Aim: To determine the association of fasting plasma glucose (FPG) level within normal range and the risk of prediabetes and type 2 diabetes in an Iranian population.

Methods: A total of 806 first-degree relatives (FDRs) of patients with type 2 diabetes who had FPG levels less than 5.6 mmol/l (100 mg/dl) in 2003 to 2005, and who did not have diabetes or impaired fasting glucose (IFG), were followed through 2010 for the occurrence of prediabetes or type 2 diabetes. At baseline and through follow-ups, participants underwent a standard 75 g 2-hour oral glucose tolerance test (OGTT).

Results: The incidence of type 2 diabetes, impaired glucose tolerance (IGT), and IFG was 9.6 (95% confidence interval (CI): 6.8-12.4), 28.7 (23.8-33.6), and 33.0 (27.7-38.2) per 1,000 person-years based on 4,489 person-years of follow-up, respectively. FPG was associated with the incidence of diabetes, IGT, and IFG. The multivariate-adjusted hazard ratios (95% CI) for diabetes, IGT, and IFG were 1.36 (1.01-1.84), 1.45 (1.10-1.91) and 1.31 (1.00-1.71), for the highest quintile of FPG compared with the lowest quintile, respectively.

Conclusions: An increase in FPG in the normal range is associated with an increase in the incidence of IGT, IFG, and type 2 diabetes. These results prove FPG in the normal range to be useful in identifying apparently healthy FDRs of patients with type 2 diabetes at risk of developing prediabetes and diabetes.

Keywords: prediabetes · type 2 diabetes · impaired fasting glucose · impaired glucose tolerance · first-degree relative