Allergy testing is more nuanced than most patients expect. A positive result doesn't always mean you'll react to something in real life — and a negative result doesn't always mean you're safe. Understanding what the tests actually measure, and what the numbers mean, helps you get the most from the results.
What allergy tests measure
Most standard allergy tests detect sensitisation — the presence of IgE antibodies specific to an allergen. IgE (immunoglobulin E) is the antibody class involved in the immediate hypersensitivity reactions that cause hay fever, hives, asthma and anaphylaxis. Sensitisation means your immune system has built a response; it doesn't always mean that response will cause symptoms every time you encounter the allergen.
Skin prick test (SPT)
A small drop of allergen extract is placed on the forearm, then a lancet creates a tiny scratch through it. If IgE antibodies on skin mast cells recognise the allergen, a raised, itchy wheal — like a mosquito bite — forms within 15–20 minutes.
A wheal diameter of 3 mm or more (above the negative control) is typically considered positive. Larger wheals generally correlate with higher sensitivity, but size is not a reliable predictor of reaction severity in real life.
Skin prick tests are fast, inexpensive and sensitive. They're usually the first-line test for airborne allergens (pollens, dust mite, pet dander) and common food allergens.
IgE blood tests (RAST / specific IgE)
Specific IgE blood tests (once called RAST, now using fluorescent enzyme immunoassays) measure the concentration of IgE antibodies to a specific allergen in your blood. Results are reported in kilounits per litre (kU/L) and graded on a class scale from 0 to 6.
- **Class 0 (<0.35 kU/L):** Absent — no sensitisation detected
- **Class 1 (0.35–0.7 kU/L):** Low — borderline, clinical significance uncertain
- **Class 2 (0.7–3.5 kU/L):** Moderate sensitisation
- **Class 3–4 (3.5–17.5 kU/L):** High sensitisation
- **Class 5–6 (>17.5 kU/L):** Very high sensitisation
The difference between sensitisation and allergy
Approximately 30–40% of people who test positive for sensitisation to common allergens never experience symptoms when they encounter them. Conversely, some people with genuine allergies test negative on standard panels — particularly for some food allergies where the relevant proteins are not well represented in commercial extracts.
The gold standard for food allergy diagnosis remains the oral food challenge — supervised consumption of the suspected food under medical supervision — precisely because test results alone are insufficient.
Component testing
Molecular (component-resolved) allergy testing goes further by identifying which specific proteins within an allergen you're sensitised to. This matters clinically: for example, a peanut-allergic patient sensitised to Ara h 2 (a stable storage protein) has a much higher risk of anaphylaxis than one sensitised only to Ara h 8 (a heat-labile cross-reactive protein common in pollen-food allergy syndrome). Component testing helps distinguish true food allergy from cross-reactivity.
How to use allergy test results
Allergy results should always be interpreted alongside your clinical history — what you've actually reacted to, how severe those reactions were, and whether they're reproducible. A positive test for an allergen you've never reacted to is less clinically significant than a positive test for something that reliably causes you symptoms. Store your allergy test results with your health record so every provider you see has an accurate, complete allergy history.