Why do you have to fast before a blood test?

Answer by  monkeyz (3150)

This really depends on the test you are getting. Some blood tests do not require fasting. Tests of cholesterol and blood sugar would be affected by consumption of food, so those particular tests would require a fast. To get an accurate idea of your general blood sugar and lipid levels, you need to know what they are before eating.

Answer by  cindy (1484)

Fasting is only required before blood tests where you want to know values before intake of food. The most common test to require this is cholesterol. This is because if you did eat something it would cause a temporary increase in cholesterol. What the doctor wants to know instead is your baseline cholesterol level.

Answer by  ZackRoca33 (206)

You only have to fast before certain blood tests - if you are having your sugar levels measured then you want "resting" sugar, which requires fasting.