It doesn't have to be extremely accurate to tell abnormal glucose levels.
I can tell if you are prediabetic II just by asking you about your diet.
Do you eat sugar, HFCS, honey, nectar, juice, molasses, maple syrup and/or white flour products (most breads, crackers and pastas) regularly?
BAM! You are suffering from metabolic syndrome and ergo ate prediabetic.
In ten years reread this answer.