If a person blood sugar is 120 mg/mL that person already have diabetes mellitus. Diabetes mellitus is a medical condition in which the level of glucose in the blood is very high because the glucose were not able to enter the body cells. The condition is usually cause by absence or inadequate level of insulin in the body.
Assuming the blood glucose is fasting blood glucose, not post prandial blood glucose, then it should be pre diabetes since the glucose level is 100-125mg/dl.
Fasting blood glucose for diabetes diagnosis should be >125mg/dl and post prandial/after eating blood glucose should be >200mg/dl. Prediabetes patient must be tested again to make sure the diagnosis, probably with blood HbA1C or oral glucose tolerance test
