Allergies cannot be diagnosed using tests alone. In fact, without a clinical history, flipping a coin is more likely to give the correct answer than an allergy test. The story of what happened is most important. When you see your doctor make sure you are as clear as you can be about the following points:
When did the reaction occur and how soon did it occur after contact with the particular trigger?
What exactly happened? If you have photos of rashes on your phone, bring them along.
If it was a food or medicine, bring the packaging along if you can with the list of ingredients.
Did the reaction need any treatment? If so what was given?
Sometimes allergy tests are not required. It may be possible to diagnose the allergy based on just the history. On other occasions, a challenge test against the food or medicine is going to be the best way of either proving whether or not the child is allergic to the substance.
Yes, you can help in 3 ways:
Think about what happened when the reaction occurred. Your doctor will want to know what happened when you got your reaction. Writing down the sequence of events leading to the reaction is useful. It can be helpful to ask others (friends, teachers, dinner lady) who were there what they saw. Photos or videos are sometimes really helpful.
Bring a list of foods or other possible allergens that could have caused the reaction. It is helpful for the doctor to know what you could have reacted to. Foods eaten within an hour or two of the reaction are likely most relevant. The packaging of suspect foods, or getting the ingredients used in the recipe can be helpful.
Remembering to bring all medicines/ inhalers.
Skin prick test results are affected by antihistamines and steroid tablets. If you are able to, antihistamines should be stopped FIVE days before your appointment.
Allergy tests are not always helpful. They can support the history when the reaction occurs within an hour or so, but should NOT be used for screening as they may lead to false positive reactions and result in children avoiding food unnecessarily. The two main types of allergy tests used in clinic are skin prick tests and blood tests.
A drop of the suspected substance causing your reaction is placed on the skin.
As not all substances your child can react to are readily available, sometimes you will be asked to bring the substance in, particularly if it is a fruit, lentil or other less common food. In this case a prick-to-prick test is performed. If more than one food is to be tested then you should bring the food in separate sealed containers like small Tupperware boxes or sandwich bags to keep the foods from touching each other.
The skin is pricked with the solution with what looks like a mini plastic pencil. It only takes only 15 minutes to get the result.
The test itself involves a series of pricks and although this can be slightly uncomfortable there are no needles! The test is usually done on the forearm. If the forearms are affected by eczema then the test can be done on the back or on the thigh.
Skin prick testing takes 15 minutes, the entire consultation can last from 30-45 minutes.
Skin prick tests are safe and at the most cause minor local irritation, which settles on its own in 30-45 minutes. Washing the skin with water can be helpful.
After the test you can go about enjoying your day as planned.
For an information sheet about skin prick tests click here.
RAST (Radioallergosorbent test) is the old-fashioned name for allergen-specific IgE tests. We rarely use radioactivity in the laboratory any more, so a more accurate name for the test is “specific IgE test”.
IgE is the antibody that triggers allergic reactions by linking up with a particularly type of white blood cell (mast cell). It is different to IgG that protects against infections and increases after vaccination.
IgE can sometimes be neutralised by IgG (particularly IgG4) antibodies, which are now known to increase in some patients after immunotherapy (desensitisation). Thus having IgE antibodies doesn’t mean for sure that you are allergic to the substance, as the IgE may be blocked by IgG.
The only way to prove whether or not you are allergic is to do a challenge with the substance. Your doctor will help you to decide if a challenge test is necessary and safe. Challenge tests are often done in hospital where we can keep an eye on you.
Even with an apparently clear history and positive allergy tests, the diagnosis of allergy can be wrong. Allergy specialists will sometimes recommend a challenge in hospital. Challenges, particularly to foods, are done increasingly frequently.
They may show that your child is not allergic. That’s great as the child doesn’t need to spend the rest of their life avoiding the food unnecessarily.
Even if the challenge causes a reaction, because we start with a tiny amount, the result is likely to be some lip swelling, hives or vomiting. More severe reactions such as wheeze are less common and can be effectively dealt with by the doctor.