how long to fast before blood work
1 Fasting blood sugar test. 2  A blood sample will be taken after an overnight fast. 3  A fasting blood sugar level less than 100 mg/dL (5.6 mmol/L) is normal. 4  A fasting blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered prediabetes.