Hemoglobin A1c (HbA1c) Blood Test Explained

HbA1c shows average blood sugar over the past two to three months by measuring sugar attached to hemoglobin in red blood cells. A result below 5.7% is usually normal, 5.7-6.4% may indicate prediabetes, and 6.5% or higher may indicate diabetes, per the ADA.

Hemoglobin A1c, often called HbA1c or A1c, is measured with a blood test that looks at hemoglobin, the protein inside red blood cells that carries oxygen. When sugar (glucose) circulates in the blood, some of it attaches to hemoglobin in a process called glycation. Because red blood cells live for about three months, the HbA1c test can show an average of blood sugar levels over that time rather than a single point-in-time reading like a fasting glucose test. This makes it a useful marker for tracking longer-term blood sugar patterns rather than the short-term ups and downs that can follow a recent meal, illness, or stress.

Clinicians often order an HbA1c test as part of routine health screening, especially for adults with risk factors such as extra body weight, a family history of diabetes, or high blood pressure. It is also used to help monitor blood sugar control in people already managing diabetes, since it reflects longer-term trends rather than day-to-day changes. HbA1c is commonly included in metabolic panels alongside fasting glucose and lipid tests, since blood sugar and cholesterol levels are often reviewed together when assessing overall metabolic health. Results are typically interpreted alongside symptoms, other lab values, and personal and family medical history rather than in isolation.

Hemoglobin A1c results are usually reported as a percentage, and laboratories compare results to standard adult reference ranges to describe normal, prediabetes, and diabetes categories. These ranges apply to adults and can vary slightly between laboratories and testing methods, so the exact numbers on a lab report may differ a little from other sources. The table below shows the reference range for this marker as reported in adult testing guidelines.

A higher-than-typical HbA1c result, generally above 5.7%, may indicate that average blood sugar levels have been higher than usual over the past two to three months. The most common reason clinicians consider is prediabetes or diabetes, conditions in which the body does not use insulin effectively or does not produce enough of it, allowing sugar to build up in the blood over time. An HbA1c of 6.5% or higher is often used as one of several criteria that can support a diabetes diagnosis, though additional testing is usually needed to confirm this.

Other factors can sometimes raise HbA1c without reflecting typical blood sugar control. Certain blood disorders that affect red blood cell lifespan, some medications, and conditions like anemia can influence results. Pregnancy, certain hormonal conditions, and long-term steroid use may also play a role in elevated readings. Because HbA1c depends on red blood cell turnover, results can sometimes be harder to interpret in people with certain chronic illnesses.

Because so many factors can influence this marker, a high HbA1c result on its own does not confirm a specific cause. A healthcare provider can help interpret the result alongside other tests, such as fasting glucose, and personal health history.

A lower-than-typical HbA1c result is discussed less often than a high one, but it can still carry meaning. It usually reflects average blood sugar levels that have been lower over the past two to three months, which may occur with very controlled eating patterns, certain low-carbohydrate diets, or medications that lower blood sugar effectively.

Conditions that shorten the lifespan of red blood cells, such as certain forms of anemia or hemolysis, can also lower HbA1c readings because there is less time for sugar to attach to hemoglobin. Blood loss, recent blood transfusions, and some inherited blood disorders may have a similar effect. In some cases, a low HbA1c may not reflect blood sugar at all, but rather a change in how red blood cells are made or how long they circulate.

Because a low HbA1c can sometimes mask episodes of very low or very high blood sugar between tests, a healthcare provider may recommend additional monitoring, such as fasting glucose checks or a glucose meter, especially for people managing diabetes.

HbA1c is often interpreted alongside other markers to build a fuller picture of metabolic health. Fasting glucose reflects blood sugar at a single point in time, while HbA1c shows a longer-term average, so the two are often reviewed together. Hemoglobin levels matter because conditions affecting red blood cells can also affect HbA1c results. Triglycerides and creatinine are sometimes checked at the same visit, since blood sugar, fat levels, and kidney function are often connected in metabolic health assessments.

Lifestyle factors such as diet, physical activity, and stress can influence blood sugar and, over time, HbA1c. Some medications can also affect the result. Because testing methods and reference ranges can vary somewhat between laboratories, it can help to compare results from the same lab over time and discuss the trend with a healthcare provider rather than focusing on a single number.

Any HbA1c result outside the typical range is worth discussing with a healthcare provider, who can consider it alongside symptoms, other lab tests, and personal and family history. This is especially true for results suggesting prediabetes or diabetes, since early lifestyle changes or treatment can make a meaningful difference. Seek urgent care if a blood sugar check shows a very low or very high glucose value along with symptoms such as confusion, extreme thirst, fainting, or difficulty breathing, since these can signal a medical emergency requiring immediate attention.

Uploading a lab report to MediWing can help translate results like HbA1c into plain language and track how a marker changes over time, which can support conversations with a clinician.