Type 2 diabetes mellitus (T2DM) is the most common type of diabetes marked by persistently high blood glucose levels (ie, hyperglycemia).¹ T2DM is characterized by insulin resistance and non-autoimmune loss of pancreatic b-cell insulin secretion. 

Diabetes can be diagnosed using tests that measure the body’s response to the presence or absence of glucose.¹ The diagnostic tests approved for diabetes include the following: fasting plasma glucose value, 2-hour glucose value during a 75-g oral glucose tolerance test, hemoglobin A1C criteria, or random glucose value with either hyperglycemic crises or hyperglycemic symptoms. In the absence of hyperglycemic crises, diagnosis of diabetes requires two abnormal test results (eg, fasting plasma glucose and hemoglobin A1C) at the same time or abnormal test results for one test, at two different times.

1) Fasting Plasma Glucose Test 

Fasting plasma glucose (FPG) levels are measured after 8 hours of fasting (ie, no caloric intake).¹ A health care professional will draw blood from a vein, and fasting blood glucose levels will be measured.² 

Impaired or abnormal fasting plasma glucose value signifies that the body experiences hyperglycemia even in the absence of caloric consumption when blood glucose levels should normally be low.³ Studies indicate that impaired fasting glucose (IFG) is attributed to decline in pancreatic b-cell function (responsible for insulin secretion), reduced insulin sensitivity of the liver (reduced responsiveness to insulin to take up glucose), and inappropriately elevated glucagon secretion (a hormone that signals that glucose should be synthesized).

Normal FPG measures below 100 mg/dL, usually ranging between 80-90 mg/dL.¹ Individuals with FPG levels measuring between 100 mg/dL to 125 mg/dL are noted to have IFG and are diagnosed with prediabetes. When FPG measures above 125 mg/dL, diabetes is diagnosed.

2) Oral Glucose Tolerance Test

Glucose tolerance test using the oral glucose tolerance test (OGTT) is performed to determine whether glucose can be used and stored properly.^1,4 Individuals will be asked to fast overnight for at least 8 hours before taking the test, then a health care professional will draw blood to measure the fasting glucose levels. A 75 g oral glucose drink will be given, then blood draws will be taken again 1 hour and 2 hours after the drink. 

Impaired or abnormal glucose tolerance signifies that the body is unable to correctly respond to rising levels of glucose in the blood.⁵ It often tells that there are two different defects: impaired insulin secretion and insulin resistance. Pancreatic b-cell dysfunction can hinder appropriate secretion of insulin in response to rising blood glucose levels. Additionally, decreased sensitivity of various tissues to insulin caused by insulin resistance can hinder uptake of glucose into cells.

Normal plasma glucose level is under 140 mg/dL after 2 hours of drinking a glucose solution.¹ Individuals with a 2-hour plasma glucose level between 140 mg/dL – 199 mg/dL during a 2-hour 75 g OGTT is noted to have impaired glucose tolerance and is diagnosed with prediabetes. When the 2-hour 75 g OGTT plasma glucose levels meet or exceed 200 mg/dL, diabetes is diagnosed.

3) A1C Test

Hemoglobin is well known as the iron-containing metalloprotein found in red blood cells that helps transport oxygen in the body.⁶ Hemoglobin A1C (A1C) is hemoglobin that has become glycosylated, or coated with glucose, in the presence of elevated intracellular glucose levels.⁷

The A1C test reflects the average glycemia over approximately 3 months and is used as the primary tool for assessing glycemic control and as a strong predictor of future diabetic complications.⁸ Along with FPG levels and OGTT, the A1C test can be used to diagnose diabetes.⁹ Individuals with normoglycemia have A1C levels below 5.7%. Individuals with levels between 5.7% and 6.4% are diagnosed with prediabetes, and individuals with levels equal to or above 6.5% are diagnosed with diabetes.