superslyskillzmcfly replied to your post: In case anyone was curious what the aftermath of…
I always thought they did those tests on people’s backs. Huh. I bet it felt like a really slow tattoo.
Warning: I am not a doctor, just someone who’s had this done and who read up about it on PubMed.
There are a couple of different versions of allergy testing. The one I had in 2001 in Massachusetts used my left forearm and was very similar to a tattoo: they pricked me lightly with a set of needles with an allergen on them. It was actually pretty much identical to a TB test, if you’ve had that done, only 20 times over. Mine tested 20 substances, including groups of common food allergens (chocolate, strawberries, etc). The downside to that type is that it’s not very specific and can result in a lot of false negatives. They just give you a mix of a handful of tree pollens, for example. Also this type doesn’t really indicate severity. It’s just a yes or a not-sure. When I did it, I got the pricks done, then had to go back after 24 to 48 hours to have the test read, much like a TB test.
This one was an intradermal titration test, where they injected increasingly concentrated allergens under my skin to see exactly how much it took to cause a reaction. It has more false positives but far fewer false negatives. It’s deeper than a tattoo and needs a moderately sized skin surface. Mine only tested environmental allergens, leaving out ingested allergens, because those were what we needed to know about. I was tested with 28 substances over 55 injection sites. Each was injected twice and some three times, sometimes in a new site and sometimes not, depending on the skin response. It broke down grasses, for example, into species. It did not test some environmental things the 2001 test did (eg cockroaches), either because those species aren’t common here or because there isn’t a good allergy injection for it—they’re doing this to see if they can fix my rhinitis, not as a true diagnostic. This one only took about 45 minutes start to finish (including set-up, printing out my results, chatting, etc) because the deeper, bigger, more concentrated injections means the skin reacts quickly. They told us to allow 2 hours but I think that's way more than most people will need to get it done.
If they are interested in more specificity in terms of exact species, or they’re doing it as a true diagnostic, sometimes they’ll re-do the first kind of test on the back because the increased surface area lets them test more things. For example, if you test positive to seafood, it can be important to know what kind of seafood. Ditto for the “grasses” category. Sometimes on children or very small adults they use the back to do the same one I had yesterday. And sometimes they do that kind of test on the back so they can do the expanded version with food allergens included. Those can include up around 200 injections and so you need a good surface area.
The third kind of test, that I have not had, is where they stick a big sticker on your back that contains bits of various things like metals and latex and cat dander and whatnot, and you wear it around for 24 to 48 hours. That one tests for dermal (contact) allergies. I react to both my own trapped sweat and the adhesive on them, though, so we’ve never bothered with that one.
Basically this is the medium sized and more sensitive (and more painful) of the “prick” tests. The large tests are done on your back, but my arm was big enough for the version they were doing today. The little, less sensitive ones are often done on the forearm.