Allergy Tests

How do you test for Allergy?

There are a few different ways of testing for allergy.

IgE Blood tests

IgE blood tests are available for a wide range of different foods, animal hairs, pollens etc.  They are not very reliable for babies and it takes a few weeks to get the results. You also need some experience in interpreting the results, as you don’t get a simple Yes/No answer – you might get a borderline result, and you can get false positives. 

Skin Prick testing

Depending on what things you are suspected of being allergic to, you may be offered skin prick testing. You should avoid taking anti-histamine medicines for at least 3 days before the test, as this will potentially reduce the usefulness of the test. An extract of whatever you may be allergic to (a food, pollen, animal hair, house dust mite etc.), or sometimes the actual material, is applied to the skin of the forearm. A very short needle is then used to gently prick the extract into the skin. This can be uncomfortable briefly, but it does not usually cause bleeding. If you are allergic, an itchy red swelling will appear after 10-15 minutes. Skin prick testing is very safe.

Patch testing is done by some dermatologists looking for contact dermatitis – the patch is applied to your back and left for 72 hours. This is not the appropriate test for someone who appears to react to something within minutes or hours.

Challenge

But there are some situations where there isn’t a test, or else IgE blood tests or skin prick tests don’t give you an answer. The only way to prove the allergy is to do a challenge, where you are intentionally exposed to whatever it is you are reacting to. There is obviously a risk that you might have a nasty reaction. Your health care professional will advise you when it is appropriate to do a challenge, and whether it is safe to do it at home, or if it should be done under supervision in hospital.

Sometimes a double blind test is done, where you are given something to eat which may or may not be what you are allergic to. This is in some ways the only perfect test, but is usually only done when symptoms are inconsistent or unusual.

“Allergy” and “intolerance” tests

There are a range of “allergy tests” that are advertised that are not recommended by health professionals on the basis that there is no scientific evidence of their validity e.g. IgG blood tests, hair analysis. Please discuss with your GP or allergy doctor/nurse before paying for any kind of allergy test, on the high street or internet! 

Allergy test (false)

Allergy test (false)

I’ve never had a reaction but I want a test

A common problem is where someone else in the family is allergic to a food or penicillin or bees, and you would like to know whether you or another member of the family is also allergic.

Unfortunately, testing in this situation is unhelpful.  If you have never been exposed to something, it is extremely unlikely that you are allergic to it (with a few exceptions) even if there is a strong family history, so a test is likely to be negative; but it doesn’t mean that you wouldn’t become allergic to it in the future!  There is also the issue that these tests can often produce borderline or false positive results, which is just confusing.

For young babies, we believe that delaying introduction of new foods (because you are scared of allergy) is likely to INCREASE the risk of allergy. So in the vast majority of cases you should aim to introduce a wide range of weaning foods including egg, nuts etc (in a form that does not pose a choking risk) from the age of 6 months, even if there is a strong family history of allergy

Can’t you just test for everything?

There are two issues here. Firstly, because allergy happens when your body becomes sensitive to specific things (sometimes just 1 thing, e.g. egg) all allergy tests are very specific. Both blood and skin testing look for allergy to specific foods, animals etc. With blood testing, some mixtures are used e.g. common foods, nuts, but you would still want to know which specific thing(s) in that mixture you were allergic to. So there is a very large of things that can potentially be tested for, and apart from the cost, you would need vast amounts of blood or skin!

Secondly, the tests we have are not perfect. That’s why the “gold standard” test is the double blinded challenge. Some people get positive test results to things they are clearly not allergic too, others get negative results to things they are clearly allergic to! So the history (what reactions happened when) is sometimes more important than the test results.