A fasting blood glucose check of 21mmol/L may be diabetes, or it may be elevated blood glucose due to other reasons. The normal range of fasting blood glucose is 3.9-6.1mmol/L. Blood glucose of 21mmol/L belongs to the elevation of blood glucose, which may be the manifestation of diabetes mellitus, but it may also be caused by other reasons, such as severe infections, hyperthyroidism and other pathological factors. The latest diagnosis of diabetes mellitus is based on the typical symptoms of “three more and one less” (drinking, urinating and eating more, and weight loss) plus a venous plasma glucose measurement of ≥11.1 mmol/L or a fasting glucose of ≥7.0 mmol/L at any time of the day, or a glucose of ≥11.1 mmol/L at 2 hours after an OGTT. mmol/L, and a diagnosis of diabetes is made when one of these three items is met. Fasting blood glucose 21mmol/L alone cannot directly diagnose diabetes, but it is a high blood glucose value that must be taken seriously, and it is recommended to consult a doctor in time, repeat the measurement of blood glucose, and clarify the reasons for the elevation of blood glucose, so as not to delay the condition.