Fasting blood glucose 20mmol/L is far beyond the normal range, elevated fasting blood glucose can be seen in diabetes mellitus, serious infections, increased intracranial pressure, such as intracranial hemorrhage, cranial trauma, etc., and it is recommended to seek medical attention in a timely manner. Clinical fasting blood glucose normal value of 3.9 ~ 6.1mmol / L, when reached or exceeded 7.0mmol / L and have typical diabetes symptoms, you can consider the diagnosis of diabetes, so the fasting blood glucose 20mmol / L has been seriously out of the normal range, need to consider the possibility of disease. It is worth noting that when blood glucose is elevated at such a high value, it is also necessary to be alert to the presence of diabetic ketoacidosis, as well as hyperosmolar hyperglycemia syndrome. The patient should be closely monitored for clinical signs such as sunken eyes, convulsions, unresponsiveness, and exhaling the odor of rotten apples, and seek prompt help from a physician. In summary, a fasting blood glucose of 20 mmol/L suggests a severe elevation of blood glucose, and prompt medical attention is recommended if this problem occurs.