Allergies affect millions of people worldwide, causing symptoms that range from mildly irritating to potentially life-threatening. If you suspect you have allergies but are unsure what is triggering your symptoms, an allergy blood test can provide valuable answers. This comprehensive guide explains how allergy blood tests work, what they can detect, and how to understand your results.

What is an Allergy Blood Test?

An allergy blood test, also known as a specific IgE blood test, measures the levels of immunoglobulin E (IgE) antibodies in your blood. IgE antibodies are produced by your immune system in response to substances it perceives as harmful – even when those substances (allergens) are actually harmless to most people.

When you have an allergy, your immune system overreacts to a specific allergen by producing IgE antibodies specific to that substance. These antibodies trigger the release of chemicals like histamine, which cause the symptoms we associate with allergic reactions.

By measuring the levels of specific IgE antibodies in your blood, allergy tests can identify which substances you are sensitised to and may be causing your symptoms.

How Does Allergy Blood Testing Work?

The most common type of allergy blood test is the specific IgE test (previously known as RAST testing). Here is how it works:

A blood sample is taken from a vein in your arm

The sample is sent to a laboratory

Your blood is tested against a panel of common allergens

The laboratory measures the amount of IgE antibodies produced in response to each allergen

Results indicate which allergens triggered an immune response and the level of sensitisation

For more information about what to expect during a blood draw, read our guide on blood testing procedures, results, and after effects.

What Can Allergy Blood Tests Detect?

Allergy blood tests can identify sensitisation to a wide range of allergens, including:

Food Allergens:

Peanuts and tree nuts (almonds, cashews, walnuts, etc.)

Milk and dairy products

Eggs

Wheat and gluten

Soy

Fish and shellfish

Sesame

Environmental Allergens:

Dust mites

Pollen (grass, tree, weed)

Mould spores

Pet dander (cat, dog, and other animals)

Cockroach allergens

Other Allergens:

Insect venom (bee, wasp)

Latex

Certain medications

At Private Blood Tests London, we offer comprehensive allergy testing panels. Visit our [allergy test page](/allergy-test) to learn more about our available options.

Allergy Blood Test vs. Skin Prick Test

There are two main methods for allergy testing: blood tests and skin prick tests. Each has its advantages:

Allergy Blood Test Advantages:

Safe for people with severe allergies or skin conditions

Not affected by antihistamines or other medications

Can be performed regardless of skin condition (eczema, dermatitis)

No risk of allergic reaction during the test

Can test for many allergens from a single blood sample

Suitable for young children who may not tolerate skin testing

Skin Prick Test Advantages:

Results available within 15-20 minutes

Generally less expensive

Can test for a wide range of allergens in one session

Your healthcare provider can help determine which testing method is most appropriate for your situation.

The Allergy Blood Test Procedure

Having an allergy blood test is straightforward and convenient:

Preparation

One of the key advantages of allergy blood testing is that no special preparation is required. Unlike skin prick tests, you do not need to stop taking antihistamines or other allergy medications before the test. You can eat and drink normally beforehand.

During the Test

The test involves a simple blood draw. A healthcare professional will collect a blood sample from a vein in your arm – the process takes just a few minutes and causes minimal discomfort.

After the Test

You can resume all normal activities immediately. The blood sample is sent to a laboratory for analysis, and results are typically available within a few days.

Understanding Your Allergy Blood Test Results

Allergy blood test results are usually reported in one of two ways:

Quantitative Results (kU/L or IU/mL):

These show the actual concentration of specific IgE antibodies in your blood:

Less than 0.35 kU/L – Negative (no sensitisation detected)

0.35-0.69 kU/L – Low level of sensitisation

0.70-3.49 kU/L – Moderate level of sensitisation

3.50-17.49 kU/L – High level of sensitisation

17.50-49.99 kU/L – Very high level of sensitisation

50.00-100.00 kU/L – Extremely high level of sensitisation

Class Results (0-6):

Some laboratories report results using a class system:

Class 0 – Negative

Class 1 – Low positive

Class 2 – Moderate positive

Class 3-6 – Increasingly high positive results

Important Considerations About Results

It is crucial to understand what allergy blood test results do and do not tell you:

Sensitisation vs. Clinical Allergy

A positive blood test indicates sensitisation – meaning your immune system has produced IgE antibodies against that allergen. However, sensitisation does not always mean you will experience symptoms when exposed to the allergen. Some people have positive test results but no clinical symptoms.

Severity of Reactions

Higher IgE levels do not necessarily predict more severe allergic reactions. Someone with moderately elevated IgE to peanuts could have more severe reactions than someone with very high levels.

False Negatives

Occasionally, people with genuine allergies may have negative blood test results, particularly if:

The allergy has developed recently

IgE levels have declined over time

The specific allergen protein was not included in the test panel

For these reasons, allergy test results should always be interpreted alongside your clinical history and symptoms.

Who Should Consider Allergy Blood Testing?

Allergy blood testing may be appropriate if you:

Experience symptoms that could be allergic (hives, itching, swelling, breathing difficulties, digestive issues)

Have had a suspected allergic reaction and want to identify the cause

Have eczema or other skin conditions that make skin prick testing difficult

Cannot stop taking antihistamines

Have a history of severe allergic reactions (anaphylaxis)

Want to monitor known allergies over time

Need testing for a young child

Managing Allergies After Testing

Once you have identified your allergens, management typically involves:

Avoidance – The primary strategy for managing allergies

Medications – Antihistamines, nasal sprays, or other treatments to manage symptoms

Emergency preparedness – Carrying adrenaline auto-injectors if at risk of anaphylaxis

Immunotherapy – Desensitisation treatment for certain allergies

When to Seek Medical Advice

At Private Blood Tests London, we provide your raw allergy test data. We recommend sharing your results with your GP or an allergy specialist who can:

Interpret results in the context of your symptoms and history

Provide a clinical diagnosis

Recommend appropriate management strategies

Refer you for specialist care if needed

Book Your Allergy Blood Test

Understanding your allergies is the first step toward better management and improved quality of life. At Private Blood Tests London, we offer comprehensive allergy testing at our CQC-registered clinic in South Kensington. Our experienced team ensures a comfortable testing experience, and your results are delivered quickly and securely.

Explore our [allergy test services](/allergy-test) and [health screening packages](/health-screening).

Take control of your allergies – book your allergy blood test today.

Allergy Blood Test: Everything You Need to Know