Allergy testing

No special preparation is needed. A blood sample will be taken from a vein in your arm. 

Unlike skin prick testing, you don’t need to stop taking antihistamines before a blood allergy test. This is one of the main advantages of blood testing. 

Before testing, your doctor should take a detailed history of your symptoms to work out what might be triggering your reactions. This helps them choose the right allergens to test for and prevents unnecessary testing. Your GP will not recommend testing if the symptoms are not consistent with an allergic reaction. If the trigger is clear and can’t be avoided (for example pollen) then testing may not be helpful. 

You may be asked about when symptoms occur, what you were doing or eating at the time, how severe the reactions are, whether they happen repeatedly and whether anyone in your family has allergies.

What the test measures 

Allergy blood tests measure specific IgE antibodies in your blood. IgE (immunoglobulin E) is a type of antibody that’s part of your immune system. 

How allergic reactions work 

Normally, IgE helps fight infections. However, in people with allergies, the immune system mistakenly makes IgE antibodies to harmless substances like pollen, certain foods, or animal fur. We call these harmless substances ​‘allergens’ when they trigger allergies. 

The first time you’re exposed to an allergen, your body may make specific IgE antibodies against it. These IgE antibodies attach to special cells in your body called mast cells (in tissues) and basophils (in blood). This process is called sensitisation. 

When you encounter the allergen again, it binds to the IgE antibodies on these cells. This triggers the cells to release chemicals like histamine, which cause allergy symptoms – sneezing, itching, swelling, and in severe cases, anaphylaxis. 

Each specific IgE antibody only reacts with one particular allergen. For example, if you’re allergic to peanuts, you’ll have peanut-specific IgE antibodies. 

What gets tested 

The laboratory measures specific IgE levels for particular allergens. Based on your symptoms and history, and whether you are seeing a GP or allergy specialist, your doctor will choose which allergens to test. 

Common allergens tested include: 

• Airborne allergens: grass pollen, tree pollen, house dust mite, mould spores, animal dander (cat, dog) 
• Food allergens: peanuts, tree nuts, milk, eggs, wheat, soy, fish, shellfish 
• Insect allergens: bee venom, wasp venom 

Tests can be done for single allergens or as panels (groups). For example, a ​‘grass mix’ panel tests several grass types at once if you have symptoms outdoors but aren’t sure which grass is the trigger. 

You may also hear these tests called RAST tests. This stands for radioallergosorbent test, an older method that’s now been replaced by newer, more accurate technologies. 

What your results mean 

Allergy test results must always be interpreted alongside your symptoms and medical history. The numbers alone don’t tell the whole story. 

Negative or normal results 

A negative result suggests your symptoms are probably not caused by an allergic reaction to the tested allergen. However, no test is perfect – a negative result doesn’t completely rule out allergy, especially if it is something you have not come across for a long time. 

If you have ongoing symptoms but negative results, you may need different allergens tested, or your symptoms might be caused by something other than IgE-mediated allergy (like food intolerance or another condition). 

Positive results 

A positive result indicates you have specific IgE antibodies to that allergen. This means you’re sensitised to it and it may be causing your symptoms. 

However – and this is crucial – not everyone with positive test results has symptoms when they encounter the allergen. You can be sensitised (have IgE antibodies) without being truly allergic (having symptoms). 

This is why your doctor needs to match the test results with your actual symptoms and experiences. 

Understanding IgE levels 

Results are usually given as a level (often in kUA/​L – kilo units of allergen-specific IgE per litre) and sometimes as a class (0 to 6), although the latter is outdated. 

Higher levels generally indicate stronger sensitisation, but the level doesn’t reliably predict how severe your reaction will be. Someone with a low positive result might have severe symptoms, whilst someone with high levels might have mild symptoms. 

The amount of specific IgE also can’t tell you whether you’ve outgrown an allergy. For this, your doctor may recommend supervised food challenges. 

Total IgE 

Sometimes doctors also measure total IgE (all IgE, not just specific types). High total IgE usually means you have elevated levels of specific IgE antibodies. This state is called being ​‘atopic’ – having a genetic tendency to develop allergic diseases. 

However, total IgE alone can’t identify what you’re allergic to – you need specific IgE tests for that. 

If results are positive 

Your doctor will help you understand what the results mean for you specifically, based on your symptoms and history. 

Allergen avoidance: 

The first step is usually avoiding the allergen where possible. Your doctor will give you specific advice based on what you’re allergic to. 

Medications: 

• Antihistamines (for mild symptoms like sneezing, itching, runny nose) 
• Nasal steroid sprays (for allergic rhinitis) 
• Asthma inhalers (if you have allergic asthma) 
• Adrenaline auto-injector (if you’re at risk of anaphylaxis) 

Emergency action plan: 

If you’ve had or are at risk of severe reactions (anaphylaxis), your doctor will provide an emergency action plan explaining what to do if you’re exposed to the allergen. You’ll be prescribed adrenaline auto-injectors and taught how to use them. 

Referral to specialist: 

You may be referred to an allergy specialist, particularly if you have severe allergies, multiple allergies, or if your allergy is affecting your quality of life significantly. 

Immunotherapy: 

For certain unavoidable allergens (grass pollen, tree pollen, house dust mite, bee and wasp venom), immunotherapy may be an option. This involves gradually exposing you to increasing amounts of the allergen to reduce your sensitivity. 

Immunotherapy is only available at specialist allergy centres. New immunotherapy treatments for specific food allergies (like peanut) are also being developed. 

Currently, there’s no clear evidence that specific IgE tests are useful for monitoring response to immunotherapy. 

If results are negative 

If tests are negative but you still have symptoms, your doctor will investigate other possibilities. 

This might include: 

• Testing for different allergens 
• Considering non-IgE mediated allergies or intolerances 
• Testing for coeliac disease (if wheat/​gluten symptoms) 
• Investigating other conditions that mimic allergies 
• Referral for specialist assessment and possible supervised challenge testing 

Blood allergy tests are generally very reliable, but several factors can influence results: 

• Medications: Unlike skin tests, antihistamines don’t affect blood test results. However, some immunosuppressive medications might affect IgE production 
• Age: Young children may not have developed IgE antibodies yet even if allergic 
• Other conditions: Parasitic infections can raise total IgE levels 
• Cross-reactivity: Some allergens are chemically similar, so IgE to one might cross-react with another 

Allergy testing often involves multiple approaches to get the clearest picture. 

Skin prick tests: 

Small amounts of allergen are placed on your skin (usually forearm) and the skin is gently pricked. Results are available in 15–20 minutes. These must be done in specialist allergy clinics under medical supervision as reactions are possible. 

Challenge testing: 

You’re given increasing amounts of a suspected allergen under close medical supervision to see if it triggers symptoms. This is particularly useful for food allergies and determining if allergies have been outgrown. Only done in specialist clinics due to risk of severe reactions. 

Patch tests: 

Used for contact dermatitis (delayed skin reactions). Small amounts of substances are placed on patches stuck to your back for 48 hours. These test different reactions (not IgE-mediated) and are used for things like cosmetic ingredients, nickel, or fragrances that cause itchy, blistered rashes. 

Full blood count: 

May show elevated eosinophils (a type of white blood cell) in allergic conditions. 

Total IgE: 

Measures all IgE antibodies. High levels suggest you’re atopic (prone to allergies) but don’t identify specific allergens. 

Allergies occur when your immune system overreacts to harmless substances. We don’t fully understand why some people develop allergies and others don’t, but genetics play a big role. 

Allergies are very common. In the UK, around 1 in 4 people will have an allergy at some point. Common allergic conditions include hay fever (allergic rhinitis), asthma, eczema, and food allergies. 

Most allergic reactions are mild – sneezing, itching, or rashes. However, some people have severe reactions called anaphylaxis. This is a medical emergency involving difficulty breathing, swelling of the throat, severe drop in blood pressure, and potential loss of consciousness. 

Although children do outgrow some allergies (particularly milk and egg), adults usually don’t. Allergies causing severe reactions (like peanut allergy) rarely disappear on their own. 

Cross-reactivity is important to understand. Some allergens are chemically similar, so if you’re allergic to one, you might react to related ones. For example, people allergic to tree pollen often develop oral allergy syndrome when eating certain fresh fruits (tingling or scratchy sensation in mouth or throat). This happens because proteins in the fruit are similar to tree pollen proteins. 

Learn more about allergies on the NHS website. Support organisations like Allergy UK and the Anaphylaxis Campaign provide helpful information and support.