The standard Oral Glucose Tolerance Test (OGTT) does not measure first phase insulin response. To measure that it would be necessary to look at blood sugar at 30 minutes after the meal. There’s a study which makes this point (Hulman, et.al. Glucose patterns during an oral glucose tolerance test and associations with future diabetes, cardiovascular disease and all-cause mortality rate. Diabetologia. 2018 Jan;61(1):101-107.).
Elevated 30 min glucose is associated with increased risk of diabetes and all-cause mortality rate independent of fasting and 2 h glucose levels. Therefore, subgroups at high risk may not be revealed when considering only fasting and 2 h glucose levels during an OGTT.
This subject is covered in this review (Cheng K, et.al. First phase insulin secretion and type 2 diabetes. Curr Mol Med. 2013 Jan;13(1):126-39.).
Pancreatic β-cells secrete insulin in a biphasic manner, first and second phase insulin secretion, and loss of first phase insulin secretion is an independent predictor of T2D onset.