Oral Glucose Tolerance Tests and Long-Term Glucose Levels

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Woerle HJ, Pimenta, WP, Meyer C, et al.: Diagnostic and therapeutic implications of relationships between fasting, 2-hour postchallenge plasma glucose and hemoglobin A1C values. Arch Intern Med 164:1627-1632, 2004.

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

Doctors use several different tests to diagnose diabetes and other blood glucose control problems. One is the fasting plasma glucose (FPG) test, which measures the level of glucose before a person eats. Another is the 2-hour postchallenge plasma glucose (PCPG), which measures how the body responds to a sugary drink.

A test called glycated hemoglobin (A1C) is a measure of long-term blood glucose control. The lower the A1C, the better. The International Diabetes Federation and the American College of Endocrinology recommend a target A1C level of 6.5%, while the American Diabetes Association suggests a target of 7%.

Why did the researchers do this particular study?

The researchers wanted to know how FPG and PCPG results relate to long-term glucose levels. They also wanted to know which one is more important to control in order to reach healthier A1C levels.

Who was studied?

Between 1986 and 2002, 404 people with normal blood A1C levels were studied.

How was the study done?

FPG, PCPG, and long-term glucose levels were measured for all participants.

What did the researchers find?

Most of the people with long-term glucose levels within a normal range of 6% to 7% had normal FPG levels but high PCPG levels. The researchers say PCPG is a more sensitive test of blood glucose problems than FPG.

What are the limitations of the study?

Other things, such as physical fitness, may influence glucose control; these were not measured. Also, tests like FPG are like a one-time snapshot and may not reflect changes in a person's blood glucose throughout the day.

What are the implications of the study?

Doctors should focus on controlling blood glucose levels after meals. The results suggest that the upper limit that is considered normal for FPG may be too high, and should be lowered to help prevent disease.