For decades, the diagnosis of T2DM has been based on plasma glucose criteria: either a fasting plasma glucose (FPG) level of 126 mg/dL or higher, a 2-hour postprandial glucose (PPG) level of 200 mg/dL or higher after ingestion of 75 g of oral glucose, or a random plasma glucose level of greater than 200 mg/dL with symptoms of diabetes mellitus such as polyuria, polydipsia, or weight loss. The use of other measures—such as HbA
1c levels, which reflect long-term glycemic exposure—has been the subject of ongoing debate. In 2008, an International Expert Committee with members representing the American Diabetes Association (ADA), the European Association for the Study of Diabetes (EASD), and the International Diabetes Foundation convened and recommended that the diagnosis of T2DM be made if the HbA
1c level is 6.5% or higher; the ADA affirmed this decision.
13,17 One limitation of measuring HbA
1c levels is that, because it is a hemoglobin-based test, anemia or hemoglobinopathies may interfere with the validity of the results. Many physicians acknowledge that measurement of HbA
1c levels is a powerful tool, but they still consider measurement of FPG and PPG levels to be the standard of testing. If HbA
1c testing is not possible, diagnosis should be based on previously recommended diagnostic methods (eg, measurement of FPG levels or 2-hour PPG levels, with confirmation by repeat testing on a separate day).
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As a practical matter, if a patient's FPG and HbA1c levels are both elevated, even though the guidelines recommend that the diagnosis not be made until the test results are confirmed, many physicians will assume T2DM is present and base their recommendations to the patient on the basis of that assumption—without necessarily giving the patient that diagnosis until they have had a chance to confirm it with repeat testing. Arriving at a correct, valid diagnosis is especially important for patients with T2DM, because any error could lead to misinformation in medical records and difficulty with healthcare insurance once the diagnosis of T2DM is a matter of record.