Oral glucose tolerance test‐based calculation identifies different glucose intolerance phenotypes within the impaired fasting glucose range

AIMS/INTRODUCTION The conventional oral glucose tolerance test (OGTT) cannot detect future diabetics among isolated impaired fasting glucose (is-IFG) nor normal glucose tolerant (NGT) groups. By analyzing the relationship between fasting (FPG) and 2-h plasma glucose (2hPG), the present study identifies is-IFG subjects liable to worsening glucose homeostasis. MATERIALS AND METHODS Oral glucose tolerance test was carried out in 619 patients suffering from obesity, hypertension or dyslipidemia, whose FPG was in the 100-125 mg/dL range. We calculated the percentage increment of 2hPG with respect to FPG (PG%) in these patients using the formula: ([2hPG - FPG] / FPG) × 100. Differences in β-cell function within is-IFG patients were assessed by estimated insulin sensitivity index (EISI), first-phase insulin release (1stPH) and 1stPH/1/EISI (1stPHcorrected). RESULTS Diabetes was diagnosed in 69 patients (11.2%), combined IFG/impaired glucose tolerance (IGT) in 185 patients (29.9%) and is-IFG in 365 patients (58.9%). Is-IFG was subdivided into PG% tertile groups: the percentage of females increased from 25% in the lowest to 45.2% in the highest tertile (χ(2) = 18.7, P < 0.001). Moving from the lowest to the highest PG% tertile group, insulin and 2hPG concentrations rose, whereas FPG, EISI, and 1stPHcorrected decreased progressively and significantly. Furthemore, PG% correlated inversely with EISI (r = -0.44, P < 0.0001) and 1stPHcorrected (r = -0.38, P < 0.0001). CONCLUSIONS Oral glucose tolerance test does differentiate the great heterogeneity in metabolic disorders of patients with FPG 100-125 mg/dL. Furthermore, PG% can expand the diagnostic power of OGTT in the is-IFG range by distinguishing metabolic phenotypes very likely to herald different clinical risks.