Blood glucose 21mmol / L may have acute complications (ketoacidosis, hyperosmolar coma, etc.) and chronic complications (diabetic nephropathy, diabetic retinopathy, etc.), it is recommended to consult a doctor in time. 1. Acute complications: excessive blood glucose can easily cause metabolic disorders in the body, leading to acute and serious complications, such as ketoacidosis and hyperosmolar coma. (1) Ketoacidosis: Blood glucose as high as 21mmol/L may lead to ketoacidosis, resulting in severe water loss, or various reflexes or even coma. (2) Hyperosmolar coma: Blood glucose as high as 21mmol/L may lead to hyperosmolar coma in patients, mainly due to severe elevation of blood glucose caused by substandard insulin dosage or sudden interruption of treatment, which may cause dehydration and neurological symptoms, hearing impairment and hallucinations, and may also induce coma. 2. Chronic complications: If blood glucose is as high as 21mmol/L for a long period of time, or fluctuates up and down significantly, it may lead to a variety of chronic complications, such as diabetic nephropathy, diabetic peripheral neuropathy, diabetic retinopathy and diabetic foot. Due to individual differences, not all people with blood glucose as high as 21mmol/L will experience the above conditions. However, if the patient does not take active and effective treatment, as the disease continues to progress, serious complications may occur. Therefore, when blood glucose is as high as 21mmol/L, it is necessary to consult a doctor promptly.