I'm prediabetic and have had several glucose tests. Absolutely you cannot eat that egg or anything else before the test. I've been told to eat nothing for 12 hours but I think before I was prediabetic it was eat nothing after midnight which makes it 8 hours if test was at 8.
You can drink water, coffee and tea without sugar but you can use non-caloric sweetner. I'm not sure about diet sodas.
If you eat, your test won't be accurate and you'll likely have to do it over. No fun. The lab tech should ask you when was the last time you ate and if it's not been sufficient time she won't draw the blood. Be honest, because if the level is even just a few numbers over 100, the next test is not only still fasting but you have to stay there, drink this awful tasting stuff and then have blood drawn in increments of time.
I've had to fast for 24 hours a few times in my life. Even after 12 hours I'd be starved and would celebrate by going to my favorite pancake restaurant and indulge. lol So I know how you feel. I had a CBC yesterday and the orders should have included fasting glucose but didn't. Dr is asking them if they can use that blood but if not, it's fasting for 12 hours all over again. Ugh!