When patients are instructed to fast for medical screenings, such as a cholesterol test, they are allowed to drink water. Other beverages, even those made by adding substances to water, such as coffee and tea, must be avoided. A fast for a cholesterol test typically lasts nine to 12 hours.
A fasting blood glucose test, which screens for diabetes, requires no food or drink, other than water, for eight hours prior to the test. A fasting lipid profile measures several substances in the blood in order to help determine an individual's risk for heart disease. Many patients schedule their fasting tests during the morning so the fasting occurs while they are sleeping.