendency to developing allergy. A positive result only indicates that the individual is sensitised to that allergen but it does not give any information of clinical reactivity.

Direct challenge, either by inhalation or ingesting allergens, can be of great diagnostic importance in certain allergic diseases such as asthma or food allergy. In addition to allergen challenge, the general hyper-responsive state of the airway associated with asthma may be evaluated by exercise or by inhalation of chemicals to which asthmatics are more sensitive than non-asthmatics.

Oral Food Challenges

Oral challenges can be performed to determine if a particular food is causing the allergic response. There are different types of oral challenges. The open label challenge where both the physician and the patient know the content of the ingested substance, or single-blinded, with only the doctor knowing the content of the challenge or double-blind and Placebo-controlled, where neither the patient nor the doctor know the content of the challenge. This Double-blind Placebo-Controlled Food Challenge (DBPCFC) is considered the "gold standard" in diagnosing IgE-mediated (true) food allergy.

Blood Tests

IgE measurement

IgE levels are often elevated in cases of allergic disease. A normal IgE level does not exclude allergy, while definitely elevated levels may be seen in non-atopic people.

Specific IgE measurement

The amount of IgE directed to a specific allergen can be measured in the laboratory using a blood sample. Specific IgE for allergens are appropriate tests in patients with a history of sensitivity to a particular allergen. Different systems to measures specific IgE are available. Specific IgE can be measured against a large range of suspected allergens.

Mast Cell Tryptase Test

Mast cells are important cell of the immune system. They store histamine and other chemicals that mediate the allergic response. Mast cell tryptase is an enzyme that is found in mast cell granules and secreted upon stimulation of the cell together with histamine and other mediators. Mast cell tryptase is released into the circulation after major allergic reactions and its measurement can be useful in helping to make the diagnosis of severe allergic reactions.

Patch Testing

Patch testing is used to diagnose T cell allergy. These reactions are mediated by a particular type of immune cells known as T cells. Clinically these reactions manifest with an eczematous rash confined to the site of contact. The cell-mediated response appears 7 to 14 days after initial sensitisation and reactivates within 2 to 5 days of re-exposure. Patch testing is used to identify the sensitising substance and thus a careful history is paramount in trying to identify potential allergens. Once identified, strict avoidance is necessary to avoid further skin reaction. The ‘memory’ for the reaction is usually carried by the skin for life.

Technique

All or part of a standard battery of allergens is applied to a patch (adhesive tape) and the patch is applied to the upper back. There are commercially available patch test strips that are smaller and easier to apply, but these are much less sensitive than the Finn Chamber method.

• The test is usually read at 48 hours and again at 72 hours.

• Experience is essential if correct interpretation and reliable results are to be obtained.

Strips of Patch test chambers applied to the strips.

Young woman with allergy test patches on her back

Results interpretation

Some common Allergens (causing contact dermatitis) used in Patch Testing:

Unproven Tests in Allergy

There are some tests that have been around for quite some time which have been shown to be unreliable in allergy diagnosis. Every few years they are re-marketed under a new brand name usually with a lot of media hype and interest.

• Applied Kinesiology - Muscle Testing

• The Leukocytotoxic (Bryan’s) test

•Cytotoxic Food Testing

• Electrodermal skin test - VEGA Test

• ELISA/ACT

• Hair Analysis

• Iridology

• Pulse Test