Both allergy blood tests and skin prick tests measure IgE sensitisation — the immune mechanism behind most allergic reactions. They aren't alternatives that give different answers; they're different ways of measuring the same thing. Which is better depends on what you need from the test.
How Each Test Works
Skin prick test (SPT): A small drop of allergen extract is placed on the forearm and the skin is briefly pricked to allow it to enter. After 15–20 minutes, the clinician measures the size of any wheal (raised bump) that forms. A wheal above a certain size indicates sensitisation.
Allergy blood test (specific IgE): A blood sample is taken and analysed in a laboratory for IgE antibodies specific to individual allergens. Results are returned as a numerical value (measured in kU/L) indicating the level of sensitisation to each allergen tested.
Head-to-Head Comparison
| Skin Prick Test | ALEX3 Blood Test | |
|---|---|---|
| Number of allergens tested | Typically 20–40 in a session | 300+ from one blood draw |
| Antihistamines required to stop? | Yes — must stop 3–7 days before | No — can take antihistamines |
| Comfort | Multiple pricks; can cause itching during the test | Single venepuncture only |
| Suitable for children | Yes, but distressing for young children | Yes, and often preferred for children |
| Suitable for severe eczema? | Difficult — inflamed skin gives unreliable results | Yes — no skin involvement |
| Anaphylaxis risk | Very small but documented | None |
| Results turnaround | 15–20 minutes (during appointment) | 3–5 days (lab analysis) |
| Component allergen testing | Limited | Yes — includes molecular components |
| Polysensitisation detection | Depends on which extracts used | Comprehensive across all categories |
What Is Component Allergen Testing?
One significant advantage of the ALEX3 blood test is its ability to identify sensitisation to specific molecular components within an allergen source — not just the whole extract. For example, with grass pollen, ALEX3 can distinguish sensitisation to Phl p 1 and Phl p 5 (the primary sensitising proteins) from cross-reactive carbohydrate determinants (CCDs) which are clinically irrelevant. This matters because CCD sensitisation produces positive results on standard tests without causing real symptoms — a source of significant false-positive confusion.
Component testing also helps predict severity. Peanut sensitisation to Ara h 2 indicates a different risk profile from sensitisation to Ara h 8, which is predominantly a cross-reaction from birch pollen allergy. Without component data, a positive peanut result gives much less clinical information.
When Skin Prick Testing Is Preferred
Skin prick testing remains valuable in specific situations — particularly when an allergist wants immediate visual confirmation during a clinical consultation, or when testing with a specific fresh food (such as fresh fruit or vegetable) that isn't available in commercial extract form. For most routine allergy assessment, blood testing now offers superior breadth and convenience.
ALEX3 Allergy Blood Test at Lambert Medical Practice
300+ allergens tested. No need to stop antihistamines. Results within days. Book at our Surbiton clinic.
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