How do you test for Allergy?
There are a few different ways of testing for allergy. Blood tests can be quite good for some kinds of allergy, but are not very reliable for babies and it takes a few weeks to get the results. Skin prick tests can be quite good, and you get a result within 15 minutes, but again are not very reliable for babies. Patch testing is done by some dermatologists looking for contact dermatitis – the patch is applied to your back and left for 72 hours.
But there are some people where testing doesn’t seem to work properly. It might be because their allergy is restricted to one part of their body eg their eyes or nose. Sometimes there is no obvious reason, and the only way to prove the allergy is to do a challenge, where you are intentionally exposed to whatever it is you are allergic to. There is obviously a risk here that you might have a nasty reaction. Sometimes a double blind test is done, where you are given something (to eat, usually) that you can’t see/taste/feel, and 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.
There are some “allergy tests” that are advertised that are not recommended by health professionals on the basis that there is no scientific evidence of their validity eg 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!
What is 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.
Can’t you just test for everything?
There are 2 issues here. Firstly, because allergy happens because your body becomes sensitive to specific things (sometimes just 1 thing, eg 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 eg 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.