I rely on severity and duration, mostly--
if it's kind of over-and-done-with in ten or fifteen minutes, then that's one thing. Unless I have diarrhea bad enough that I actually feel
lightheaded. That is a warning sign that blood pressure may be impacted, or that fluid balance has been due to rate of fluid loss. KWIM?
My DH points out (he's got his own walnut and soy allergies, btw);
it's the difference between "Oh. Sh**."
and something more like "OH!!!! SH**!! SH**!! HELP!"
(Yes, we have an odd sense of humor about FA at our house-- might as well laugh about it, right?)
The latter is an epinephrine moment, and the former is probably a wait-and-see without anything else happening.
Any cramping like menstrual cramping? THAT is a hallmark symptom of anaphylaxis for me personally. I
always know when I get that going on that I'm in trouble and in for a more severe reaction than if it were entirely GI.
Some of those other symptoms can be pretty subtle. They are things that as reasonable adults, we'd be inclined to ignore or just tolerate as "Yeah, whatever," but in conjunction with a suspected systemic allergic reaction going on, they become a lot more ominous. For example, a stuffy nose or itching-- anywhere-- along with diarrhea would be reason enough to use epinephrine. The reason is that that adds up to two body systems.
Some symptoms are serious enough that they warrant epinephrine even with nothing else going on, of course. But some of them are merely annoying unless you view them in context. The reason is that most laypersons don't have a pulse oximeter and BP cuff just laying around ready to use during a reaction, and without them, you're just
guessing that airways and cardiovascular function are both unimpaired. Those are the things that kill, and unfortunately, once a reaction becomes systemic... (that is, more than a local, contact response) it's a little unpredictable.
GI symptoms are peculiar in that they
may be systemic symptoms, but with a food ingestion, they
can be merely "localized" ones, as well. It sort of depends on severity and onset.