Allergy Blood Tests: IgE, RAST & Allergen Panels
Almost 1 in 5 Australians has an allergic disease. Blood tests can identify specific triggers — but understanding what they can and can't tell you is essential for avoiding unnecessary dietary restrictions.
How Allergy Blood Tests Work
Allergy blood tests measure immunoglobulin E (IgE) — the antibody your immune system produces in response to allergens. When you are exposed to an allergen you are sensitive to, your body makes specific IgE antibodies that bind to mast cells. On subsequent exposure, the allergen cross-links these IgE molecules, triggering mast cell degranulation and the release of histamine, leukotrienes, and other inflammatory chemicals. This cascade produces the symptoms we recognise as allergy: sneezing, itching, hives, swelling, and in severe cases, anaphylaxis.
Blood tests detect the IgE antibodies circulating in your bloodstream. A total IgE measures all IgE regardless of source — a general screen for atopic tendency. Specific IgE tests measure antibodies directed at individual allergens — peanut, dust mite, cat dander, and hundreds of others. The modern platform used by most Australian pathology labs is ImmunoCAP, which replaced the older RAST (radioallergosorbent test).
In Australia, allergic diseases affect approximately 4.6 million people. The Australasian Society of Clinical Immunology and Allergy (ASCIA) is the peak body that sets guidelines for allergy diagnosis. ASCIA emphasises that allergy test results must always be interpreted alongside clinical history — a positive test alone does not equal clinical allergy, and a negative test does not completely exclude it.
Types of Allergy Blood Tests
Total IgE
What it measures: Total IgE measures the overall level of immunoglobulin E antibodies in your blood. IgE is the antibody class responsible for allergic reactions — when an allergen enters the body, IgE bound to mast cells triggers the release of histamine and other chemicals that cause allergy symptoms. A high total IgE indicates the immune system is primed for allergic responses, but it does not identify which specific allergen is the trigger. Total IgE is also elevated in parasitic infections (common worldwide but rare in Australia), atopic dermatitis (eczema), allergic bronchopulmonary aspergillosis, and some immune deficiency syndromes.
How to interpret: Normal range: approximately 0–100 kU/L in adults, but varies significantly with age. Children typically have lower levels that increase through adolescence. A high total IgE (above 200 kU/L) strongly suggests an atopic (allergy-prone) constitution but does not diagnose a specific allergy. A normal total IgE does NOT rule out allergy — you can have significant specific IgE to a particular allergen while total IgE is normal.
Important limitations: Total IgE is a screening test, not a diagnostic test. It tells your doctor that allergy is likely but cannot identify the specific trigger. Approximately 20% of people with confirmed allergies have a normal total IgE. Conversely, smokers, patients with parasitic infections, and those with eczema may have elevated total IgE without classical allergies. Specific IgE testing is needed to identify individual triggers.
Specific IgE (RAST / ImmunoCAP)
What it measures: Specific IgE testing measures antibodies against individual allergens — house dust mites, specific pollens, cat dander, peanut, egg, milk, and hundreds more. The modern test is called ImmunoCAP (manufactured by Thermo Fisher), which has largely replaced the older RAST (radioallergosorbent test), though many doctors and patients still use the term "RAST test." Your blood is exposed to purified allergen extracts, and the amount of IgE that binds is measured. This tells your doctor whether your immune system has been sensitised to that specific allergen.
How to interpret: Results are reported as kU/L with a class system: Class 0 (below 0.35 kU/L) is negative. Class 1 (0.35–0.70) is low positive. Class 2 (0.70–3.50) is moderate. Class 3 (3.50–17.50) is high. Class 4+ (above 17.50) is very high. Higher levels generally correlate with greater likelihood of clinical allergy, but the relationship is not linear — some people with Class 2 results have severe reactions, while others with Class 4 results tolerate the allergen without symptoms.
Important limitations: A positive specific IgE result means sensitisation (your immune system recognises the allergen) but does NOT necessarily mean clinical allergy (that you will have symptoms on exposure). Approximately 50–60% of people with positive specific IgE to foods can eat that food without symptoms. This is why allergy testing should always be interpreted alongside your clinical history — never eliminate foods based solely on a blood test result. False negatives can occur if you are tested too early after sensitisation.
Allergen Component Testing (Molecular Allergy Diagnostics)
What it measures: Component testing is a newer, more precise form of specific IgE testing that measures antibodies to individual proteins within an allergen source rather than the whole extract. For example, peanut contains multiple allergenic proteins — Ara h 2 is associated with severe, systemic reactions (anaphylaxis risk), while Ara h 8 is a cross-reactive protein that usually causes only mild oral symptoms. Similarly, egg allergy can be directed against ovomucoid (Gal d 1, heat-resistant, meaning all forms of egg trigger reactions) or ovalbumin (Gal d 2, heat-sensitive, meaning baked egg is often tolerated). This information is clinically transformative because it guides risk assessment and dietary advice.
How to interpret: Results are reported in kU/L similar to standard specific IgE. The clinical significance depends on WHICH component is positive. For peanut: positive Ara h 2 indicates high anaphylaxis risk; positive Ara h 8 alone suggests oral allergy syndrome (mild, pollen-related). For egg: positive Gal d 1 suggests all egg forms will trigger reactions; positive Gal d 2 alone suggests baked egg may be tolerated. Your allergist will interpret component results in the context of your clinical history.
Important limitations: Component testing is more expensive than standard specific IgE and is not always available at general pathology providers in Australia. It is typically ordered by allergists and immunologists, not GPs. Medicare bulk bills some component tests when ordered by a specialist with a clinical indication, but others may incur out-of-pocket costs of $50–$150. Not all allergen sources have well-characterised components yet.
Common Allergen Panels Available in Australia
| Panel | Allergens Tested | Common Use |
|---|---|---|
| Inhalant / Respiratory Panel | House dust mite, cat dander, dog dander, ryegrass, Bermuda grass, Alternaria mould, Aspergillus, cockroach | Hay fever, allergic rhinitis, asthma, chronic sinusitis |
| Common Food Panel | Cow’s milk, egg white, wheat, soy, peanut, tree nuts (cashew, almond, walnut, hazelnut), fish, shellfish (prawn, crab) | Suspected food allergy in children and adults, eczema triggers |
| Paediatric Panel | Cow’s milk, egg, peanut, wheat, soy, tree nuts, fish, house dust mite, cat, dog | Initial allergy screening in infants and young children |
| Insect Venom Panel | Bee venom, wasp venom, jack jumper ant (Myrmecia pilosula) | History of anaphylaxis after insect sting. Jack jumper ant is unique to south-eastern Australia. |
| Drug Allergy Panel | Penicilloyl G, penicilloyl V, amoxicilloyl, cefaclor, chlorhexidine, latex | Suspected drug allergy. Limited availability — specialist referral usually required. |
Blood Test vs Skin Prick Test: Which Is Better?
| Feature | Blood Test (Specific IgE) | Skin Prick Test (SPT) |
|---|---|---|
| How it works | Measures specific IgE antibodies in blood sample | Allergen applied to pricked skin; wheal measured after 15 minutes |
| Where performed | Any pathology lab — blood draw only | Allergy clinic only (specialist setting) |
| Time to results | 2–5 business days | 15–20 minutes (immediate) |
| Affected by antihistamines | No — antihistamines do NOT affect blood IgE | Yes — must stop antihistamines 3–7 days before |
| Risk of allergic reaction | None (blood sample in lab) | Very small risk of systemic reaction (supervised setting) |
| Suitable for severe eczema | Yes — unaffected by skin condition | No — extensive eczema prevents accurate reading |
| Sensitivity | Slightly lower than skin prick for some allergens | Slightly higher sensitivity overall |
| Number of allergens per visit | Unlimited (limited by cost, not procedure) | Typically 20–40 allergens per session |
| Cost (Australia) | Bulk billed (4 allergens per GP request) or private ($15–40 per allergen) | Specialist consultation fee applies ($80–$250 gap after Medicare rebate) |
| Best for | Young children, patients on antihistamines, severe eczema, remote areas | Initial comprehensive assessment, immediate results, clinical correlation |
Food Allergy vs Food Intolerance: Different Tests Entirely
One of the most common sources of confusion in allergy testing is the difference between food allergy and food intolerance. They involve completely different immune mechanisms and require different tests:
Food Allergy (IgE-mediated)
Immune system produces IgE antibodies against food proteins
Symptoms occur within minutes to 2 hours of eating
Can cause anaphylaxis (life-threatening)
Diagnosed by: specific IgE blood test or skin prick test
Even tiny amounts of food can trigger a reaction
Common triggers: peanut, tree nuts, egg, milk, shellfish
Food Intolerance (non-IgE)
NOT an immune-mediated response (usually enzyme deficiency)
Symptoms are delayed (hours to days) and dose-dependent
Never causes anaphylaxis
Diagnosed by: elimination diet + oral challenge (gold standard)
Small amounts of the food are often tolerated
Common triggers: lactose, fructose, food additives, salicylates
What to Ask Your GP
Script for your GP appointment:
“I've been experiencing hives / hay fever / eczema flares / suspected food reactions / insect sting reactions and I suspect allergies. Could we test specific IgE for my suspected triggers? I understand Medicare covers up to 4 allergens per request. If I need broader testing, could you refer me to an allergist?”
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Information sourced from the Australasian Society of Clinical Immunology and Allergy (ASCIA) and the Royal College of Pathologists of Australasia (RCPA). SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment.