This is an “office “procedure performed in the doctor’s rooms. It involves putting drops of reagents of suspected foods/aeroallergens on the adult or child’s forearm (sometimes the back in young children) and pricking through the drops with the tip of a lancet. These tests are slightly “irritating” but not painful. Sometimes young children struggle a bit as they do not like to be held still! After 15 minutes the doctor will read the result of the test. If the allergen causes a “hive” to form then this indicates that the patient is “sensitised” to that allergen. In those with being tested for suspected food allergies this does not necessarily mean the person is allergic- small hives are sometimes false positives. Your allergy doctor will interpret the tests taking into consideration the skin test size and the patient’s history, to come up with a prediction of whether the patient is allergic or not.
The bigger the skin test, the greater the chance of a true immediate type allergy. Please note that the size of the skin test does NOT predict how severe the reaction will be! Some food-allergic people with a very large skin test experience a mild rash when they ingest the food, others with a relatively small skin prick test can have anaphylaxis.
The good thing about skin tests is that they are cheap and excellent tests, and the results are obtained during the consultation with the doctor, who can then give the appropriate advice. However, the patient needs to be off antihistamines for at least 3 days before a skin test, otherwise the antihistamine can affect the result. Skin tests may be tricky in some patients with severe eczema or a very reactive skin. In this scenario your doctor may decide to do blood tests instead. Skin tests are exceptionally safe and usually only cause the small swellings on the arm, which generally disappear within 30 minutes or so. Every now and again, patients have a “reaction” to the skin test in the form of a rash or even lip swelling- this is very rare. We also very rarely see a mild flare of eczema hours to days after a skin test.
In the case of drug allergies, we sometimes need to use slightly deeper skin tests called “intradermal” tests, which involves a tiny injection into the top layer of the skin with the drug to look for a skin reaction.
A blood sample can be taken to check for the antibody E against certain food or aero-allergens. Newer tests called ‘component” test can also be performed in some cases to more accurately predict a patient’s risk of food allergy.
Blood tests do not require the patient to be off antihistamines and are completely safe.
However, they are more expensive than skin tests and the results take a few days to come through, so you may not receive “all the answers” during the initial consultation. As for skin tests, a positive result should be interpreted in expert hands as it may not necessarily mean a true allergy.
Food challenges play an important part in food allergy diagnosis, especially in cases in which the history and allergy tests disagree with each other, or do not yet confirm the allergy. Only very few specialists and centres perform food challenges. In a food challenge, the doctor/nurse will give the child tiny, and then steadily increasing, amounts of the food in question every 15-20 minutes or so, watching the patient very very carefully for signs of a reaction.
If the patient has a reaction, the challenge is stopped, appropriate treatment given to deal with reactions, and the patient is observed for at least 2 hours to make sure they have settled completely. If the patient manages to reach the “top dose”, they are also observed for a further 2 hours to check for late reactions.
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