In the United States today, diabetes mellitus is a public health nightmare.Consider the following:

• Of the estimated 15.6 million persons nationwide who have diabetes, a projected 5.4 million persons are unaware that they have the disease; individuals may remain undiagnosed for an average of five to 10 years.

• Diabetes has the highest direct costs for health care of any disease category: the National Institutes of Health estimated that the total cost was $91.1 billion in 1995, and diabetes is responsible for one in every seven health care dollars spent in the United States.

• Diabetic retinopathy is the number one cause of blindness in working-age adults in the United States, but an estimated 90 percent of vision loss associated with diabetes is preventable.

• Diabetic nephropathy is the number one cause of end-stage renal disease in the United States, but an estimated 90 percent of future cases of end-stage renal disease related to diabetes are preventable.

• Diabetes is the number one cause of non-traumatic amputations in the United States, but an estimated 85 percent of cases of limb loss associated with diabetes are preventable.

• Health risks in patients with diabetes can be dramatically reduced by careful glucose control, aiming for near-normal levels of hemoglobin A_1c (HbA_1c), but national surveys demonstrate that only 12 percent of patients achieve these near-normal levels and that the average HbA_1c level among diabetic patients is 9.1 percent (normal: 3.9 to 6.0 percent); even worse, the majority of patients do not have HbA_1c measurements performed at all.

The benefits of glycemic control have been unambiguously demonstrated in both type 1 diabetes mellitus (formerly known as insulin-dependent diabetes mellitus, or IDDM) and type 2 diabetes mellitus (formerly known as non–insulin-dependent diabetes mellitus, or NIDDM).^1,2 Over the past several years, the introduction of newer pharmacologic agents has made disease management easier and permitted development of treatment algorithms that facilitate the attainment of the requisite control.³ However, to take advantage of such treatment, patients need to be diagnosed.

Unfortunately, as pointed out by Mayfield⁴ in this issue of American Family Physician, the previous diagnostic criteria were based mainly on the use of the oral glucose tolerance test, which was inconvenient enough that it had limited use in clinical practice. As a consequence, the default criterion for diagnosis became the fasting plasma glucose level alone, which was pegged at a plasma glucose level of 140 mg per dL (7.8 mmol per L). Two problems occurred with using this level as a cutoff point. First, it simply was too high, based on retinopathy risk (see Figure 1 in the Mayfield⁴ article). Second, this cutoff point did not correspond with the oral glucose tolerance test level recommended for a diagnosis of diabetes. The unfortunate consequence was that patients with severe enough disease to lead to complications remained undiagnosed.

These factors led the American Diabetes Association (ADA)⁵ and the World Health Organization (WHO)⁶ to commission expert committees to examine the available data and make recommendations that might allow persons with diabetes to be more easily diagnosed in clinical practice. The recommendations entail a major shift in the way diabetes is diagnosed. The expert committees performed analyses showing that lowering the fasting plasma glucose cutoff point to 126 mg per dL (7.0 mmol per L) or higher would result in two things. First, the change would acknowledge that the risk of retinopathy begins at a lower fasting plasma glucose level than is now used for diagnosis. Second, most persons with undiagnosed diabetes would be identified, without much risk of a false-positive diagnosis. Thus, 126 mg per dL (7.0 mmol per L) becomes a surrogate for an oral glucose tolerance test two-hour value of 200 mg per dL (11.1 mmol per L). This change in the cutoff point does not increase the number of persons with diabetes. Rather, it increases the number of persons with known diabetes. That is why it is a crucial public health measure.

The old criteria used a fasting plasma glucose level of 115 mg per dL (6.4 mmol per L) or lower for normal. In contrast, the new criteria use a fasting plasma glucose level of 110 mg per dL (6.1 mmol per L) or lower for normal. Persons who have fasting plasma glucose levels between 110 and 125 mg per dL (6.1 and 6.9 mmol per L), which are too high to be considered altogether normal, are now defined as having “impaired fasting glucose.” This group of persons is considered to be at increased risk of diabetes, similar to those with impaired glucose tolerance, who have two-hour oral glucose tolerance test values of 140 to 199 mg per dL (7.8 to 11.0 mmol per L).

Measurement of HbA_1c is not currently recommended for the diagnosis of diabetes, although some studies have shown that the frequency distributions for HbA_1c have characteristics similar to those of the fasting plasma glucose test and the two-hour plasma glucose test. However, both HbA_1c and fasting plasma glucose (in type 2 diabetes) have become the measurements of choice in monitoring the treatment of diabetes, and decisions on when and how to implement therapy are often made on the basis of HbA_1c levels. The revised criteria are for diagnosis and are not treatment criteria or goals of therapy. No change was made in the ADA recommendations of a fasting plasma glucose level of 120 mg per dL (6.7 mmol per L) or lower and HbA_1c levels of 7.0 percent or lower as treatment goals.

Widespread adoption of the new criteria may have a large impact on the number of persons actually diagnosed with diabetes. Presently, about 30 to 40 percent of adults with diabetes in the United States are undiagnosed, but many might now be diagnosed if the simpler fasting plasma glucose test were always used. The use of the fasting plasma glucose test facilitates screening for type 2 diabetes.

Screening is important for a variety of reasons. Hyperglycemia is important in the pathogenesis of the specific complications of diabetes mellitus—microangiopathy (retinopathy and nephropathy) and neuropathy. Meticulous glycemic control slows the course of development of diabetic complications.^1,2 Prolongation of normoglycemia should reduce the risk of diabetic complications. Studies suggest that, in the earlier stages (impaired fasting glucose and impaired glucose tolerance), interventions such as diet and exercise may forestall the evolution of type 2 diabetes.⁷ Screening for type 2 diabetes is now easy because only a simple fasting plasma glucose test is required. The more cumbersome oral glucose tolerance test is no longer the primary screening tool. Screening and early diagnosis of type 2 diabetes should be highly cost effective. All adults over age 45 should be screened every three years. All individuals at higher risk (based on obesity, ethnicity, etc.) should be screened annually, starting at an earlier age.