Allergy tests are a great tool to help diagnose what you are allergic to but it is important you have the right kind of allergy testing, from a reliable provider. You can get referred for food allergy testing or to check if you have a penicillin allergy or other allergies by your GP for free on the NHS.
As a mum of two children with environmental and food allergies, and author of Living With Allergies, it is something I have researched into and experienced firsthand. Here is what you need to know:
Allergy testing for delayed (Non-IGE) food allergies
Unfortunately, allergy tests will only show up IgE-mediated allergies that cause symptoms such as hives, swelling or breathing difficulties. They are are unable to diagnose Non-IgE, or delayed reactions that occur hours or days after ingestion. These can only be diagnosed through a clincal history and elimination diet, which your GP or a dietician can support you with.
Skin prick tests (SPTs)
Skin prick tests are a quick and straightforward way to help diagnose IGE mediated allergies to food, pollen, animals or medication like a penicillin allergy by detecting the level of IgE antibodies in the skin.
At the allergy clinic they will have a range of allergens that have been extracted into a liquid format. Sometimes, if your allergen is less common, is to a fruit or vegetable, or they want to test it in a certain form, they may ask you to bring a fresh sample from home.
The allergist will then label your arm or back for all the different allergens they are testing and also do a negative and positive control test. This checks that your body is not overreacting or suppressed by medication. You will usually be asked to stop taking antihistamines up to five days before your tests so they do not affect the results.
A small drop of the liquid or the fresh produce is put on your skin and then the skin is pricked briefly with a sharp probe so it can enter the bloodstream. These will then be left for 15-20 minutes to see if the skin reacts. Your allergist will measure the size of the bump and weal, the red area around it, to determine how much you reacted. A weal of 3mm diameter is generally considered a positive reaction. The larger the weal, the more likely you are to be allergic to that substance but this cannot predict the severity of a reaction.
A small amount of steroid cream will help ease any itching and the weals should disappear within an hour. Rarely, skin prick tests can cause more severe reactions and these will be treated immediately in the clinic.
Patch tests are used to see whether you are allergic to particular chemicals or substances and are often used to identify causes of skin allergies. The allergens are applied to your back and left on for up to 48 hours to check for any reactions.
Blood tests (RAST tests)
Blood tests, also known as RAST (radioallergosorbent) tests, can be a good way to test for multiple different allergies at one time and show the amount of IgE antibodies your body produces to certain substances.
The blood will need to be sent to the lab for analysis, so it takes longer to get the results and they are more expensive to perform, so skin prick tests are usually the first choice. However, blood tests may be done if skin prick tests are not available and this is often the case in regional centres where there isn’t access to a specialised allergy clinic. Blood tests are also good if the skin is too sensitive or covered in eczema, or if people cannot stop taking antihistamine. They may also be used if a solution for a specific allergen is not available or if someone has had such a severe reaction in the past, skin prick tests are considered too dangerous.
Different scales are used to report the results of RAST tests but the higher the levels, the more likely someone is to have an allergic reaction. Although like the skin prick tests, it cannot necessarily predict the severity of a reaction. Many people will often have slightly elevated IgE levels if they suffer from asthma and eczema, so a low positive may not indicate a true allergy. They should always be interpreted alongside a medical history.
Allergy test results
Unfortunately, it is not as simple as a positive result indicating an allergy and a negative result meaning you don’t have one. Allergy tests can often produce ‘false positives’, meaning your results are positive for a food or substance you actually have no problem with. This may be because your body is sensitised to it but does not react, or the test is picking up proteins that cross react with other similar substances – such as someone who is peanut allergic reacting to other legumes. It is also possible to have a ‘false negative’, where the test is not showing an allergy you know you react to, although this is less common. This is why it is important for any tests to be interpreted by a trained allergist who can also take into account a patient’s previous reactions and medical history.
Food allergy tests vs food intolerance tests
Food allergy tests should not be confused with food intolerance tests. IGG tests and others such as the York Test are not reliable and are not the same as true allergy tests that check for IGE antibodies They may suggest a multitude of food sensitivities and can mean you unnecessarily restrict your diet.
You can find more useful information in my book Living With Allergies
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