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Oral Glucose Tolerance Test (OGTT): Undeniably the First Choice Investigation of Dysglycaemia, Reproducibility can be Improved
Type 2 diabetes mellitus accounts for ≈90–95% of those with diabetes, about 50% of those with type 2 diabetes are unaware and it can remain undiagnosed for up to 12 years, ≥25% of people have evidence of microvascular complications at diagnosis. The consequences of diabetes can be reduced by screening and early interventions. Urinalysis as a screening test is limited by its low sensitivity ranging from 21% and 64%, though has high specificity (>98%), it has a place where no other procedure is available. Fasting plasma glucose though recommended as a universal screening and diagnostic test for diabetes mellitus, a changed in the diagnostic criteria was made when this did not give corresponding hyperglycaemic impact compared to the OGTT results, bringing a complex and variable effect on the prevalence of diabetes and on subjects diagnosed. To date the searching to finding the corresponding FPG to what is normal or IGT is still ongoing. FPG testing poorly identify early signs of dysglycaemia. This is due to the difficulty ensuring compliance with instructions about fasting, FPG represents glucose handling during the moment of fasting period only and is affected easily by short-term lifestyle changes, FPG has diurnal variation, higher in the morning than in the afternoon, these may cause serious misclassifications. OGTT do indicates the pathophysiology responsible for diabetes better as it provides information on what happens in the postprandial state when the functional capacity of pancreatic β-cell is crucial. It accurately detects changes in post-prandial glycaemia that tend to precede changes in fasting glucose. OGTT is the gold standard for the diagnosis of GDM and the only means of identifying people with IGT and WHO placed emphasis on the OGTT as the “gold standard”, in diagnosis of dysglycaemia. Reproducibility can be improved remarkably when patient preparation, a forvarable atmosphere during the procedure, standardized sampling protocol, sample handling, and analysis are given high attention. Measurement of A1c equals the assessment of hundreds of FPG levels and also captures postprandial glucose peaks. Regrettably, it has been shown that 44% of people with newly diagnosed diabetes with OGTT had A1c <6.0% and that a stronger correlations with plasma glucose is better in subjects with known diabetes, but not in the general population. A1C values just above the upper limits of normal require OGTT to be correctly interpreted; it is not available in many part of the world. Finally, A1c can not diagnose IFG and IGT to disclose high-risk subjects for diabetes. In conclusion an OGTT is undeniably the best test in investigation of dysglycaemia, either with the intention of testing for pre-diabetes, type 2 diabetes, or for gestational diabetes mellitus.
Part of the book: Type 2 Diabetes