Glycated hemoglobin (GHb) is a product formed by the combination of blood glucose with hemoglobin (another term for hemoglobin) in red blood cells, and its synthesis rate is proportional to the concentration of sugar in the environment to which the red blood cells are exposed (i.e. the higher the blood glucose the higher the glycated hemoglobin). The formation of glycated hemoglobin is irreversible, and its concentration is related to the life span of red blood cells (120 days on average) and the average concentration of blood glucose during that period, and is not subject to daily fluctuations in the magnitude of plasma glucose concentration, nor is it affected by exercise or food, so glycated hemoglobin is a reflection of the average blood glucose concentration over the past 8 to 12 weeks, which can provide a reliable laboratory indicator for assessing blood glucose control. Usually, HbAlc in our laboratory test is a subcomponent of glycated hemoglobin, and the glycated hemoglobin is reflected by testing HbA1c, while HbAlc and GHb are now often considered synonymous in clinical practice. The 2010 ADA (American Diabetes Association) guidelines have included HbAlc ≥ 6.5% as one of the diagnostic criteria for diabetes. Compared to our guidelines which use OGTT (Oral Glucose Tolerance Test) as a diagnostic criterion, HbAlc is simpler and easier to use, with stable results and less variability, and is not affected by the timing of eating and short-term lifestyle changes. Why does China not adopt HbAlc as a diagnostic criterion for diabetes? It is because HbAlc testing is not yet common in China, the standardization of testing methods is not enough, and the relevant instruments and quality control do not yet meet the current standards for diabetes diagnosis. However, for hospitals that adopt standardized testing methods with strict quality control and normal reference values of 4.0%-6.0%, HbAlc ≥6.5% can be used as a reference for the diagnosis of diabetes mellitus. HbAlc is one of the main indicators of the level of long-term glycemic control. For most non-pregnant adult patients with type 2 diabetes, a reasonable HbAlc control goal is <7.0%.