Hi....., Fasting blood sugar (FBS) measures blood glucose after you have not eaten for at least 8 hours. It is often the first test done to check for prediabetes and diabetes....and the range is within 100mg/dl... 2-hour postprandial blood sugar measures blood glucose exactly 2 hours after you start eating a meal. This is not a test used to diagnose diabetes...and the range is within 140mg/dl.....