Allergy Test — Purpose, Procedure & Normal Values | MyMedicPlus

Allergy tests identify specific substances (allergens) that trigger an abnormal IgE-mediated immune response in sensitised individuals. When the immune system mistakenly targets harmless substances — pollen, food proteins, animal dander, insect venom, or drugs — it produces immunoglobulin E (IgE) antibodies that cause symptoms ranging from mild sneezing and urticaria to life-threatening anaphylaxis. The three main testing modalities are skin prick tests (SPT), which introduce minute quantities of allergen extracts into the superficial skin layer; patch tests, which evaluate delayed-type (Type IV) contact hypersensitivity over 48–96 hours; and specific IgE blood tests (ImmunoCAP, RAST, ISAC microarray), which quantify allergen-specific antibody levels in serum. These tests are performed by allergists, clinical immunologists, and dermatologists in outpatient and specialist clinic settings. Results underpin allergen avoidance counselling, emergency action plan development, and eligibility assessment for allergen-specific immunotherapy.

Clinicians order allergy tests to confirm suspected sensitivities identified through clinical history. Indications include perennial or seasonal hay fever (allergic rhinitis) from pollen, dust mites, or pet dander; allergic asthma triggered by environmental allergens; atopic eczema flare-ups; chronic urticaria or angioedema; food allergy reactions to peanut, tree nuts, shellfish, milk, eggs, wheat, or sesame; insect venom allergy (bee, wasp); drug hypersensitivity (penicillin, NSAIDs, biologics); latex allergy in healthcare workers; and occupational allergies in bakers, hairdressers, and laboratory workers. Results guide personalised avoidance strategies, epinephrine auto-injector prescriptions, pharmacotherapy selection, and eligibility for subcutaneous or sublingual allergen immunotherapy to reduce long-term sensitivity.

Stop antihistamines before any skin test: first-generation agents (diphenhydramine, chlorphenamine) for at least 3 days; second-generation agents (cetirizine, loratadine, fexofenadine) for 5–7 days; H2-receptor blockers (famotidine) for 3 days. Tricyclic antidepressants and systemic corticosteroids also suppress skin reactivity — discuss stopping these with your allergist before discontinuing any prescription medicine. Blood IgE tests (ImmunoCAP) are unaffected by antihistamines and serve as an alternative when medications cannot safely be withheld. Avoid using body lotion, cream, or topical corticosteroids on the test site the morning of the test. Wear a short-sleeved or loose top to allow forearm or back access. Bring a complete medication list, a record of previous allergic reactions, and any prior allergy test results to your appointment.

For a skin prick test, a nurse or technician marks a grid on your forearm or back and places a small drop of each allergen extract — house dust mite, cat and dog dander, grass and tree pollen, mould spores, food proteins — beside each mark, alongside a histamine positive control and a saline negative control. A sterile lancet pricks through each drop without drawing blood. After 15–20 minutes, any raised, itchy wheals are measured in millimetres. For a patch test, adhesive panels containing contact allergens (metals, fragrances, preservatives, rubbers) are applied to the upper back and worn for 48 hours; readings are taken at 48 and again at 72–96 hours after removal. Blood IgE tests require a single venous blood draw; the serum is sent to a specialist laboratory for allergen-specific IgE quantification using fluorescent enzyme immunoassay, with results returned within 1–7 business days.

Skin prick test: a wheal measuring ≥3 mm greater in diameter than the negative (saline) control is considered a positive result. Reaction size grading from 1+ (wheal 3–5 mm) to 4+ (wheal >15 mm with pseudopods) roughly correlates with degree of sensitisation. Blood IgE test reference ranges (WHO/ISAC): Class 0 (<0.35 kU/L) — negative, sensitisation unlikely; Class 1 (0.35–0.69 kU/L) — borderline; Class 2 (0.70–3.49 kU/L) — moderate sensitisation; Class 3 (3.50–17.49 kU/L) — high sensitisation; Classes 4–6 (≥17.5 kU/L) — very high to extremely high. Total serum IgE above 150 kU/L suggests generalised atopy. Critically, a positive result confirms immunological sensitisation only — it does not prove clinical allergy. Correlation with the patient's full exposure history and, when needed, a medically supervised oral food challenge is essential before advising strict allergen elimination or prescribing immunotherapy.

Skin prick tests may cause local redness, swelling, and itching at the test site lasting 30–60 minutes; topical hydrocortisone cream and oral antihistamines relieve this promptly. Delayed systemic reactions are possible within 30 minutes of testing — all allergy testing is conducted in a clinical setting equipped with adrenaline (epinephrine) and full resuscitation facilities. True anaphylaxis from a skin prick test is extremely rare, estimated at fewer than 1 in 1 million test applications. False-positive skin test rates reach approximately 50–60%, so sensitisation demonstrated by any test must always be correlated with the patient's clinical history before any diagnosis is made. False negatives occur when antihistamines have not been adequately stopped, allergen extract potency is low, or the responsible allergen is absent from the panel tested. ISAC component-resolved diagnostics improve specificity by identifying the precise allergenic molecule. Blood IgE tests carry only the minor risk of bruising at the venepuncture site and are preferred when anaphylaxis risk is high, skin disease is severe, or antihistamines cannot be discontinued.