Blood Plasma Quality Control by Plasma Glutathione Status Fasting for Blood Test: How Many Hours Do You Really Need? Fasting before blood test. Is it necessary for every test? Many people fast unnecessarily before routine blood tests. Only specific tests like Lipid Profile and Fasting Blood Sugar require 8 to 12 Fasting for blood work typically involves avoiding food and drink (except water) for eight to twelve hours to ensure accurate results, particularly for tests like glucose, cholesterol, and triglycerides. It is important Fasting requirements for Chemistry tests.