Diagnosis of food allergies is sometimes very straightforward. If clearly allergic symptoms like urticaria or anaphylaxis occurred immediately after eating a single suspect food such as a nut or a prawn, your doctor may ask a few questions and then proceed straight to a skin or blood test to confirm diagnosis.
In other cases, the link between a food and a reaction is not as obvious, for example, if symptoms were less specific (like headache or diarrhea), occurred some time after eating, or were prompted by a food with many ingredients, such as a curry or pie. And food sensitivities, as distinct from true allergies, can be much harder to pin down. In these cases, your doctor will probably take a detailed medical history of your symptoms: such as when they occur, how long they last, what treatments have worked, with what food they might be associated, how much you ate before the reaction began, how the food was prepared, and what other foods you were eating at the same time as the one you may suspect made you ill. You will also probably be asked to keep a food diary to track everything you eat.
These steps may enable your doctor to reach a diagnosis with no further testing; in this case, he or she will probably put you on an elimination diet, in which you eliminate the suspect food for a week or two and see if the symptoms disappear. Again, the food diary will come in handy in tracking whether this works. If your symptoms disappear when you eliminate the food, then reappear when you eat it again, it’s almost certain that the food is causing the symptoms.
To confirm a food allergy, your doctor may carry out medical tests that measure the allergic response. These include:
▪ Skin prick test. The skin test lancet is dipped into the suspect fresh food, then scratched or pricked into the skin on your forearm or back. Any swelling or redness, called a weal, indicates a local allergic reaction, meaning you have IgE on your skin’s mast cells specific to the food being tested.
▪ RAST blood test. If you have severe reactions to the suspect food that lead to anaphylaxis, or if you develop severe skin rashes or hives, your doctor will probably suggest a blood test such as the RAST test to measure the presence of food-specific IgE in your blood. It takes about a week to get results.
While an immediate allergic reaction to food, backed up by a positive skin prick or RAST test is usually sufficient to diagnose IgE-mediated food allergy, skin tests cannot be used to predict allergic reactions to foods; many patients who test positive to particular foods do not react when they actually eat them. Occasionally a skin test is misleading if the wrong food was tested.
▪ Double-blind food challenge. This involves placing various suspect and non-suspect foods in individual opaque capsules. You swallow a capsule, and the doctor waits to see if you have a reaction, then you continue the process until you’ve taken all the capsules. The study is considered ‘blinded’ because neither you nor the doctor know what food the capsules contain until the test is finished. (Alternatively, the suspect food may be disguised in other foods.) Someone with a history of severe reactions can’t be tested this way, and, because the test takes time, it’s fairly expensive. Studies show that fewer than half of all people who suspect that they have a food allergy test positive in a double-blind food challenge.
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