When fasting before a blood test, an individual is allowed to consume water and take usual medications unless instructed otherwise by the health care provider ordering the test. No other food or beverages should be consumed, according to Quest Diagnostics.
Drinking plenty of water is important when fasting before a blood test because an individual is not eating during this time and because being well-hydrated allows blood to be drawn from the veins more easily, claims Quest DIagnostics. Dehydration can make blood work more difficult, states Dr Abdallah Karam.
Blood tests that require fasting typically are those testing for blood sugar or lipids. Testing for blood sugar to determine prediabetic or diabetic conditions, for example, require a standard blood sugar level. Consumption of foods containing sugar before a blood test causes the test to show a higher level of blood sugar, possibly indicating to a doctor that an individual is diabetic. Fasting before the test ensures measurement of a standard blood sugar level is measured, allowing for a more accurate diagnosis, according to Dr. Abdallah Karam.
Blood tests measuring lipids are similarly affected by food consumption prior to blood being drawn. Normal cholesterol levels can be elevated enough through food consumption prior to testing to indicate that an individual requires aggressive treatment, states Dr. Abdallah Karam.