News & Research

Association of A1C and fasting plasma glucose levels with diabetic retinopathy prevalence in the U.S. population, by Yiling J. Cheng and colleagues. Diabetes Care 32:2027–2032, 2009

What is the problem and what is known about it so far?

Doctors often use a lab test called the fasting plasma glucose (FPG) test to find out whether someone has diabetes. According to the American Diabetes Association, people who have an FPG level of at least 126 mg/dl have diabetes. This cut-off point was selected, in part, because it is linked to the presence of diabetic eye disease, one of the earliest complications to develop in people with diabetes. Researchers and doctors are now considering whether the A1C test, which measures blood glucose over several months, might also be used to diagnose diabetes. Unlike FPG, A1C tests do not require patients to fast overnight and their results are less affected by short-term changes in eating and exercise. However, no specific A1C cut-off point has been set to diagnose diabetes.

Why did the researchers do this particular study?

The researchers wanted to look at how many people with A1C and FPG levels have diabetic eye disease. They wanted to see which test was more linked to eye disease and could therefore be a better test to diagnose diabetes.

Who was studied?

The study included 1,066 adults who participated in a large national health study called the National Health and Nutrition Examination Survey (NHANES).

How was the study done?

The researchers looked at participants’ A1C and FPG results and the results of the eye disease studies done as part of NHANES.

What did the researchers find?

There was a sharp increase in diabetic eye disease for those with A1C levels of 5.5% or higher and for those with FPG levels of 126 mg/dl and higher. Eye disease was more strongly linked to A1C level than to FPG level.

What were the limitations of the study?

Although eye disease is a complication of diabetes, some people without diabetes still get same type of eye disease. Relying on the presence of eye disease to set test levels to diagnose diabetes may not be completely accurate. Some of the people in this study were already taking medicines for diabetes, which may have affected the results. However, the researchers took this into account when looking at the results. A1C is not always measured the same way, and different methods of measuring A1C could give different results.

What are the implications of the study

Debate continues about which test is best to diagnose diabetes. For now, people who have an FPG of 126 mg/dl or higher have diabetes. This study suggests that diabetes could also be diagnosed with an A1C level of 5.5% or higher. However, more study is needed before doctors can start using the A1C test to diagnose diabetes.

FOR MORE INFORMATION

Test for Diabetes and Pre-Diabetes

Diabetic Eye Disease