There are two main groups of blood tests that are significantly affected by fasting: blood sugar tests and lipid profile. More specifically, these tests are for LDL and HDL cholesterol, triglycerides and fasting blood sugar. 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 eats something just one hour before the blood draw, it might signal to their doctor that they are diabetic as the blood sugar would be high. If that patient didn't admit of the food he had before the test, 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.
A fasting for 8 hours will be ideal in tests like this. But 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. Other usual tests which require fasting include Iron blood tests and Gamma Glutamyl Transferase test (GGT). 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.