Getting your blood drawn requires a trip to the lab, so health care providers often request that several tests be performed on one blood specimen. Common blood work for multiple issues includes glucose tolerance tests to check for diabetes and lipid panels to assess your cardiovascular risk. When your doctor prescribes a battery of blood tests to evaluate your health, you may have to forgo food and beverages for a period of time. This process is called fasting.
Fasting Blood Tests
If some of the tests do not entail fasting and others do, you will have to fast. So if your blood sample will be evaluated for a total blood count, which doesn't require fasting, as well as for glucose, lipids, vitamin A, gastrin or other substances that the laboratory must detect in an absence of dietary elements, you will need to fast. Glucose blood specimens may be drawn at random after eating or after a fast. Your doctor will provide laboratory instructions about how to fast if you must.
The basic definition of fasting is to consume nothing but water during a specified time frame. Laboratories may or may not adhere to this strict dietary limitation, but most require you to avoid brewed beverages such as coffee and tea, soda, juice and all other drinks that add organic or synthetic substances to water. For fasting glucose and lipid profile tests, you are not allowed to eat any food or drink anything except water for 8 to 12 hours, according to patient instructions issued by the blood lab.
When to Start Fasting
Plan to arrive for your blood draw early in the morning. Count backwards from the appointed time by the allotted hours to figure out the start time of the fast. If you must fast overnight, eat a regular meal beforehand and stop eating when the fast begins. After that time, do not drink regular or herbal tea, regular or decaffeinated coffee, regular or diet soda, or anything other than water unless instructed by your doctor or the laboratory.
When to Stop Fasting
Knowing how and when you will end your fast can help you stick with it overnight. If the lab provides walk-in service rather than set appointments, you may have to wait for your turn. Prepare the night before by bringing a healthy snack and an insulated bottle of tea or coffee, if you usually drink some in the morning. Wait until after the technician draws your blood sample and then enjoy food and drink. To avoid fainting or other problems associated with low blood sugar, do not fast for more than 14 hours.