What Is Glucose Tolerance?

Glucose tolerance tests are performed to determine whether the body is able to use and store glucose properly.¹ Normal plasma glucose level is under 140 mg/dL after two hours of drinking a glucose solution, meaning less than 140 mg (ie, milligram is 1/1000 gram) of glucose is present in a deciliter (ie, deciliter is 1/10 liter) of blood. Measuring glucose tolerance is one of the diagnostic tests for diabetes.²

How Is Glucose Tolerance Measured?

Glucose tolerance is usually measured using the oral glucose tolerance test (OGTT).² Individuals will be asked to fast overnight for at least 8 hours before taking the test. A health care professional will then draw blood to measure the fasting glucose levels, then a 75 g oral glucose drink will be given. Blood draws will be taken again 1 hour and 2 hours after the drink. Those undergoing the exam are advised to consume a regular mixed diet (ie, >150 g of carbohydrates) 3 days before the OGTT as carbohydrate restriction and fasting can give falsely high glucose levels with OGTT.

What Is Impaired Glucose Tolerance? 

Impaired glucose tolerance (IGT) in a diagnostic sense is defined as a 2-hour plasma glucose level between 140 mg/dL – 199 mg/dL during a 2-hour 75 g OGTT. IGT is one of the ways to diagnose prediabetes.² When the 2-hour 75 g OGTT plasma glucose levels meet or exceed 200 mg/dL, diabetes is diagnosed. Diagnosis of diabetes, however, requires two abnormal test results (eg, OGTT and hemoglobin A1C) at the same time or abnormal test results for one test, at two different times.

Impaired or abnormal glucose tolerance signifies that the body is unable to correctly respond to rising levels of glucose in the blood.³ 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.