The A1c test and glucose measurements are two distinct ways to assess blood sugar levels. While glucose provides a snapshot of the sugar currently in the bloodstream, the A1c test offers a broader view of average blood sugar control over an extended period. These tests are both important tools in understanding how the body manages sugar.
What Are A1c and Glucose?
Glucose is a simple sugar that serves as the body’s primary source of energy, derived from the carbohydrates in food. When food is consumed, glucose enters the bloodstream, prompting the pancreas to release insulin, a hormone that helps glucose move from the blood into cells for energy. Blood glucose levels fluctuate throughout the day based on factors like eating, exercise, and stress, and can be measured at home using a finger-prick blood glucose meter or continuously with a continuous glucose monitor (CGM).
In contrast, the A1c test, also known as glycated hemoglobin or HbA1c, measures the average amount of glucose attached to hemoglobin in red blood cells over approximately the past two to three months. When glucose enters the bloodstream, it naturally binds to hemoglobin in a process called glycation. Since red blood cells typically live for about three months, the A1c test reflects the average blood glucose levels over their lifespan, providing a long-term indicator of blood sugar control. This test is usually performed by a laboratory blood draw, though some point-of-care versions exist.
Relating A1c to Average Glucose
The A1c test does not directly measure a current glucose level, nor is it calculated from a single blood sugar reading. Instead, the A1c result is reported as a percentage, which correlates to an estimated average glucose (eAG) level. This eAG represents the average blood glucose concentration over the two to three months prior to the test, expressed in the same units (milligrams per deciliter, mg/dL, or millimoles per liter, mmol/L) used for daily blood glucose monitoring. Healthcare providers use this relationship to translate the A1c percentage into a more familiar average glucose value.
A commonly used formula to convert A1c percentage to estimated average glucose in mg/dL is: eAG (mg/dL) = 28.7 × A1c (%) – 46.7. For example, an A1c of 7% corresponds to an eAG of approximately 154 mg/dL. This eAG is an estimation, providing a general picture of average blood sugar control rather than a precise daily calculation.
While A1c reflects the average, it does not show daily fluctuations or “glycemic variability” in blood sugar levels. A person with stable blood sugar readings throughout the day and another with wide swings between high and low glucose levels could potentially have the same A1c percentage. Frequent high and low blood sugar excursions, even if the average is within target, can pose health risks. Therefore, A1c provides a valuable long-term overview, but it is often complemented by daily glucose monitoring to capture these short-term patterns and variations.
Why A1c Matters for Health
The A1c test helps healthcare professionals diagnose and manage blood sugar-related conditions. It is used to detect prediabetes, a state where blood sugar levels are higher than normal but not yet high enough for a diabetes diagnosis. An A1c result between 5.7% and 6.4% indicates prediabetes, signaling an increased risk for developing type 2 diabetes and cardiovascular concerns.
Beyond diagnosis, A1c helps monitor diabetes management and guide treatment decisions. For most adults with diabetes, a target A1c of less than 7% is often recommended, though individual targets vary based on age and other health conditions. Consistently elevated A1c levels indicate prolonged high blood sugar, which increases the risk of developing diabetes-related complications over time. These complications can include damage to nerves, kidneys, and eyes, and an increased risk for heart disease. Regular A1c testing, typically every three to six months for those with diabetes, helps healthcare teams assess treatment effectiveness and make necessary adjustments.
What Can Influence A1c Results
Several factors can affect the accuracy or interpretation of A1c results, independent of a person’s average blood sugar control. Conditions impacting red blood cell lifespan are particularly relevant, as the A1c test relies on glucose attached to hemoglobin over the red blood cell’s life. For instance, certain anemias, such as iron-deficiency anemia, can lead to falsely high A1c results, while conditions causing increased red blood cell turnover, like sickle cell disease or thalassemia, can result in falsely low A1c readings.
Recent significant blood loss or blood transfusions can also influence A1c levels by introducing newer red blood cells with less time for glucose to attach, potentially lowering the A1c result. Kidney or liver disease may also affect A1c accuracy. Certain medications, including some corticosteroids, antipsychotics, and specific HIV treatments, can impact blood sugar levels and thus affect A1c readings. Pregnancy can also alter A1c results. When interpreting A1c results, healthcare providers consider these and other individual circumstances to ensure the most accurate assessment of long-term blood sugar control.