Whether and for how long a blood test requires fasting depends on the purpose of the blood test. If the test is for blood type, viral markers, or two-and-a-half pair test, fasting is not required and can be done at any time. If the test is for fasting blood glucose measurement, 8-12 hours of fasting is required. Checking fasting blood glucose usually requires patients to fast after ten o’clock the night before. Fasting is to avoid the influence of diet and other factors on some components within the blood, because food is absorbed into the blood by the gastrointestinal tract and then transported through the blood to the whole body, thus affecting the accuracy of the test results. Also, after eating, various digested food chyme particles are quickly absorbed into the blood, clouding the blood and causing “fatty blood” to affect the test. Therefore, for some tests, fasting for 8-12 hours can ensure the accuracy. In addition, blood tests for kidney function, liver function, blood sedimentation, glucose tolerance test, serology, immunology, etc. all require 8-12 hours fasting.