Do You Know the Difference Between Impaired Fasting Glucose and Impaired Glucose Tolerance?

Abstract & Commentary

Synopsis: The new criterion for FPG will identify many more individuals who are at risk for developing diabetes.

Source: The Expert Committee On Diagnosis and Classification of Diabetes Mellitus. Diabetes Care. 2003;26:3160-3167.

The current recommendations for the diagnosis of diabetes and for lesser degrees of impaired glucose regulation, ie, impaired fasting glucose and impaired glucose tolerance (IFG & IFT) were established by an International Expert Committee in 1997. The committee recommended several changes to the diagnostic criteria for diabetes and for lesser degrees of impaired glucose regulation. The following major changes addressed:

• The 1997 report recommended that the cut point for the diagnosis of diabetes be changed from a fasting blood glucose of 140 mg/dL or above, to a level of 126 mg/dL or above. This change was based on data that showed an increase in the prevalence and incidence of diabetic retinopathy beginning at approximately 126 mg/dL, as well as to reduce the discrepancy that existed in the number of cases detected by the FPG cut point of 140 mg/dL and the 2-h value in the oral glucose tolerance test (OGTT) of 200 mg/dL or higher.

• The normal fasting glucose was defined as 110 mg/dL.

• The HbA1c was not recommended as a diagnostic tests, because of a lack of standardization measures in various laboratories

• The OGTT (FPG and 2-h post glucose value) "was recognized as a valid way to diagnose diabetes." The use of the test for this purpose was discouraged because of inconvenience, lack of reproducibility, and greater cost. The diagnostic category IGT "was retained to describe people whose FPG was < 126 mg/dL but whose 2-h PG after 75 g of glucose was 140-199 mg/dL."

• The range of FPG levels between 110-126 mg/dLwas named impaired fasting glucose (IFG). Since 1997, many new data related to the diagnosis of diabetes have been published. Impaired glucose tolerance has been associated with cardiovascular disease (CVD) risk factors and CVD events, whereas IFG is much less strongly associated with CVD events and CVD mortality. In addition, clinical trials have shown that progression of IFG to diabetes could be delayed or prevented by lifestyle modification (diet and exercise), metformin, and acarbose.

In light of these and other new observations, a reconstructed International Expert Committee has made the following recommendations:

• The 2-hour criterion of 200 mg/dL identifies a larger population as having diabetes than the previous criterion of 140 mg/dL or higher. The committee lowered the FPG to 126 mg/dL to reduce this discrepancy. The committee recommended that this category not be changed.

• The data indicate that a larger number of individuals with IGT will eventually develop diabetes than the IFG. To make the 2 measurements more uniform, the committee recommended that the IFG level be reduced to 100 mg/dL rather than 110 mg/dL. The cut point for IGT remains at 140-199 mg/dL.

Although the HbA1c has become more standardized in the United States, many countries have not been able to standardize the assay. Also, the HbA1c may be affected by pregnancy, transfusions, uremia, and hemolytic anemia. The committee therefore recommended that the HbA1c be used as a monitor of therapy rather than for a definitive diagnosis of diabetes.

The FBG and the 2-h PG remain the tests of choice for the diagnosis of diabetes. The k2-h PG is a more sensitive assay in most populations but the FPG is more reproducible, less costly and more concise.

Table: Diagnostic Thresholds for Diabetes and
Lesser Degrees of Impaired Glucose Regulation

Comment by Ralph R. Hall, MD, FACP

The extensive discussion of the pathophysiology and rationale for these recommendations, in this article, are well worth reading. The review needs little discussion except to note that the new criterion for FPG will identify many more individuals who are at risk for developing diabetes. This is the population we should work with to attempt to delay or prevent diabetes and CVD.

Dr. Hall, Emeritus Professor of Medicine University of Missouri-Kansas City School of Medicine, is Associate Editor of Internal Medicine Alert.