If you've ever visited your doctor and required blood test, he or she may have told you to return on another day to have the blood drawn when you haven't eaten overnight. Although it may seem as if the doctor is doing this to make your life inconvenient, there are some real medical reasons for doing these blood tests after a short fast (not eating).
There are two main groups of test that are significantly affected by fasting state, blood sugar and lipids. More specifically, these tests are for LDL and HDL cholesterol, and triglycerides. On a daily basis, the normal range for blood sugar and lipids undergoes changes hour to hour depending on recent ingestion of food and drink. By fasting, one is getting a baseline sugar and lipid value that is not influenced by a recent meal.
In addition, the guidelines for the definition of diabetic and pre-diabetic are based on looking at standardized situations. A fasting state is one of those situations. For example, a fasting blood sugar greater than 125 mg/dl would define a patient as diabetic. However, if that patient snuck a granola bar one hour before the blood draw, it might signal to their doctor that they are diabetic as the blood sugar would be high. A blood sugar of 125 mg/dl one hour after eating a carbohydrate load is normal - but in the truly fasting state it is not. If that patient didn't fess up, their physician would believe that they are likely diabetic.
Lipids follow the same guidelines. All of the normal values, and the values used to determine high risk and need for treatment, are based on FASTING values for LDL and HDL cholesterol and triglycerides. If a patient is not in the fasting state the cholesterol values could be elevated by as much as 20 points. This could make one falsely fit into criteria for more aggressive treatment.
An important point to remember is to stay well hydrated with plain water even in the fasting state. Many blood draws can be difficult on patients who are dehydrated and plain water effectively counteracts this condition.
Always remember that a fasting state is needed for many blood tests to compare apples to apples. The most valuable information from your blood draw containing blood sugar and lipid parameters will likely often be in the truly fasting state. To get the most valuable, accurate information from your blood draw, don't sneak even a breath mint.
If you have further questions or concerns about these blood tests and what you need to do to make them as accurate as possible, talk to your doctor or nurse. They will be more than happy to give you any information you need to ensure that your tests truly reflect your current state of health.