Abstract: Objective To observe the characteristic of glycosylated hemoglobin A_1c (HbA_1c) for diagnosing type 2 diabetes mellitus (T2DM) and its distribution characteristic inpopulation with normal fasting plasma glucose(FPG). Methods A total of 729 subjects with normal FPG were enrolled, and uric acid(UA), triglyceride(TG), total cholesterol(TC), high-density lipoprotein cholesterol(HDL-C) and low-density lipoprotein cholesterol (LDL-C) were determined. Oral glucose tolerance test(OGTT) was administered to 247 patients, including 164 cases with T2DM, 41 cases with impaired glucose tolerance, 18 cases with impaired fasting glucose and 24cases with normal glucose tolerance. HbA_1c was determined by turbidimetric inhibition immunoassay, and the receiver operating characteristic(ROC) curve was used to determine the cut-off point of HbA_1c and FPG for diagnosing T2DM according to the results of OGTT and clinical diagnosis. The distribution characteristics of HbA_1c with different sex and with different age in same sex were analyzed and observed comparatively.Results The cut-off point of HbA_1c for the diagnosis of T2DM was 6.36% with a sensitivity and specificity of 86.50% and 90.60% respectively, the positive predictive value and negative predictive value were 94.63% and 76.50% respectively, and the area under ROC curve was 0.944 [95% confidence interval (CI):0.917-0.971]. The area under ROC curve of FPG was 0.957 (95% CI:0.932-0.981), the sensitivity and specificitywere 85.90% and 93.80% when FPG>7.0 mmol/L for the diagnosis of T2DM. The HbA_1c and HDL-C in females with normal FPG were significantly higher than those in males with normal FPG (P=0.000).Hemoglobin (Hb), FPG, UA and TG in males were higher than those in females (P=0.000, 0.020, 0.000 and 0.000).HbA_1c, FPG, TC and LDL-C were positively correlated with age. After 60 years old, HbA_1c in females increased significantly, there was increasing trend of HDL-C in males, and there was decreasing tread of HDL-C in females. Conclusions The cut-off point of HbA_1c for diagnosing T2DM is 6.36% by turbidimetric inhibition immunoassay. In order to avoid diabetes mellitus, elder people should monitor HbA_1c termly.

Key words: Glycosylated hemoglobin A_1c, Type 2 diabetes mellitus, Turbidimetric inhibition immunoassay, Diagnosis