The A1C test, also known as glycated hemoglobin or HbA1c, is a diagnostic and monitoring tool for blood sugar control. It provides a long-term measure of glucose management, offering a retrospective view that a single finger-prick test cannot. The test directly links a percentage result to an average blood sugar level, which is useful for assessing treatment effectiveness and risk of complications.
What A1C Measures and Why It Matters
The A1C test measures the percentage of hemoglobin in your red blood cells that has sugar attached to it, a process called glycation. Hemoglobin is the protein responsible for carrying oxygen throughout the body. When glucose is in the bloodstream, it binds to this protein.
The percentage of “sugar-coated” hemoglobin provides an average snapshot of your blood glucose levels over an extended period. This long-term view is possible because red blood cells have a typical lifespan of about 120 days. As a result, the A1C test reflects the average glucose exposure over the preceding two to three months.
This measurement differs from a standard daily blood glucose reading, which offers only a momentary measurement of sugar in the bloodstream. A single reading can fluctuate widely based on recent meals, stress, or physical activity. In contrast, the A1C gives a comprehensive picture of how consistently blood sugar has been managed over time. This makes it useful for diagnosing prediabetes and diabetes, and for tracking treatment progress.
Converting Blood Sugar to A1C: The 150 mg/dL Example
The A1C result is reported as a percentage, but healthcare providers often convert this figure into Estimated Average Glucose (eAG). The eAG is expressed in milligrams per deciliter (mg/dL) and represents the average daily blood sugar level that correlates with the A1C percentage. This conversion helps patients relate their long-term A1C percentage to the numbers they see on their home blood glucose meters.
The conversion uses a formula to bridge the gap between the two metrics: eAG (mg/dL) = (28.7 x A1C) – 46.7. When examining a sustained average blood sugar of 150 mg/dL, the corresponding A1C value is approximately 7.0%. For comparison, an eAG of 140 mg/dL corresponds to 6.5%.
If an individual’s average blood sugar levels consistently hover around 150 mg/dL over a two-to-three-month period, their A1C test result is expected to be near 7.0%. This conversion is a standardized calculation that allows for a clear understanding of the percentage in terms of daily blood sugar control.
Interpreting A1C Results and Clinical Ranges
The A1C percentage is placed within specific clinical ranges to guide diagnosis and treatment targets. A result below 5.7% is considered normal and indicates that blood sugar levels are well-managed.
A reading that falls between 5.7% and 6.4% is classified as prediabetes. This range signals elevated glucose levels that increase the risk of progressing to type 2 diabetes and developing associated complications.
A result of 6.5% or higher on two separate tests is the standard threshold used to diagnose diabetes. For most adults with diabetes, a target goal of 7.0% or less is generally recommended to minimize long-term health risks. Maintaining high A1C levels over time is linked to the development of microvascular and macrovascular complications, including damage to the eyes, kidneys, and nerves, as well as an increased risk for cardiovascular events.
Variables That Influence Your A1C Reading
While the A1C test is a reliable indicator of long-term blood sugar, certain physiological factors can cause the reading to be misleading. Conditions that affect the red blood cell lifespan can artificially lower or raise the A1C result. For instance, a shortened red blood cell lifespan due to conditions like hemolytic anemia or recent blood loss will yield a falsely low A1C.
Conversely, conditions that prolong the red blood cell lifespan, such as iron deficiency anemia or vitamin B12 deficiency, can lead to a falsely elevated A1C reading. Certain genetic variations in hemoglobin, known as hemoglobin variants, can also interfere with the accuracy of the test, depending on the laboratory method used.
Healthcare providers must be aware of concurrent health issues or medications, including certain HIV treatments or high-dose vitamins C and E, that could skew the A1C result. If the A1C result does not align with daily blood glucose monitoring, alternative tests like fructosamine or glycated albumin may be used to assess average glucose levels more accurately.