How Allergy Works
Allergic reactions involve a specific type of antibody called IgE (Immunoglobulin E). When you are first exposed to an allergen (e.g. peanut protein), your immune system can become "sensitised" — it produces specific IgE antibodies that attach to mast cells. On subsequent exposure, the allergen bridges these IgE antibodies, triggering mast cells to release histamine and other inflammatory chemicals, causing allergy symptoms (hives, itching, sneezing, anaphylaxis).
Total IgE Levels
| Category | Total IgE Level (IU/mL) | Interpretation |
|---|---|---|
| Normal | <100 IU/mL | No significant atopic tendency in most adults |
| Mildly elevated | 100 – 400 IU/mL | Mild atopic tendency; may have allergic condition |
| Significantly elevated | 400 – 2,000 IU/mL | Significant atopy; consider eczema, allergic asthma, rhinitis |
| Very high | >2,000 IU/mL | Severe atopy, or consider parasitic infection (especially intestinal worms in endemic areas), or hyper-IgE syndrome |
Note: A normal total IgE does NOT exclude specific allergies. Some patients have specific IgE sensitisation with normal total IgE.
Specific IgE (RAST / ImmunoCAP) Class System
| Class | Specific IgE Level (kIU/L) | Interpretation |
|---|---|---|
| Class 0 | <0.35 | Undetectable — no sensitisation detected |
| Class 1 | 0.35 – 0.7 | Very low — doubtful clinical relevance |
| Class 2 | 0.7 – 3.5 | Low — possible sensitisation; correlate with symptoms |
| Class 3 | 3.5 – 17.5 | Moderate — clinically relevant sensitisation likely |
| Class 4 | 17.5 – 50 | High — clinically relevant; significant allergy |
| Class 5 | 50 – 100 | Very high — severe sensitisation |
| Class 6 | >100 | Very high — severe sensitisation; anaphylaxis risk |
Important: A positive specific IgE (sensitisation) does NOT always mean clinical allergy. Up to 50% of people with positive IgE may tolerate the food without symptoms. Always interpret results alongside the clinical history. An allergist can arrange controlled food challenges to confirm clinical allergy.
Common Allergens Tested
- Inhalant allergens: House dust mite (Der p1, Der f1), cat dander (Fel d1), dog dander (Can f1), grass pollens, tree pollens, mould (Alternaria, Cladosporium)
- Food panel (children): Peanut (Ara h 2 component), hen's egg (white/yolk), cow's milk, wheat (Tri a 19 component), soy, tree nuts
- Food panel (adults): Shellfish, fish, peanut, sesame, tree nuts
- Venom: Bee venom (Api m1), wasp venom (Ves v5) for stinging insect allergy
- Drug allergy: Beta-lactam antibiotic (penicillin) IgE testing
Blood Test vs Skin Prick Test — Comparison
| Feature | Blood Test (Specific IgE) | Skin Prick Test (SPT) |
|---|---|---|
| Effect of antihistamines | No effect — can test while on antihistamines | Must stop antihistamines 3–5 days before |
| Anaphylaxis risk | None — blood test only | Very rare but possible; done where resuscitation available |
| Suitable for severe eczema | Yes — no skin needed | Difficult if skin is affected |
| Speed of results | Days (lab turnaround) | Immediate (15–20 minutes) |
| Sensitivity | Slightly less sensitive than SPT | Slightly more sensitive; gold standard for inhalant allergens |
| Allergen range | Wide range available as panels | Limited to available allergen extracts |
Questions to Ask Your Doctor
- Should I test total IgE first, or go directly to specific allergens?
- Which specific allergens should be tested based on my symptoms?
- A positive IgE result was found — does this mean I'm definitely allergic to this food?
- Should I be referred to an allergist for a supervised food challenge?
- Do I need to carry an adrenaline auto-injector (EpiPen) based on my results?
Medical Disclaimer: This information is for educational purposes only. Allergy testing should be interpreted by a qualified allergist in conjunction with clinical history. Do not undertake food challenges without medical supervision.