Allergy Testing Explained: Skin Prick Tests, Blood Tests and Food Challenges

A positive allergy test does not always mean you are allergic. The most reliable answer comes from combining the right test with a careful history of what actually happens when you are exposed.

Imagine your child breaking out in hives after eating a peanut biscuit. You are terrified it will happen again. An allergy clinic uses a tiny skin prick, a blood test and your story to work out exactly what is safe.

Why testing is done

Allergy tests help confirm or rule out allergies to foods, pollen, dust mites, animals, insect stings, medicines and latex. They guide what to avoid, whether an adrenaline auto-injector is needed and whether treatments such as immunotherapy might help.,Testing is most useful when it is guided by symptoms. Testing for a long list of allergens without a clear history often produces positive results that do not mean anything, which can lead people to avoid foods unnecessarily.

Skin prick testing

A drop of liquid containing a possible allergen is placed on the forearm or back, and the skin is gently pricked through it with a tiny lancet. It feels like a light scratch. After about 15 minutes, a raised, itchy bump (wheal) shows a reaction. The size is measured and compared with control tests.,Skin prick tests give quick results and are usually done in specialist clinics, where staff can treat a rare severe reaction.

Stopping antihistamines first

Antihistamines block skin prick test reactions and can make results falsely negative. You will usually be asked to stop them for several days before the test; check how long with your clinic. Do not stop other medicines, such as asthma inhalers, unless advised. Some antidepressants can also affect results, so bring a list of everything you take.

Specific IgE blood tests

A blood test measures IgE antibodies to specific allergens. It is useful when skin testing is not possible, for example in people with widespread eczema, those who cannot stop antihistamines, or very young children.,Component tests can look at specific proteins within a food. For example, with peanut, certain components are linked to more severe reactions, while others are associated with milder symptoms or cross-reactions with pollen.

What a positive result means

A positive test shows sensitisation, meaning the immune system has made IgE antibodies to that substance. It does not prove that eating or touching it will cause a reaction. Many people are sensitised without being allergic. This is why results must be interpreted by someone experienced, alongside your history.

Patch testing

Patch testing is used for delayed reactions such as allergic contact dermatitis, where the skin reacts to substances like nickel, fragrances, preservatives or rubber chemicals. Small amounts of allergens are taped to your back for about 48 hours, and your skin is checked over several days.

Food challenges

An oral food challenge is the most accurate way to confirm or rule out a food allergy. Under close medical supervision, you or your child eats gradually increasing amounts of the food, while staff watch for any reaction.,It is often used to check whether a child has outgrown an allergy, or when test results are unclear. It must only be done in a setting where reactions can be treated quickly.

Tests to avoid

• IgG food intolerance tests, which are not recommended by allergy experts for diagnosing allergy or intolerance
• Hair analysis
• Kinesiology or muscle strength testing
• Vega or bioresonance tests
• These can lead to unnecessary diet restrictions and missed diagnoses

Signs of anaphylaxis

• Swelling of the throat or tongue
• Difficulty breathing, wheezing or a hoarse voice
• Feeling faint, confused or collapsing
• Pale, clammy skin
• Use an adrenaline auto-injector if prescribed and call emergency services immediately

Allergen immunotherapy

For some allergies, treatment can change how the immune system responds. Allergen immunotherapy involves regular exposure to small, gradually increasing amounts of an allergen, as injections or tablets or drops under the tongue, usually over three years.

It is used for severe hay fever, dust mite allergy and insect sting allergy, and in some countries for peanut allergy in children through oral immunotherapy. It must be supervised by an allergy specialist, because reactions can occur.

Living with a food allergy

• Read food labels every time, as recipes change
• Ask about ingredients when eating out
• Carry two adrenaline auto-injectors if prescribed, and check expiry dates
• Make sure family, school and friends know your action plan
• Wear medical ID

Allergies that are often misunderstood

Many people believe they are allergic to penicillin because of a rash in childhood, but most of them are not truly allergic when tested. Being wrongly labelled can mean receiving less effective or broader antibiotics, which contributes to antibiotic resistance. Some hospitals offer penicillin allergy testing to remove incorrect labels safely.

Similarly, many people avoid foods because of symptoms such as bloating or tiredness, which are not caused by allergy. Food intolerances are real and can be uncomfortable, but they do not involve the immune system in the same way and are not diagnosed with allergy tests.

Frequently asked questions

Are allergy tests safe for children?

Yes, when done by trained staff. Skin prick tests can be done in babies.

Can allergies change over time?

Yes. Some childhood allergies, such as milk and egg, are often outgrown, while others like peanut often persist.

Do I need to stop my asthma inhaler?

Usually not. Always check with your clinic.

But what about home allergy test kits?

Many home kits measure IgG or test a long list of foods without medical interpretation. Results can be misleading. Speak to a healthcare provider before cutting foods from your diet.

Related: Hives

Sources: British Society for Allergy and Clinical Immunology, World Allergy Organization, American Academy of Allergy, Asthma and Immunology, National Institute for Health and Care Excellence.