Managing blood sugar is central to modern health, especially for those with prediabetes or diabetes. Daily finger-prick tests only provide a snapshot of blood glucose at a single moment, making long-term assessment difficult. Healthcare professionals rely on two metrics for a comprehensive view of glucose management: Hemoglobin A1c (A1C) and Estimated Average Glucose (eAG). These tools translate short-term fluctuations into a reliable long-term average, providing context to evaluate treatment effectiveness.
Defining the Metrics A1C and eAG
The A1C test measures the percentage of red blood cells that have sugar attached, a process called glycation. Hemoglobin, the oxygen-carrying protein inside red blood cells, binds with circulating glucose. Since red blood cells typically live for two to three months, the A1C test captures the average blood sugar level over that entire period. This measurement is expressed as a percentage, reflecting the amount of glycated hemoglobin.
The A1C percentage provides a stable, long-term indicator of glucose control, unlike single blood glucose readings which fluctuate based on meals or activity. The Estimated Average Glucose (eAG) is the A1C percentage converted into the units used for daily monitoring, such as milligrams per deciliter (mg/dL). The eAG translates the abstract percentage into a familiar average daily blood sugar number, standardizing the language for patients and clinicians.
The Direct Conversion and Clinical Reference Ranges
An average blood sugar level of 122 mg/dL is equivalent to approximately 5.9% A1C. This conversion is derived from a standardized formula linking the percentage of glycated hemoglobin to the average plasma glucose level. A reading of 122 mg/dL is higher than optimal but not yet diagnostic of diabetes.
The clinical community uses specific A1C ranges to define metabolic states. A normal A1C result is below 5.7%. The range between 5.7% and 6.4% is classified as prediabetes, indicating an elevated risk for developing type 2 diabetes. A result of 6.5% or higher on two separate tests confirms a diagnosis of diabetes. Therefore, a 5.9% A1C places an individual in the prediabetes category, requiring intervention.
Why A1C Is Used for Diagnosis and Monitoring
Healthcare providers rely on the A1C test for both initial diagnosis and ongoing management because it provides an objective measure of long-term glucose exposure. Unlike a fasting plasma glucose test, which can be manipulated by recent diet changes, the A1C is stable. Its reflection of the full three-month period reduces the influence of day-to-day variations, making it a reliable tool for screening and diagnosing metabolic conditions.
For individuals with diagnosed diabetes, the A1C test routinely monitors treatment effectiveness. It shows whether lifestyle changes, medication, or insulin therapy successfully keep blood sugar levels within a target range. When glucose control is stable, the test is typically performed every six months. If targets are not met or therapy is adjusted, testing is often increased to every three months. The A1C value guides doctors in adjusting a patient’s treatment plan to reduce the risk of long-term complications.
Conditions That Can Affect A1C Accuracy
While the A1C test is valuable, certain medical conditions can interfere with its accuracy. The test relies on the typical lifespan of red blood cells, so any condition affecting red blood cell turnover can skew the result. Conditions that shorten red blood cell lifespan, such as hemolytic anemia or significant recent blood loss, can lead to a falsely low A1C reading.
Conversely, conditions like severe iron deficiency anemia or B12 deficiency can result in a falsely high A1C value. Other factors, including chronic kidney disease, liver disease, and pregnancy, also affect the test’s reliability. Hemoglobin variants, common in certain ethnic groups, may also interfere with specific laboratory testing methods. In these situations, healthcare professionals may opt for alternative tests, such as fructosamine or glycated albumin, for a clearer picture of average glucose control.