Fasting blood glucose test. Do not eat or drink anything except water for 8 to 10 hours before a fasting blood glucose test. These are used to diagnose diabetes, a condition caused by too much glucose (sugar) in the blood.ron blood tests are usually taken in the morning before you eat anything. You should also avoid taking iron pills or tablets for 24 hours before your test. Your body absorbs iron very quickly from food or pills, so this can raise your iron levels and affect the test results.