Hemoglobin (Hb) is the primary protein in red blood cells, transporting oxygen from the lungs to the body’s tissues. The most abundant form in adults is Hemoglobin A (HbA), accounting for 95 to 98% of all hemoglobin. In a health context, HbA refers to its glycated variant, measured by the Hemoglobin A1c (HbA1c) test. This measurement is the standard method for evaluating long-term glucose control.
Hemoglobin A and the Glycation Process
The science behind the A1c test involves glycation, a natural, non-enzymatic chemical reaction. Glucose molecules circulating in the bloodstream spontaneously bind to the HbA protein inside red blood cells. The amount of glucose attached is directly proportional to the concentration of sugar in the blood at that time.
Once glucose binds to hemoglobin, the bond remains permanent for the red blood cell’s entire lifespan. Since red blood cells circulate for about 120 days (three months), the HbA1c measurement reflects the average blood glucose level over this period. The test offers a retrospective view of blood sugar management, unlike a simple finger-prick glucose test.
The measurement targets the A1c subtype, the most common fraction of glycated hemoglobin. A higher percentage indicates that red blood cells were exposed to higher average glucose concentrations over the preceding weeks. This allows the HbA1c test to serve as a reliable marker of long-term glycemic exposure and overall diabetes management.
The HbA1c Test: Purpose and Measurement
The HbA1c test is a straightforward diagnostic and monitoring tool. Its primary utility is twofold: diagnosing pre-diabetes and diabetes, and monitoring the effectiveness of treatment plans. It provides a comprehensive picture of glucose control that daily blood sugar checks alone cannot achieve.
The procedure is minimally invasive, usually requiring a simple blood draw or a fingerstick. Unlike many other blood glucose tests, the A1c measurement does not require the patient to fast beforehand. The result is reported as a percentage, representing the proportion of total hemoglobin that has glucose attached.
For instance, a result of 7.0% indicates that seven percent of the hemoglobin protein is glycated. Healthcare providers use this percentage to assess the patient’s risk of developing complications associated with consistently high blood sugar. The test offers a standardized, objective measure for tracking glucose control.
Decoding Your A1c Results
The HbA1c percentage result is translated into standardized clinical categories for diagnosis and treatment goals. For non-pregnant adults, a result below 5.7% is considered normal, suggesting blood sugar has been well-controlled over the past three months.
A result between 5.7% and 6.4% indicates pre-diabetes, signaling a higher risk for developing type 2 diabetes and requiring lifestyle modifications or further monitoring. This range suggests that average blood glucose levels are elevated, but not high enough for a formal diabetes diagnosis. A result of 6.5% or higher meets the diagnostic criterion for diabetes.
To make the percentage more intuitive, the A1c result can be converted into an Estimated Average Glucose (eAG) value. The eAG is expressed in the same units—milligrams per deciliter (mg/dL) or millimoles per liter (mmol/L)—used for daily blood glucose monitoring. This conversion helps individuals understand their A1c in a context familiar to their regular self-checks.
For example, an HbA1c of 7.0% roughly correlates to an eAG of 154 mg/dL. This correlation is derived from the A1C-Derived Average Glucose (ADAG) study. The eAG provides a practical bridge between the long-term percentage and the day-to-day blood sugar numbers patients track.
Conditions That Influence HbA1c Accuracy
The A1c test’s accuracy depends on red blood cells having a normal lifespan of about 120 days. Conditions that shorten the survival time of these cells can lead to an artificially low HbA1c result because the hemoglobin has less time to bind with glucose. This represents measurement interference, not a change in the patient’s actual glucose levels.
Common interfering factors include hemolytic anemia, where red blood cells are destroyed prematurely. Recent significant blood loss (e.g., hemorrhage or frequent donation) also lowers the A1c value by introducing new, non-glycated red blood cells. Other types of anemia, particularly those related to iron, Vitamin B12, or folate deficiencies, can also affect the test’s reliability.
Chronic kidney disease and end-stage liver disease alter red blood cell turnover and survival, leading to misleading A1c readings. Inherited hemoglobin variants (hemoglobinopathies), such as sickle cell trait, can also interfere with the laboratory’s chemical measurement method. In these cases, alternative tests like glycated albumin or fructosamine may be used to assess long-term glucose control.