From the Guidelines
Fasting HbA1c is generally considered better than fasting glucose for diagnosing prediabetes, as it provides a more comprehensive picture of glycemic status over the past 2-3 months. According to the most recent study from 2025 1, an HbA1c range of 5.7–6.4% identifies individuals at high risk for diabetes and cardiovascular outcomes. This is supported by another high-quality study from 2023 1, which recommends using HbA1c levels between 5.7% and 6.4% to define high risk for developing diabetes.
Key Points to Consider
- HbA1c measures average blood sugar over the past 2-3 months, providing a more comprehensive picture of glycemic status compared to fasting glucose, which only captures a single point in time.
- An HbA1c between 5.7% and 6.4% indicates prediabetes, while fasting glucose between 100-125 mg/dL suggests impaired fasting glucose.
- The advantage of HbA1c is that it doesn't require fasting, has less day-to-day variability, and better reflects overall glucose control.
- Certain conditions like anemia, hemoglobinopathies, or recent blood loss can affect HbA1c results, making fasting glucose more appropriate in these cases.
Clinical Implications
Healthcare providers often use both HbA1c and fasting glucose tests together, sometimes adding an oral glucose tolerance test for a complete evaluation of glucose metabolism. As noted in a study from 2014 1, preventive interventions are effective in groups of people with HbA1c levels both below and above 5.9%, and an HbA1c range of 5.7–6.4% is associated with a similar diabetes risk to the high-risk participants in the Diabetes Prevention Program. Therefore, HbA1c is the preferred test for diagnosing prediabetes due to its ability to provide a more accurate and comprehensive picture of glycemic status.
From the Research
Comparison of Fasting HbA1c and Fasting Glucose for Prediabetes
- Fasting HbA1c and fasting glucose are both used to diagnose prediabetes, but their effectiveness as screening tools is still being researched 2, 3.
- A study published in JAMA found that prediabetes is defined by a fasting glucose level of 100 to 125 mg/dL, a glucose level of 140 to 199 mg/dL measured 2 hours after a 75-g oral glucose load, or glycated hemoglobin level (HbA1C) of 5.7% to 6.4% or 6.0% to 6.4% 2.
- Another study published in the Annual review of public health noted that five different definitions of prediabetes are used in current practice, which are based on different cut points of HbA1C, fasting glucose, and 2-h glucose, and that a major challenge for the field is a lack of guidance on when one definition might be preferred over another 3.
Effectiveness of HbA1c as a Screening Tool
- A study published in Clinical biochemistry found that the recommended threshold HbA1c value of 6.5% did not give the optimal combination of negative predictive value (NPV) and positive predictive value (PPV) compared to a threshold HbA1c value of 7.0% 4.
- The same study found that the optimal HbA1c value for the diagnosis of diabetes is 7.0%, but even at this HbA1c the positive predictive value (PPV) is suboptimal and may cause up to 12% of patients without diabetes to be classified as having diabetes mellitus 4.
Comparison of HbA1c and Fasting Glucose in Relation to Exercise and Lifestyle Modification
- A study published in Advances in experimental medicine and biology found that exercise as a regular physical activity at a medium to vigorous intensity is found to be an efficient influencer that would switch back most of the known type 2 diabetes mellitus factors toward healthier positions, including improved insulin sensitivity and reductions in glycosylated hemoglobin (A1C) 5.
- Another study published in JAMA found that structured exercise training that consists of aerobic exercise, resistance training, or both combined is associated with HbA1c reduction in patients with type 2 diabetes, and that structured exercise training of more than 150 minutes per week is associated with greater HbA1c declines than that of 150 minutes or less per week 6.