Under normal circumstances, the reference range of fasting blood glucose is 3.9~6.1mmol/L. Although fasting blood glucose 6.55mmol/L is beyond the normal value, it does not reach the diagnostic standard of diabetes mellitus, so fasting blood glucose 6.55mmol/L may not be diabetes mellitus. The diagnosis of diabetes mellitus needs to be combined with clinical symptoms and glucose tolerance test to make a comprehensive judgment. The latest diagnosis of diabetes mellitus is based on the typical symptoms of “three more and one less” (drinking, urinating, eating and weight loss), plus the measurement of intravenous plasma glucose ≥11.1mmol/L at any time of the day, fasting blood glucose ≥7.0mmol/L, or 2-hour glucose ≥11.1mmol/L after a glucose tolerance test, which can lead to the diagnosis of diabetes mellitus. Diabetes mellitus is diagnosed when one of these three items is met. Fasting blood glucose 6.55mmol/L may be physiologic transient elevation, such as strenuous exercise, emotional excitement, etc., but also may be prediabetes. Fasting blood sugar 6.55mmol / L patients need to do further examination, under the guidance of specialized physicians to clarify the cause of elevated blood sugar.