ALEX3 Allergy Blood Test in Wimbledon: A GP Answers FAQs

If you think you or your child may have an allergy, you have probably come across the ALEX3 allergy blood test. It is a laboratory test that measures specific IgE antibodies to a large number of allergens from a single blood sample. But what does ALEX3 actually test for? Is it as reliable as a skin-prick test? Can you take antihistamines beforehand? And does a positive result mean you are definitely allergic?

As a GP practising at Mayfield Clinic Wimbledon, these are questions I hear regularly from patients considering allergy testing. Below, I have answered the ones that come up most often.

What is the ALEX3 allergy blood test, and what does it check for?

ALEX3 is a laboratory blood test that measures specific IgE antibodies associated with IgE-mediated allergic reactions. The ALEX3 profile tests around 300 allergens, together with total IgE. It covers a wide range of potential allergens, including grass, tree and weed pollens, house dust mites, cats, dogs and other animals, moulds, bee and wasp venoms, latex, milk and egg, peanut and tree nuts, wheat and other cereals, soya and other legumes, fish and shellfish, and a range of fruits and other foods.

ALEX3 also includes molecular allergen components. Rather than only testing an extract from an allergen source, component testing can identify IgE directed against individual proteins within that source. This can sometimes provide additional information about a person's pattern of sensitisation and may help a suitably trained clinician assess the clinical significance of a result. For example, some people sensitised to birch pollen may also have IgE to related proteins in certain raw fruits or nuts, which can be associated with symptoms such as itching or tingling in the mouth.

However, a component result does not by itself predict exactly how severe an allergic reaction will be. Results need to be considered alongside the person's symptoms, the timing of previous reactions and their exposure history. At Mayfield Clinic Wimbledon, your clinical history helps determine whether ALEX3 is appropriate, or whether a more targeted allergy blood test or another form of assessment may be more suitable.

Is ALEX3 as reliable as a skin-prick test, and can I take antihistamines?

Both specific IgE blood testing and skin-prick testing can be used to investigate suspected IgE-mediated allergy. The appropriate test depends on the clinical history, the individual circumstances and what testing is suitable and available.

NICE guidance for children and young people with suspected food allergy recommends choosing specific IgE blood testing and/or skin-prick testing based on the allergy-focused clinical history and what is suitable, safe and acceptable. NICE also recommends interpreting the results in the context of the clinical history.

An allergy blood test has some practical advantages. Antihistamines do not affect the measurement of specific IgE in the blood in the way they can affect skin-prick testing. It can be useful if you have eczema or other skin problems that make skin testing difficult. A large number of allergens can be tested from one blood sample, and because a blood test does not expose you to the allergen, it cannot itself provoke an allergic reaction.

Skin-prick testing, usually carried out in a specialist allergy clinic, gives a result during the appointment and is particularly useful when one specific allergen is suspected. If you are having a skin-prick test, you may be asked to stop antihistamines beforehand because they can suppress the skin response. You generally do not need to stop antihistamines simply to have an ALEX3 blood test.

The important point is that neither test replaces a good allergy history, and a test result should not be interpreted on its own.

Can the ALEX3 test tell me if I have a food intolerance?

No. ALEX3 is an allergy test, not a general food intolerance test. It measures IgE antibodies and is designed to assess IgE-mediated allergy. These reactions commonly develop within minutes to a few hours of exposure and can cause symptoms such as hives or an itchy rash, swelling, sneezing or a runny nose, wheezing or breathing difficulties, vomiting, and in severe cases, anaphylaxis.

Food intolerance is different. Symptoms such as bloating, abdominal discomfort or digestive upset after eating can have many possible causes, including lactose intolerance, irritable bowel syndrome and other gastrointestinal conditions. Coeliac disease requires separate investigation and is not diagnosed using ALEX3. There is also a distinction between IgE-mediated and non-IgE-mediated food allergy, which require different approaches to assessment.

You may also come across commercial "food intolerance" tests that measure IgG antibodies against foods. These should not be confused with IgE allergy testing. NICE advises against serum-specific IgG testing for diagnosing food allergy, as well as other unvalidated approaches such as hair analysis and applied kinesiology.

ALEX3 is also not a test for drug allergy. Suspected allergies to medicines such as penicillin require a different clinical assessment and, where appropriate, specialist allergy investigation.

What does a positive ALEX3 result actually mean?

A positive specific IgE result indicates sensitisation to the allergen tested. It does not necessarily mean that you have a clinically relevant allergy to that substance. Sensitisation and clinical allergy are not the same thing. With a broad test such as ALEX3, it is possible to find sensitisation to substances that you eat or come into contact with regularly without experiencing any symptoms.

This is why allergy test results need to be interpreted in the context of your clinical history, rather than treating every positive result as a diagnosis. NICE specifically recommends this approach for suspected food allergy in children and young people.

When your results are available, we may consider what happened when you encountered the suspected allergen, how quickly your symptoms developed, what symptoms you experienced, whether you have subsequently eaten or encountered the substance without a reaction, and whether the blood test result fits your clinical history.

A negative result does not always completely rule out allergy. If the clinical history strongly suggests an IgE-mediated allergy despite a negative test, further assessment or specialist referral may be appropriate.

If a food has previously caused a significant or immediate allergic reaction, do not try to reintroduce it at home simply because an allergy blood test is negative. Any planned food challenge or reintroduction should only take place when advised and in an appropriate setting.

Similarly, we would not normally recommend eliminating foods that you currently tolerate simply because they have produced a positive blood test. This is particularly important in children. Depending on the results and clinical history, the next step may be practical avoidance advice, treatment for allergic symptoms, further investigation or referral to an allergy specialist where appropriate.

How do I get an ALEX3 allergy blood test in Wimbledon, and how long do results take?

The first step is a GP consultation. Taking a proper allergy-focused history is an important part of deciding whether ALEX3 is actually the right investigation for you. At Mayfield Clinic Wimbledon, we can discuss your symptoms, possible triggers and previous reactions.

Depending on your history, we may consider ALEX3, a smaller targeted specific IgE blood test, referral for skin-prick testing, or another investigation or specialist referral. If ALEX3 is appropriate, the blood sample can usually be taken at the clinic and sent to the laboratory.

Results typically take a few days, although the exact timing can vary depending on sample processing and laboratory turnaround. We will inform you when your results are available and provide information about what they show. Test fees are charged separately from the GP consultation. Current prices are listed on our website and the cost will be confirmed before testing.

You can find out more about our allergy testing in Wimbledon or browse our wider range of blood tests in Wimbledon.

Important information

This article provides general information about allergy blood testing and is not a substitute for individual medical advice. If someone develops swelling of the lips, tongue or throat, difficulty breathing, feels faint or collapses, call 999 immediately rather than booking an allergy test. Anyone who has previously experienced a severe allergic reaction should tell us when booking, as they may require specialist allergy assessment.

ALEX3 can be used in children, but for suspected food allergy we may consider testing the specific foods suggested by the allergy-focused history rather than automatically performing a broad panel. Our GP can advise on the most appropriate approach, including the practicalities of taking a blood sample from younger children.

Information is correct at the time of writing. Test availability, turnaround times and prices may change.

I am Dr Umer Malik, MBBS, BSc, MRCGP, MPH, and I have 17 years of postgraduate clinical experience, including 9 years as a fully qualified GP. I currently practise at Mayfield Clinic Wimbledon, where I regularly assess patients with allergic symptoms and help decide when allergy testing may be appropriate.

If you would like to discuss whether ALEX3 or another allergy investigation is right for you, you can book an appointment at Mayfield Clinic Wimbledon.