It depends on the blood test, how much it will affect it. You will need to call the lab first thing in the morning and ask them, or you can tell them when you go in. Fasting usually means just that though, and you need to go by the rules to get the most accurate test. So be prepared to have to starve a second time, more than likely. It's better to do that than have an abnormal result and need more involved testing just for a piece of bread and cheese.