Allergy diagnosis: Why test? • Avoid trigger allergens • Specific immunotherapy for certain allergens • Exclude allergy • Unnecessary pharmacotherapy • Unnecessary avoidance

Diagnostic tests for allergies

: In vivo tests • Skin prick tests • Intradermal tests • Patch tests • Autologous serum skin test • Provocation tests • Food challenge test

In vitro tests: • • • •

Total IgE (RAST tests) (CAST tests) Leukocyte Histamine Release Test • Phagocytosis inhibition test • ECP .

Skin Testing Indications : Refractory Allergic Rhinitis Refractory Asthma Before initiating immunotherapy Food Allergy Insect allergy or drug allergy

Contraindications : Severe Eczematous Dermatitis Dermatographism History of Anaphylaxis Very young (< 2 years) Unable to discontinue Antihistamines Uncontrolled Asthma (Peak expiratory flow < 75%)

Preparation – Have Epinephrine available – Discontinue Antihistamines • Second generation Antihistamines: 2 weeks before • First generation Antihistamines: 3 days prior to test • Other Antihistamine-type agents to stop before test – Phenothiazine – Tricyclic Antidepressants – Anticholinergic Medications – H2 Receptor blocking medications (e.g. Ranitidine) – Inhaled Corticosteroids do not affect test – Short-term Systemic Corticosteroids do not affect test

 Test a lot of antigens  Use Controls  Positive Control: Histamine  Negative Control: Glycerol-Saline  Read test 15 minutes after application of allergen  Positive if wheal diameter 3 mm > negative control

Interpretation All Tests negative: No further testing needed One or more inhalants positive: Avoid & Consider Immunotherapy If Positive Control is Negative : If Negative Control is positive :  Dermatographism  Try RAST testing

Skin prick testing Advantages • Gold standard for allergy diagnosis • Cheap & Quick • Visible results . • Can be used for allergens where in vitro testing not available • Can be done in presence of steroids & cromoglycate

Disadvantages

• Requires trained staff & resuscitation facilities • Not possible in severe dermatographism or eczema • Some drugs prevent reactions

INTRADERMAL SKIN TESTING  Intradermal skin testing is practised in some situations . But it is uncomfortable than skin prick.  it is more sensitive, is more likely to lead to false positive results.  For this reason, it is more commonly used for evaluation of patients with sensitivity to antibiotics or insect venom.

Patch Test • Suspected allergens placed on patient’s back induce a type IV hypersensitivity reaction via specific T lymphocytes which is read after 72 hours to determine allergic reaction Indications for Patch Test • • • • •

Atypical Eczema & non-immediate skin reactions Allergic Contact Dermatitis Occupational asthma & dermatitis Drug Reactions, especially delayed Non-immediate Food Reactions

Autologous serum skin test

Side-effects and risks of skin testing Skin tests are slightly uncomfortable, but usually well tolerated. Local itch and swelling normally subsides within 1-2 hours.  More prolonged or severe swelling may be treated with an oral antihistamine, topical corticosteroid cream and an ice pack. Occasional patients will experience feel dizzy or light-headed and need to lie down.  Severe allergic reactions from allergy testing in asthma are very rare.

The bronchial provocation test  Bronchial provocation tests (BPTs) are widely used to identify bronchial hyperresponsiveness (BHR) .  Histamine and methacholine, given by inhalation, are the most commonly used as well as other allergens.  Exercise and eucapnic hyperventilation tests are also used, particularly for identifying persons with exercise-induced bronchoconstriction .

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Diagnosis of food allergy

food challenge test The gold standard for food allergy diagnosis. These challenges take place under close medical supervision . The test is aborted if an adverse reaction occurs at any stage.

In vitro tests: • • • • • •

Total IgE (RAST tests) (CAST tests) Leukocyte Histamine Release Test Phagocytosis inhibition test ECP .

Total Serum IgE Concentration Levels of total IgE antibody is often (but not always) raised in people with allergies A total IgE level is indicated in the evaluation of asthmatic patients being considered for therapy with monoclonal antibody to IgE. It may be indicated for those patients: • internal parasites. • allergic bronchopulmonary aspergillosis, • immune deficiency disease characterized by increased IgE levels (e.g., Wiskott-Aldrich syndrome, hyper-IgE staphylococcal abscess syndrome)

Specific IgE : RAST (radio allergo-sorbent test ) These tests detect antigen-specific IgE antibodies in the patient's serum. INDICATION  skin testing is not easily available,  when skin condition such as severe eczema or dermographism prevent accurate testing  in children younger than two years of age.  when the patient is taking medications that interfere with accurate testing.

RAST tests vs Skin prick tests RAST • Quantitative values • Not influenced by medication • Cross-reactions may occur • Fruit allergens labile • More expensive • False results

SPT • Results available immediately • Cheaper than RAST • Difficult to standardise • Dependant on technique • Results influenced by medication • Cannot be performed skin disease on site • Small risk of systemic reactions

Cellular Allergen Stimulation test (CAST) • Cellular test, based on the determination of sulfidoleukotrienes (LTC4, LTD4,LTE4) produced by IL-3 primed basophils stimulated by allergens in vitro • The CAST assay has been used in allergy diagnosis, such as inhalation allergies, allergies to insect venoms, foods, occupational allergens and various drugs. • the value of this diagnostic test, arises when other diagnostic tests are not reliable, and in non-IgEmediated hypersensitivity reactions

Mast cell Tryptase •The increased levels of tryptase can normally be detected up to three to six hours after the anaphylactic reaction. Levels return to normal within 12 - 14 hours after release

Leukocyte Histamine Release Test  It measures the amount of histamine released in-vitro.  An allergen extract is added to the peripheral blood leukocytes of the individual being tested.  Histamine is normally released as a consequence of the interaction of allergen with cell-bound IgE antibodies.  If an individual is hypersensitive to a specific antigen, the leukocytes will release the histamine in-vitro.

PHAGOCYTOSIS INHIBITION TEST This test is not yet approved globally and is still considered investigational

• testing a substance by addition to the leukocyte -Candida mix instead of histamine, The results are expressed as the percent of cells found having phagocytosed candida In their cytoplasm.

Practice Points • History + Skin-prick testing remain the "gold standard" for identifying clinically relevant allergens.· • In vitro tests offer advantages in some clinical situations . • A positive RAST in the absence of a consistent history indicates sensitisation, not allergy. • Mast cell tryptase is a useful marker of anaphylaxis in situations where the diagnosis is unclear. • Elevations of IgE and eosinophilia are non-specific. • in the meantime we do not accurately know the extent of non-IgE reactions. • food challenge test is the "gold standard" for food allergy diagnosis

References: • • • • • • •

www.allergy-diaasoliman-egypt.net www.webmd.com www.nationalasthma.org www.allergy.org www.food-allergy.org www.wrongdiagnosis.com www.allergy-clinic.co.uk