The diagnosis of diabetes mellitus is made primarily through the findings of elevated glucoses on fasting laboratory samples, random glucoses during outpatient or inpatient care, and, much less commonly now, after formal oral glucose tolerance testing (OGTT). In 1997, the American Diabetes Association (ADA) revised its diagnostic criteria to rely solely on a fasting plasma glucose value greater than 126 mg/dL (7.0 mmol/L), rather than on a fasting glucose over 140 mg/dL or a 2-h oral glucose tolerance test plasma glucose value over 200 mg/dL, as had been recommended by the 1980-1985 World Health Organization (WHO) diagnostic criteria and the ADA 1979 criteria for diabetes.
The new ADA criteria also recommended two other diagnostic classes. Impaired fasting glucose (IFG) is defined as a fasting plasma glucose (FPG) between 110 mg/dL (6.1 mmol/L) and 126 mg/dL (7.0 mmol/L); normal fasting glucose is defined as a fasting plasma glucose less than 110 mg/dL. The OGTT is not recommended for routine diagnosis of glucose intolerance or diabetes. The 1998 preliminary report of WHO essentially endorses the ADA 1997 recommendation, with the exception that they advocate the use of OGTT.
The new ADA 1997 criteria change the incidence of diabetes by age, sex, and ethnicity, resulting in a significant increase in the number of individuals diagnosed with diabetes mellitus while perhaps excluding significant numbers of individuals who would have gained the diagnosis through postchallenge glucose elevations. From all the population data available, the criteria of a 2-h plasma value greater than 200 mg/dL (11.1 mmol/L) would be met by almost all patients who met the older fasting value of 140 mg/dL (7.8 mmol/L). However, it would also be met by many individuals with a lower fasting value.
The new fasting value of 126 mg/dL (7 mmol/L) is reported to be more in agreement with the diagnosis of diabetes by the 2-h post-OGTT plasma glucose value of 200 mg/dL. However, the data suggest this may not be the case. At least 25 studies have examined the impact of the new 1997 ADA criteria with the older 1985 WHO criteria. These reports indicate that 11% to 80% of the individua1s diagnosed with diabetes mellitus by the WHO criteria will be missed if the diagnosis is solely based on the lower FPG, which appears to particularly include the elderly. The observations leading to the use of 200 mg/dL level and the difficulties with this level suggest further revisions may be needed. Using data from the NHANES III survey, comparisons of the results of FPG with the 2-h post-OGTT plasma glucose level are more than 50% discrepant.
The ADA’s 1997 report has stated that the justification for the cut point for the 2-h post-OGTT glucose level of 200 mg/dL is derived, in part, from the evidence that the prevalence of microvascular complications increases dramatically at this point. In addition, the 2-h plasma glucose value following an OGTT from many large populations has a bimodal distribution. The nadir intersection of the two modes is known as the antimode and it shifts to the right with advancing age. The 200 mg/dL level represents the average level of the antimodes from several large population studies (Pima Indians, Naruans, Samoans, Mexican-Americans, and East Indians). However, the antimodes from these populations arc quite variable (range, 143-310 mg/dL) and do not support the average 200 mg/dL level. They do, however, increase with age.
Many of the reported studies show that the 1997 ADA diagnosis standards do not result in equal sensitivity for fasting and 2-h glucose levels, especially in older individuals. Although the use of fasting plasma glucose alone for diabetes diagnosis may simplify testing, the WHO criteria would identify a much greater percentage of elderly subjects with diabetes or impaired glucose testing and move ahead the diagnosis by 5 to 8 years over criteria based on fasting glucose levels. As diabetes prevention efforts increase, a move to a more sensitive bias in diagnosis may be needed.
The majority of newly diagnosed diabetic patients (> 90%) emerge from the population with impaired glucose tolerance (IGT), no matter how it is defined, and are at higher risk for cardiovascular disorders.