Allergy Skin Prick Testing

Transcription

1 Allergy Skin Prick Testing

2 What is allergy? The term allergy is often applied erroneously to a variety of symptoms induced by exposure to a wide range of environmental or ingested agents. True allergy occurs when the immune system becomes activated so that one of the following occurs:- 1) Specific antibodies of the immunoglobulin E (IgE) class are produced (immediate type-1 allergy typically causing hay fever and asthma). 2) An inflammatory response mediated by T-lymphocytes (delayed hypersensitivity causing, for example, contact dermatitis/eczema). 3) Precipitating antibodies and immune complexes are deposited in the alveoli resulting in a chronic inflammatory reaction (e.g. farmer s lung, bird fancier s disease). These should be distinguished from other conditions such as migraine, ME, IBS etc. which could be caused by a chemical intolerance. Immediate (Type-1) Allergy Type 1 immediate allergy is one of the most common conditions encountered in the developed world, affecting around 20% of the population. Symptoms vary from mild hay fever through severe asthma and eczema to potentially fatal systemic reactions (anaphylaxis). See table below. Allergic rhinitis (e.g. hay fever) Asthma Pollens (timothy grass, birch, etc.), dust mite Inhalation faeces Danders (cat, dog, etc.), dust mite faeces Inhalation Bronchial constriction, increased mucus production, airway inflammation Bronchial constriction, increased mucus production, airway inflammation Food allergy Tree nuts, peanuts, shellfish, milk, eggs, fish Oral Vomiting, diarrhoea, pruritis, urticaria, anaphylaxis (rarely) Systemic anaphylaxis Drugs, venom, peanuts Intravenous (either directly Oedema, raised vascular or following permeability, tracheal oral occlusion, circulatory absorption collapse, death into the blood) Fig. 1 IgE-mediated allergic reactions Individuals who suffer from type-1 allergy are predisposed to producing IgE antibodies on exposure to various agents (allergens) and have an inherited abnormality known as atopy. They may be described as being atopic, or having atopic allergy. 2 of 7

3 Once produced by the activated immune system, IgE antibodies become coated on the surface of cells called mast cells (sensitisation). These cells are widely distributed throughout the body. When the atopic individual next comes in contact with specific allergens to which they have been sensitised, these bind to the specific IgE antibodies on the mast cells. This stimulates the mast cells to release histamine and other pharmacologically active substances (see Figure). These cause local vasodilation and inflammation, giving rise to the typical symptoms of the immediate allergic response. Antigen Presenting Cell Clinical effects B cell asthma eczema hay fever anaphylaxis T.helper cell (After Brostoff, Scadding, Male & Roitt) Fig. 2 Type 1 hypersensitivity Diagnosis of Type-1 Allergy: the Skin Prick Test Intracutaneous, or skin prick testing, is essential, both to identify patients with IgE-mediated allergy and to determine sensitivity to different allergens so that appropriate advice and treatment may be given In the skin prick test, aqueous extracts of allergens are introduced into the surface layer of the skin. If the patient has been previously sensitised to the allergens tested, then the mast cells in the skin will respond in the same way as those in the airways causing a local transient inflammatory reaction. This appears as a raised yellowish wheal surrounded by a red flare. The reaction develops within about 10 minutes and fades after about 1 hour. Skin prick tests should not be performed in isolation, but in conjunction with a carefully taken case history to indicate which allergens should be chosen for testing. The test may be performed on patients of all ages although it should be borne in mind that the skin is often less reactive in young children and in the elderly. (For more detailed information see Brydon, 2000) 3 of 7

4 The Importance of Allergen Quality Most skin prick testing solutions are now manufactured to very high quality standards. Patients may be sensitised to different allergic components in even the most simple single pollen extracts. It is therefore important that as many of the different individual allergic proteins as possible are present in the skin testing solution. In order to achieve this, these proteins are characterised and identified serologically Of equal importance is the need to ensure that batchto-batch variation in the potency of each skin testing solution is minimised and also that the potency of different allergens does not vary too widely. These requirements are met through measurement of potency of different allergen dilutions against a histamine standard in panels of selected patients. The final working concentration for skin testing is selected and expressed in histamine equivalence or biological units. Important Considerations before Skin Prick Testing Contra-indications Skin prick tests should not be performed on areas of active skin disease. During pregnancy and beta-blocker therapy, skin tests should be avoided where possible. Side Effects Allergic side effects are uncommon. Highly sensitised patients may experience strong local reactions and occasionally a late local reaction may occur 6-12 hours after the immediate reaction has faded. Systemic reactions are extremely rare. However, best practice requires that epinephrine (adrenaline), 1mg/ml for injection must be available during testing. Influence of Medication Antihistamines and corticosteroids inhibit the skin reaction. Patients should not take medicines containing antihistamines for at least three days before testing. Long-acting antihistamines can affect the skin test response for up to one month after consumption. Systemic steroids should be discontinued the day before the test. Topical steroids on the test area should be avoided. 4 of 7

5 Performing a skin prick test Step 1 Rest the arm near the edge of a table. Using a skin marking pen, mark and code the inside of the forearm, either directly or onto Scotch Magic Tape (with + and - for the controls). The test site should be from >5cm above the wrist to >3cm from the cubital fossa, with the tests 2-3cm apart. Step 2 Apply one drop of each allergen solution on to the skin next to it s code number. Step 3 Prick the skin through the drop holding the Allergopharma lancet vertically. Keep a constant pressure for 1 second then remove the lancet keeping it vertical. Don t press too lightly which might result in too small a reaction or too hard, which might cause bleeding. 5 of 7

6 Step 4 Blot the forearm dry. Do not wipe dry as this might cause cross-contamination between the allergens. Step 5 Wait for minutes, but observe the reactions at regular intervals to see how they progress. Step 6 After the time has elapsed, read the reactions. Draw around each wheal with a fine skin marker pen. Measure the diameter of each wheal directly or transfer the marked wheals and codes to a permanent report form* using Scotch Magic Tape. *We can provide customised report forms free of charge. 6 of 7

7 Interpretation of Skin Prick Test Results Interpret the results of the test after 15 minutes. Positive (histamine) and negative (saline) controls should always be measured when skin testing. There should be no reaction to the negative control. The histamine control should produce a wheal at least 4-5 mm in diameter. (If the histamine gives a poor response, then the patient has probably recently taken an antihistamine. This will render the test of limited use since the allergen-induced reactions will also be affected.) A wheal size of 3mm or greater to any allergens should be recorded as clinically significant. A wheal size of less than 3mm is unlikely to be clinically significant. Measure a wheal across its longest diameter. Please note: a positive reaction indicates only that the patient has been sensitised to the allergen in question. The size of the wheal does not relate to the severity of the allergic response. Further reading Skin Prick Testing in Clinical Practice. Mary J. Brydon, NADAAS, 2000 (ISBN ) (Highly recommended) Practical guide to skin prick tests in allergy to aeroallergens. European Position Paper. Allergy 67 (2012) Clinical Immunology. Brostoff, J., Scadding, G.K., Male, D. Roitt, I M. Chapter 17. Mosby (ISBN ) Immunobiology (5 th edition). Janeway, C.A., Travers, P., Walport, M. Shlomchik, M. Chapter 12. Churchill Livingstone (ISBN ) Good Allergy Practice. Royal College of Physicians /Royal College of Pathologists, 1994 (ISBN ) European Allergy White Paper. UCB Institute of Allergy 1997 (ISBN ) 7 of 7