Waste of money?

So a couple of weeks ago, I paid to get a Spot Test, which was to test for food allergies. I know some of my food allergies, but I still get a rash sometimes when I've been good at avoiding the things I'm allergic to, so I was hoping to figure out what else I may have overlooked. I finally got the test back today, and was pretty disappointed with the results. According to this spot test, I'm not allergic to ANY foods, not even sulfites. Here's the thing. I KNOW I'm allergic to sulfites... how do I know this? Well, whenever I eat something with sulfites in it, my faces breaks out in a horrible itchy rash. When I avoid sulfites, no horrible itchy rash. So either they messed up my blood sample, or their test is bogus? I dunno... but I'm pretty disappointed.