FPG. The FPG measures the concentration of glucose in the plasma (liquid) portion of the blood after a person has fasted for at least eight hours. Currently, a plasma glucose level over 126 milligrams per deciliter (mg/dl) is called diabetes, and, according to American Diabetes Association criteria, a level from 100 mg/dl to 125 mg/dl is called prediabetes.
OGTT. The OGTT involves two measurements of glucose in blood plasma: the first after a person has fasted for at least eight hours, the second two hours after the person has ingested 75 grams of glucose. A two-hour level of 200 mg/dl or higher is called diabetes, and a level from 140 mg/dl to 199 mg/dl is called prediabetes. (When the OGTT is used to detect gestational diabetes, or diabetes that occurs during pregnancy, different amounts of glucose and more frequent blood tests may be used, and the blood glucose levels at which diabetes is diagnosed are different, too.)
Random plasma glucose. This test measures blood glucose without regard to when a person last ate. A level of 200 mg/dl in a person who also has symptoms of diabetes can mean a person has diabetes. Generally, the diagnosis is confirmed with an FPG or an OGTT.
A1C. The A1C test measures the percentage of hemoglobin molecules in the blood that have glucose molecules attached to them. Currently, a test result of 6.5% or higher is diagnostic of diabetes.
From test to diagnosis
As early scientists no doubt discovered, everyone has some glucose in their blood. So how do you know whether a person has diabetes?
Just as methods of diagnosing diabetes have changed over the years, so have beliefs about what level of blood glucose should be called diabetes. In 1979, for example, the World Health Organization and the National Diabetes Data Group issued criteria for the diagnosis of diabetes stating that a fasting plasma glucose test result of 140 mg/dl or higher indicated diabetes. For the oral glucose tolerance test, a two-hour blood glucose level of 200 mg/dl or higher meant diabetes.
Almost 20 years later, in 1997, the American Diabetes Association updated these criteria, stating that a fasting plasma glucose test result of 126 mg/dl or higher indicated diabetes. But they retained the diagnostic level of a two-hour reading of 200 mg/dl or higher for the oral glucose tolerance test (for people who are not pregnant).
Why the change? High blood glucose may or may not cause symptoms, depending on how high it is and how accustomed a person has become to living with elevated blood glucose. (This is why people with Type 2 diabetes often have the condition for many years before they are diagnosed.) But whether or not high blood glucose is causing noticeable symptoms, it is potentially damaging a variety of bodily systems and setting the stage for long-term diabetes complications. The change from 140 mg/dl to 126 mg/dl when using the fasting plasma glucose test was largely based on studies showing that the number of cases of retinopathy (a form of diabetes-related damage to the eye) increased at fasting plasma glucose levels of 126 mg/dl and higher.
As researchers continue to conduct studies in which they measure participants’ blood glucose levels and whether and when these people develop diabetes-related complications, it’s possible the diagnostic cut point will change again.
In 2002, the term “prediabetes” became more widely used to describe blood glucose levels that are higher than normal but not high enough for a diagnosis of diabetes. Many people with prediabetes go on to develop Type 2 diabetes, but it is possible to take preventive action. There is evidence that increasing physical activity and improving eating habits can delay or prevent the onset of Type 2 diabetes in those with prediabetes.