What are normal saliva ammonia levels? Are you testing the ammonia right away or after some time? I recall there is some mechanism by which ammonia levels increase in saliva over time if not swallowed.
When was the last time you had some basic bloodwork done? If you are really concerned about toxic levels of ammonia, you need to check the blood, and while you're at it check liver enzymes and kidney function. This is all standard CBC/blood chemistry stuff. Nothing too fancy.
The body produces ammonia and uses it for various processes, but the level is kept in check by metabolic and excretory processes. If those processes are dysfunctional, and there truly is a toxic buildup of ammonia, you'll need to seek medical attention.
I would be interested to have you test the aquarium kit on a few people and see what kind of results you get. Perhaps the kit is not that sensitive/specific?
I'm trying to come up with a way that there is not a toxic level of ammonia, b/c that's bad news.