How to test for allergies
Allergy testing can be undertaken in many different ways, and it’s hard to know which method of testing will work best for you without knowing a little more about your options. Visiting your GP can be a useful start but they may not always be able to help, and there are many other ways to test for allergies that may provide you with more information.
Skin prick testing
Skin prick testing is the most common allergy test performed in an allergy clinic by specially trained staff. It is a simple, safe and quick test, providing results within 15-20 minutes. This will enable you to receive a diagnosis and management plan at your appointment.
The skin prick test introduces a tiny amount of allergen into the skin, eliciting a small, localised allergic response, in the form of a wheal (bump) and flare (redness) at the site of testing. These tests can be carried out on all age groups, including babies. The test feels like writing with a pen on the skin
- Skin prick testing is usually carried out on the inner forearm, but in some circumstances may be carried out on another part of the body, such as the back or thigh. For example, there is a larger area on the back or thigh to perform testing on a baby, similarly, for those with troublesome eczema. The test can be performed on any clear patch of skin
- The test allergens are selected following a discussion with your clinician and based on your history
- As few as 3 or 4 or up to about 25 allergens can be tested
- The skin is coded with a marker pen to identify the allergens to be tested
- A drop of the allergen (extract) solution is placed on the skin
- The skin is then pricked through the drop using the tip of a lancet – this can feel a little sharp but should not be painful and should not bleed.
The patient needs to avoid taking anti-histamines and certain other medications before the test. Long acting antihistamines (those that do not cause drowsiness) should be stopped for five days; short acting antihistamines can be stopped 48 hours beforehand. Many cough mixtures contain an antihistamine; therefore please tell your consultant who is performing the test, any medication that you have taken.
Intradermal testing (ID)
- Intradermal tests are used to investigate allergies to some medications e.g. penicillin and venoms of bee and wasp. The test involves a small injection of often diluted medications or venoms into the dermis of your skin.
- ID is carried out on the inner forearm
- The skin is coded with a marker pen to identify the medications/venoms to be tested
- A drop amount of the medication/venom is injected into the skin
- The results are ready within 15-20 minutes
- In case of negative intradermal skin tests to medication, the consultant may suggest a challenge with a particular medication to rule out a possibility of an allergic reaction.
Blood tests measure the amount of Immunoglobulin E (IgE) antibody circulating in the blood. The test is carried out on a small sample of blood, usually taken from a vein in the arm in the usual way. The sample is then sent to a laboratory and the results are available in 7 to 14 days.
These tests are particularly useful when skin prick testing is impractical, for example, when the patient has extensive eczema. They may also be used for someone who cannot stop taking anti-histamine medications for any period of time, and so would not be suitable for a skin prick test. Blood tests can also be used to confirm skin prick test results. For example, before a food challenge test.
There are a number of different blood tests for IgE available:
- Specific IgE (previously known as a RAST): this measures the amount of IgE to a specific food allergen, (a protein that can cause a reaction), such as a peanut or an egg. However, the test can give an elevated result without the patient having any symptoms (this is called “sensitisation”; it affects one-third of the population) and the elevated IgE is harmless. When an elevated result is seen in conjunction with symptoms to that allergen, we can term the condition an “allergy” and measures should be taken. Therefore, specific IgE testing should only be requested against an allergen against which the patient has complained of symptoms and random testing is not recommended.
- Component resolved IgE testing: also known as component resolved diagnosis (CRD) – this is a very recent development in specific IgE testing, whereby the laboratory can detect IgE to specific pieces of an allergen. Early evidence demonstrates that for some food allergens, such as peanut, a positive CRD to a particular part of the peanut protein may be more likely to indicate severe rather than mild allergy. Certain tests are therefore now available in specialist clinics using CRD methods.
Challenge Testing (Provocation test)
Like all medical tests, allergy tests have their drawbacks and are not perfect. Skin prick testing and blood tests are not always correct, so the only way to be certain that an allergy is present is to give the patient the food or medications in question under carefully controlled conditions. This is known as an allergy challenge.
The challenge test is usually offered for one of three reasons:
- This test may be used after skin prick and blood tests have suggested a food allergy but the results are inconclusive.
- This test may be offered when it is suspected that a person (often a child) has grown out of a particular food allergy, as is often the case with childhood allergies.
- Challenge testing is the main test to establish allergy to medications. Blood tests and skin tests are often not reliable in case of drug allergy and the only way to rule out or confirm allergy to a particular drug is to receive it again in graded doses under supervision. The medication can be administered orally e.g. penicillin, subcutaneously (small injections into soft tissues on your upper arm) local anaesthetics, intravenously (into your vein), intramuscularly (into your muscles on your upper arm).
Challenge tests are always undertaken in hospital under close medical supervision where resuscitation equipment and emergency medication are available in case a severe reaction occurs. This is a precaution, but one that is taken very seriously by staff involved.
To check for a reaction, small amounts of the allergen are given to the patient at set times. Sometimes, a small amount is first placed on the lips (this is called a lip dose). If no reaction is seen some of the allergen-containing food is then eaten. Care is taken over the amount of allergen eaten, and the timings, so that any reactions can be monitored carefully.
Sometimes the food is disguised in a ‘double-blind’ trial so that neither the tester nor the patient knows whether it is the allergen or a placebo (a harmless substance) which is being eaten. In this way, reactions caused by the idea of eating a risky food, can be avoided.
If at any time there is a reaction, the challenge will be discontinued and appropriate avoidance advice will be given.