From the Guidelines
A patient can have low Fasting Blood Sugar (FBS) with a Hemoglobin A1c (HbA1c) in the prediabetic range due to several physiological patterns, including postprandial hyperglycemia, early insulin resistance, reactive hypoglycemia, or dawn phenomenon. This discrepancy typically occurs when a person experiences normal or low blood glucose levels while fasting but has significant blood glucose elevations after meals. Since HbA1c reflects average blood glucose levels over approximately three months, these post-meal spikes can raise the overall HbA1c despite normal fasting values.
Common Causes
- Early insulin resistance: the body maintains fasting glucose through increased insulin secretion but struggles with glucose loads
- Reactive hypoglycemia: blood sugar drops below normal after initially spiking post-meal
- Dawn phenomenon: morning cortisol causes early glucose elevation before the fasting test
- Certain medications like steroids: causing glucose fluctuations throughout the day As noted in the 2017 U.S. Department of Veterans Affairs/U.S. Department of Defense clinical practice guideline 1, many factors affect measurement of HbA1c besides the level of glycemia, including conditions that alter erythrocyte life span and hemoglobin variants.
Diagnostic Considerations
For accurate diagnosis, patients should consider continuous glucose monitoring or oral glucose tolerance testing to capture these variations. Lifestyle modifications focusing on complex carbohydrates, regular meal timing, and exercise can help manage this condition by reducing post-meal glucose excursions. The American College of Physicians guidance statement update from 2018 1 emphasizes the importance of individualized approach to achieve the lowest possible HbA1c with the least amount of hypoglycemia and medication burden.
Management
The 2014 Diabetes Care report 1 suggests that an A1C range of 5.7–6.4% identifies individuals with high risk for future diabetes, to whom the term prediabetes may be applied. Individuals with an A1C of 5.7–6.4% should be informed of their increased risk for diabetes as well as cardiovascular disease and counseled about effective strategies, such as weight loss and physical activity, to lower their risks. Certain diabetes medications, such as GLP1-agonists and SGLT2-inhibitors, can achieve reductions in HbA1c without hypoglycemia and favourable CVD outcomes, though with increased medication cost 1.
From the Research
Understanding the Relationship Between Fasting Blood Sugar and HbA1c
The question of how a patient can have low Fasting Blood Sugar (FBS) with a Hemoglobin A1c (HbA1c) in the prediabetic range can be approached by examining the differences in what these two measurements indicate about blood glucose control.
- HbA1c as a Marker of Average Blood Glucose: HbA1c measures the average blood glucose levels over the past 2-3 months, providing a long-term indicator of glucose control 2, 3, 4. It is widely used for diagnosing and monitoring diabetes and prediabetes.
- Fasting Blood Sugar (FBS): FBS, on the other hand, measures blood glucose at a single point in time, typically after an overnight fast. It reflects the body's ability to manage glucose levels during fasting periods.
Factors Influencing the Discrepancy
Several factors can contribute to the discrepancy between low FBS and an HbA1c in the prediabetic range:
- Glucose Variability: Individuals with prediabetes or diabetes can experience significant glucose variability throughout the day, with higher levels after meals and lower levels during fasting periods 5.
- Hypoglycemic Events: Frequent or severe hypoglycemic events, especially in older adults or those on certain medications, can lead to lower average FBS readings while HbA1c levels remain elevated due to overall higher glucose levels over time 6.
- Diagnostic Thresholds: The thresholds for diagnosing prediabetes and diabetes differ between FBS and HbA1c. Prediabetes is defined by an HbA1c of 5.7% to 6.4% or an FBS of 100 to 125 mg/dL. The discrepancy can arise when a patient's FBS is below the threshold but their HbA1c indicates prediabetes 3.
Clinical Implications
Understanding the relationship and potential discrepancies between FBS and HbA1c is crucial for accurate diagnosis and management of blood glucose levels.
- Comprehensive Assessment: Both FBS and HbA1c should be considered in the assessment of glucose control, as they provide complementary information about a patient's glycemic status 2, 4.
- Individualized Care: Treatment plans should be tailored to the individual, taking into account their specific glucose control profile, lifestyle, and risk factors for complications 5.