HbA1c as a marker of prediabetes: A reliable screening tool or not?

Increasing global prevalence of type 2 diabetes (T2D) has resulted in concerted efforts to improve predictors for development of this obesity-related disorder. Establishing markers that identify prediabetes, an intermediary state of glycaemia above that of healthy individuals but below frank T2D, is an important focus. International cut offs have long been based on the 2 h WHO-defined oral glucose tolerance test (OGTT), but more recent use of the quicker and cheaper marker of glycated haemoglobin (HbA1c) has become widespread in clinical practice and public health. The definition of people with prediabetes in turn has expanded from those with impaired glucose tolerance (IGT) to include individuals with impaired fasting glucose (IFG) and/or raised HbA1c. Whilst HbA1c has been recommended since 2010 for both T2D and prediabetes screening, concerns have been raised over validity particularly for identifying those who will later develop T2D. Depending on criteria, HbA1c may identify only 50% with abnormal OGTT or misclassify those with normal physiology. Models predicting average time intervals for progression to T2D from prediabetes are commonly limited by ethnic, racial and gender differences, and different criteria further result in variable estimates of prediabetes prevalence and impact those eligible for lifestyle interventions. Whilst HbA1c may provide a good marker of frank T2D, some recommend its use in prediabetes only in conjunction with fasting plasma glucose (FPG). This review updates current opinion on HbA1c as an effective screening method for categorising high-risk prediabetic individuals and those requiring fast track into lifestyle modification programs.