The Hemoglobin A1c (A1C) test measures the percentage of hemoglobin—a protein within red blood cells—that has sugar molecules attached (glycation). This blood measurement provides a picture of your average blood glucose level over the preceding two to three months. For most adults without diabetes, a normal A1C level is considered below 5.7%, while the target for many adults with diabetes is typically less than 7%. The A1C is a standard tool for monitoring glucose management plans. While the dangers of a high A1C are well-known, a result below the healthy range can also signal underlying problems requiring immediate medical attention.
What a Low A1C Indicates
A low A1C result, often considered below 5.0%, is not automatically a sign of optimal health. It frequently points to chronic or recurrent low blood sugar, known as hypoglycemia, defined by blood glucose levels falling below 70 mg/dL. An A1C this low may indicate that an individual, particularly one with diabetes, is experiencing frequent or prolonged periods of low glucose.
The immediate consequence of persistent hypoglycemia is neuroglycopenia, where the brain does not receive enough fuel to function correctly. Early symptoms include shakiness, sweating, rapid heart rate, and anxiety as the body attempts to raise glucose levels. If not corrected, low blood sugar can rapidly progress to severe symptoms like confusion, slurred speech, seizures, and loss of consciousness. For individuals managing diabetes, a significantly low A1C often suggests that their medication regimen, such as insulin or sulfonylureas, may be overly aggressive and needs immediate adjustment.
Underlying Medical Conditions That Drive A1C Down
When a low A1C is not the result of medication, it can signal a physiological problem causing persistent low blood glucose. The body’s ability to maintain glucose balance relies heavily on counter-regulatory hormones and the liver’s function. A deficiency in these systems can lead to a consistently low average blood sugar level.
Adrenal Insufficiency
Adrenal insufficiency is a condition where the adrenal glands do not produce enough cortisol. Cortisol is a hormone that counteracts insulin, playing a role in promoting glucose production in the liver and making cells less sensitive to insulin. A lack of this hormone impairs the body’s ability to raise blood sugar when it drops, leading to recurrent hypoglycemia.
Severe Liver Disease
Severe liver disease, such as cirrhosis, drives A1C down by impairing glucose production. The liver is the primary organ responsible for releasing glucose into the bloodstream through two processes: breaking down stored glycogen (glycogenolysis) and creating new glucose from non-carbohydrate sources (gluconeogenesis). When liver function is severely compromised, its capacity to perform these functions is diminished, resulting in true hypoglycemia, especially during periods of fasting.
Insulinoma
In rare cases, a low A1C may be caused by an insulin-producing tumor in the pancreas called an insulinoma. This tumor secretes an excessive and unregulated amount of insulin, which constantly drives glucose out of the bloodstream and into cells. The resulting state of hyperinsulinemic hypoglycemia is chronic, leading to a significantly low A1C reading that accurately reflects the body’s low average glucose state.
Non-Glucose Related Factors That Skew A1C Results
A low A1C result does not always reflect truly low blood sugar; sometimes, it is an artifact of the measurement process related to red blood cell turnover. The A1C test measures the percentage of hemoglobin glycated over the typical 120-day lifespan of a red blood cell. Any condition that shortens this lifespan means red blood cells are removed from circulation before they can accumulate a full complement of glucose.
Conditions that cause increased red blood cell turnover, such as hemolytic anemia, acute or chronic blood loss, or kidney failure, will lead to a falsely low A1C value. In these scenarios, the blood is constantly refreshed with younger, non-glycated red blood cells, artificially lowering the measured percentage. Genetic variants of hemoglobin, such as those found in sickle cell trait or thalassemia, can also interfere with the chemical assay depending on the specific testing method used by the laboratory.
Certain medications can also induce a falsely low A1C by promoting subclinical hemolysis, which is the subtle destruction of red blood cells. Examples of such drugs include dapsone, the antiviral medication ribavirin, and some antiretroviral therapies. These factors physically alter the hemoglobin or the lifespan of the red blood cell, making the A1C unreliable as a marker of actual glucose control.
Steps After Receiving a Low Result
If an A1C result is unexpectedly low, the first step is to consult a healthcare provider for a thorough diagnostic workup. The goal is to differentiate between true hypoglycemia risk and interference with the test measurement. This process includes a detailed review of all current medications, supplements, and any recent episodes of illness or blood loss.
To verify actual glucose control, the provider may order alternative tests not dependent on red blood cell lifespan or hemoglobin type. Fructosamine or glycated albumin tests measure glucose attached to serum proteins, providing a more immediate snapshot of blood sugar levels over two to three weeks. Continuous Glucose Monitoring (CGM) provides minute-by-minute data on blood sugar trends, allowing the clinician to identify the frequency and severity of low blood sugar events. If true hypoglycemia is confirmed, further investigation is warranted to check for underlying conditions like adrenal insufficiency or an insulinoma.