The "Fasting Blood Glucose" test is the primary test given for diagnosing diabetes. It tests your blood to see if there is an elevated blood sugar level after an overnight fast (not eating anything after midnight). A value above 140 mg/dl on at least two occasions typically means that a person has diabetes.
Normal people have fasting sugar levels that generally run between 70-110 mg/dl. That, too, depends upon the facility. At Kaiser, they want our normal to be between 70-100!
They want you to run another test to see if you have a high level on at least two occasions. You might consider going on a low-sugar diet now. Try only sugar-free items (be careful to read what the sugar substitute is!). My mother has diabetes and her doctors approved Splenda for her sugar substitute. Many items use Splenda (as opposed to sweet n low or that blue one). Exercise is also a good way to control your blood glucose level if you are able to do so.
I cannot give you advice on waiting to do this other test. I would suggest that you let your doctor know about your schedule and ask him/her if it is imperative that you do it immediately!
Best Of Luck
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