During an allergy challenge test, a patient with a suspected allergy is given small amounts of the substance to which they might be allergic, in order to observe the reaction. Because of the risk of a potentially life-threatening reaction, this must only ever be done in a hospital, under close medical supervision, with emergency treatment at hand.
Only one substance can be tested for at each appointment, and the appointment may last for between 2 and 5 hours. Therefore, this test will usually only be done as a last resort where other tests such as patch tests, skin prick tests or blood tests, none of which are 100% reliable, have already been done and the results are inconclusive.
Occasionally an allergy challenge is used where patients have severe asthma, to test whether particular airborne particles might trigger an asthma attack.
An allergy challenge may sometimes be undertaken to find out whether somebody has outgrown an allergy.
Before taking the test
In the case of an allergy challenge to test for food allergies, you will probably be advised not to take any medication, such as antihistamines for a period of 3 to 7 days before the challenge, as these can affect the results. Never stop taking medication which you have been prescribed without speaking to your doctor first.
It is important that if you have ever had a severe allergic reaction, or have other health problems which may be aggravated by the challenge that the allergist conducting the tests is fully informed of this before the test starts.
What will happen during the test?
Before the challenge starts, after an initial health check, it is possible that you might have a cannula inserted into your arm, so that it will be easy to administer emergency medication if needed.
During the challenge, if you are being tested for a food allergy, you will initially be given very small quantities of the food to ingest orally. In some cases a tiny amount of the food is just wiped against your lips. This first dose will be an amount that would not normally be enough to cause a reaction. The doctor will watch you for a period of time, probably about 10 minutes, to see if anything happens before giving you a slightly larger dose to ingest. This pattern will be repeated, probably over several hours. If at any point you start to have a reaction, the test will be stopped immediately and medication administered if necessary. You will then be given advice on how to deal with the allergy.
There are three ways of doing the challenge, single-blind testing, double-blind testing or an open challenge.
- During single-blind testing: the doctor knows what substance is being tested, but the patient does not. This helps ensure that any reaction is not somehow brought on, or made more severe by a belief in the patient’s mind that they are allergic to that particular substance.
- Double-blind testing is where neither the doctor nor the patient knows what substance is being tested for, or whether it is a placebo (a harmless substance which is not an allergen, used to test the accuracy of the test). This is most often used during medical research trials, so that the person recording the results is not affected by unintended bias.
- An open challenge is where both doctor and patient are aware what substance is being used in the test. This method might be used to see whether somebody who was known to have an allergy has outgrown it.
If you are being tested to see whether you are allergic to a particular drug such as penicillin or insulin, the drug challenge might be administered orally (as above), or subcutaneously (under the skin) or intravenously (by injection).
In the rare circumstances where an allergy challenge is being done to test whether particular airborne particles trigger asthma in a particular individual the substance is administered in the form of a fine mist which will be inhaled through specialist equipment.
Is the allergy challenge test safe?
Allergy challenges which are done in hospital or in a doctor’s surgery, under close medical supervision have an excellent safety record. However, this is the only safe way to conduct the test. Many allergies carry the risk of anaphylaxis, a severe, often sudden reaction where the body goes into shock. Anaphylaxis can be life-threatening. In most cases any reaction is likely to be mild, for example the patient might break out in hives, which can be treated with antihistamines. In hospital, the doctors will have the expertise and medication at hand to treat the symptoms of anaphylaxis, should it occur.
The results of the allergy challenge
Unlike many of the other tests for diagnosing allergies, an allergy challenge is almost always conclusive.
If the result is positive and you are diagnosed as having an allergy to the allergen used in the test, your doctor will discuss an action plan with you. This is most likely to involve a discussion of how best you can avoid the substance that you are allergic to, as well as any medication you need to take if you accidentally come into contact with the substance in the future.