High serum fructosamine may be diabetes mellitus, but high serum fructosamine is not a definitive diagnosis of diabetes mellitus. Fructosamine can be produced by the reaction between plasma albumin and glucose, and the amount formed correlates with blood glucose concentration and duration. Since the half-life of albumin in the blood is 19 days, fructosamine can reflect the average blood glucose level of diabetic patients in the last 2-3 weeks, and is an indicator of the recent condition of diabetic patients. The diagnosis of diabetes mellitus is based on the typical symptoms of “three more and one less” (drinking more, urinating more, eating more, and losing weight) plus the measurement of venous plasma glucose ≥11.1mmol/L, fasting blood glucose ≥7.0mmol/L, or 2-hour blood glucose ≥11.1mmol/L after glucose tolerance test, which can be diagnosed as diabetes mellitus if one of them meets the diagnosis of diabetes mellitus. One of them is enough for a diagnosis of diabetes mellitus. The China Type 2 Diabetes 2020 Prevention and Control Guidelines include glycated hemoglobin in the diagnostic criteria, and glycated hemoglobin ≥6.5% can be used as an additional diagnostic criterion for diabetes, but diabetes cannot be ruled out if it is lower than 6.5%. The diagnosis of diabetes requires consultation with a medical professional.