A 72 hour fast, fortunately does not always last 72 hours. You start fasting and fast up until blood sugars to drop to less than 50. Once they do, they draw labs and can use that information to determine if an insulinoma is likely. The test can last up to 72 hours and is stopped at that point if sugar does not drop. Sugars are checked hourly. A mixed meal test starts with fasting labs and then you eat and take sugars and other labs every 30 min. Thanks.