Pitfalls of HbA1c in the Diagnosis of Diabetes.
Many healthcare providers screen high-risk individuals exclusively with an HbA1c despite its insensitivity for detecting dysglycemia. The two cases presented describe the inherent caveats of interpreting HbA1c without performing an oral glucose tolerance test (OGTT). The first case reflects the risk of over-diagnosing type 2 diabetes (T2D) in an older African American male in whom HbA1c levels, although variable, were primarily in the mid- prediabetes range (5.7-6.4% [39-46 mmol/mol]) for many years although the initial OGTT demonstrated borderline impaired fasting glucose (IFG) with a fasting plasma glucose (FPG) of 102 mg/dl [5.7 mmol/L]) without evidence for impaired glucose tolerance (IGT) (2-hour glucose >140-199 mg/dl ([7.8 -11.1 mmol/L]). As subsequent HbA1c levels were diagnostic of T2D (6.5-6.6% [48-49 mmol/mol]), a second OGTT performed was normal. The second case illustrates the risk of under-diagnosing T2D in a male with HIV having normal HbA1c levels over many years who underwent an OGTT when mild prediabetes [HbA1c = 5.7% (39 mmol/mol)] developed which was diagnostic of T2D. To avoid inadvertent mistreatment, it is therefore essential to perform an OGTT, despite its limitations, in high-risk individuals particularly when glucose or fructosamine and HbA1c values are discordant. Innate differences in the relationship between fructosamine or fasting glucose to HbA1c are demonstrated by the glycation gap or hemoglobin glycation index.