What is the probability of passing out/becoming unconscious after a fasting blood test - i was told i'm hypoglycemic and need tests done-?

Be monitored: Postprandial syndrome ("reactive hypoglycemia") is more of a clinical diagnosis, but if you're to be tested for fasting hypoglycemia, perhaps to rule an insulinoma in or out, have someone with you to resuscitate you should you pass it. It does happen and a competent physician knows how to prevent you from being in danger. Any doubts, get with an endocrinologist.