Hemoglobin A1c, also called A1c or glycated hemoglobin, is hemoglobin with glucose attached. The A1c test evaluates the average amount of glucose in the blood over the last 2 to 3 months by measuring the percentage of glycated (glycosylated) hemoglobin.
As part of a health checkup or when you have risk factors for or symptoms of diabetes; after first diagnosis with diabetes, every 3-4 months or about 120 days to ensure that your glycemic goals are met and/or maintained or when your therapy plan has changed
A blood sample is obtained by inserting a needle into a vein or a drop of blood is taken from a finger by pricking it with a small, pointed lancet.
Hemoglobin A1c, also called A1c or glycated hemoglobin, is hemoglobin with glucose attached. The A1c test evaluates the average amount of glucose in the blood over the last 2 to 3 months by measuring the percentage of glycated hemoglobin in the blood.
Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). There are several types of normal hemoglobin, but the predominant form – about 95-96% – is hemoglobin A. As glucose circulates in the blood, some of it spontaneously binds to hemoglobin A.
The higher the level of glucose in the blood, the more glycated hemoglobin is formed. Once the glucose binds to the hemoglobin, it remains there for the life of the red blood cell – normally about 120 days. The predominant form of glycated hemoglobin is referred to as A1c. A1c is produced on a daily basis and slowly cleared from the blood as older RBCs die and younger RBCs (with non-glycated hemoglobin) take their place.
An A1c test may be used to screen for and diagnose diabetes or risk of developing diabetes. Standards of medical care in diabetes from the American Diabetes Association (ADA) state that diabetes may be diagnosed based on A1c criteria or blood glucose criteria (e.g., the fasting blood glucose (FBG) or the 2-hour glucose tolerance test).
A1c is also used to monitor treatment for individuals diagnosed with diabetes. It helps to evaluate how well your glucose levels have been controlled by treatment over time. For monitoring purposes, an A1c of less than 7% indicates good glucose control and a lower risk of diabetic complications for the majority of people with diabetes.
However, the ADA and the European Association for the Study of Diabetes (EASD) recommend that the management of glucose control in people with type 2 diabetes be more "patient-centered." It is recommend that people work closely with their healthcare practitioner to select a goal that reflects each person's individual health status and that balances risks and benefits.
How is the test used?
Screening and diagnosis
The hemoglobin A1c test may be used to screen for and diagnose diabetes and prediabetes in adults.
The A1c test, however, should not be used for:
- Screening for cystic fibrosis-related diabetes
- Diagnosis of gestational diabetes in pregnant women
- Diagnosis of diabetes in children and teens
- People who have had recent severe bleeding or blood transfusions
- People with chronic kidney disease or liver disease
- People with blood disorders such as iron-deficiency anemia and vitamin B12 deficiency anemia
- Individuals with some hemoglobin variants (e.g., sickle cell disease or thalassemia)
Only A1c tests that have been referenced to an accepted laboratory method (National Glycohemoglobin Standardization Program certified) should be used for diagnostic or screening purposes. Currently, point-of-care tests, such as those that may be used at a healthcare practitioner's office or a patient's bedside, are not accurate enough for use in diagnosis but can be used to monitor treatment (lifestyle and drug therapies).
The A1c test is also used to monitor the glucose control of people with diabetes over time. The goal of those with diabetes is to keep their blood glucose levels as close to normal as possible. This helps to minimize the complications caused by chronically elevated glucose levels, such as progressive damage to body organs like the kidneys, eyes, cardiovascular system, and nerves. Unlike glucose results, which provide information about the glycemic status of a person strictly for the time of blood collection, the A1c test result gives a picture of the average amount of glucose in the blood over the last 2-3 months. This can help people with diabetes and their healthcare practitioners know if the measures that are being taken to control their diabetes are successful or need to be adjusted.
A1c is frequently used to people help newly diagnosed with diabetes determine how elevated their uncontrolled blood glucose levels have been over the last 2-3 months. The test is ordered several times until an optimal glucose level is achieved.
When is it ordered?
Screening and diagnosis
- Increased thirst and drinking fluids
- Increased urination
- Increased appetite
- Blurred vision
- Slow-healing infections
The A1c test may also be considered in adults who are overweight with the following additional risk factors:
- Physical inactivity
- First-degree relative (sibling or parent) with diabetes
- High-risk race/ethnicity (e.g., African American, Latino, Native American, Asian American, Pacific Islander)
- High blood pressure (hypertension)
- Abnormal lipid panel (low HDL cholesterol and/or high triglycerides)
- Women with polycystic ovary syndrome
- History of cardiovascular diseases
- Other clinical conditions associated with insulin resistance
The American Diabetes Association (ADA) recommends to begin A1c testing at age 45 for overweight or obese people; if the result is normal, the testing should be repeated at a minimum of 3-year intervals, with consideration of more frequent testing depending on initial results and risk status, or when classical signs or symptoms of increased blood glucose levels are observed.
People who are not diagnosed with diabetes but are determined to be at increased risk for diabetes (prediabetes) should have their A1c level tested at least yearly.
Depending on the type of diabetes that a person has, how well that person's diabetes is controlled, and on the healthcare practitioner's recommendations, the A1c test may be measured 2 to 4 times each year. The ADA recommends A1c testing for people with diabetes at least twice a year if they are meeting treatment goals and under stable glycemic control. When someone is first diagnosed with diabetes or if optimal glucose levels are not achieved, A1c may be ordered quarterly.
What does the test result mean?
In screening and diagnosis, some results that may be seen include:
- A person who does not have diabetes: A1c result less than 5.7% (39 mmol/mol)
- Diabetes: A1c level is 6.5% (48 mmol/mol) or higher
- Increased risk of developing diabetes in the future (pre-diabetes): A1c of 5.7% to 6.4% (39-46 mmol/mol)
For monitoring glucose control, A1c is currently reported as a percentage and, for most people with diabetes, it is recommended that they aim to keep their hemoglobin A1c below 7%. The closer they can keep their A1c to the American Diabetes Association (ADA)'s therapeutic goal of less than 7% without experiencing excessive low blood glucose (hypoglycemia), the better their diabetes is in control. As the A1c increases, so does the risk of complications.
However, if you have type 2 diabetes, you may select an A1c goal in consultation with your healthcare practitioner. The goal may depend on several factors, such as length of time since diagnosis, the presence of other diseases as well as diabetes complications (e.g., vision impairment or loss, kidney damage), risk of complications from hypoglycemia, limited life expectancy, and whether or not the person has a support system and healthcare resources readily available.
For example, a person with heart disease who has lived with type 2 diabetes for many years without diabetic complications may have a higher A1c target (e.g., 7.5%-8.0%) set by their healthcare practitioner, while someone who is otherwise healthy and just diagnosed may have a lower target (e.g., 6.0%-6.5%) as long as low blood sugar is not a significant risk.
The A1c test report also may include the result expressed in SI units (mmol/mol) and an estimated Average Glucose (eAG), which is a calculated result based on the hemoglobin A1c levels. The estimated Average Glucose (eAG) reflects indirectly the glucose level over a period of 2-3 months before the A1c measurement.
The purpose of reporting eAG is to help a person relate A1c results to everyday glucose monitoring levels and to laboratory glucose tests. The formula for eAG converts percentage A1c to units of mg/dL or mmol/L. (For details, see below.)
It should be noted that the eAG is still an evaluation of a person's glucose over the last couple of months. It will not match up exactly to any one daily glucose test result. The ADA has adopted this calculation and provides a calculator and information on the eAG on their DiabetesPro web site. The NGSP web site also provides a calculator to convert hemoglobin A1c in SI units mmol/mol into percentage.
Is there a home test for hemoglobin A1c?
Yes. If you have already been diagnosed with diabetes, a home test may be used to help monitor your glucose control over time. However, a home test (point-of-care test) is not recommended for screening or diagnosing the disease. There are FDA-approved tests that can be used at home. If you are interested in learning more, visit the article on Home Tests and ask your healthcare provider.
Why are my A1c and blood glucose different?
Beyond the difference in units used to report them, the A1c represents an average over time while your blood glucose reflects what is happening in your body now. Your blood glucose will capture the changes in your blood sugar that occur on a daily basis, the highs and the lows. Each blood glucose is a snapshot and each is different. The A1c is an indication that "in general" your glucose has been elevated over the last few months or "in general" it has been normal. It is inherently not as sensitive as a blood glucose. However, if your day-to-day glucose control is stable (good or bad), then both the A1c and blood glucose should reflect this. It is important to remember the time lag associated with the A1c. Good glucose control for the past 2-3 weeks will not significantly affect the A1c result for several more weeks.
In addition to this, it is also important to remember that glycated hemoglobin and blood glucose are two different but related things. For unknown reasons, some peoples' A1c may not accurately reflect their average blood glucose.
How is estimated Average Glucose (eAG) calculated?
The ADAG (A1c-Derived Average Glucose) formula that is used to calculate the eAG from your hemoglobin A1c (A1c) result is:
28.7 X A1c (%) – 46.7 = eAG (milligrams/deciliter, mg/dL)
An example of this is an A1c of 6%. The calculation for this would be:
28.7 X 6 – 46.7 = 126 mg/dL
for an estimated average glucose of 126 mg/dL.
What this means is that for every one percent that your A1c goes up, it is equivalent to your average glucose going up by about 29 mg/dL.
Is A1c reported the same way around the world?
For monitoring purposes, the way that the A1c is reported is in the process of changing. Traditionally, in the United States, the A1c has been reported as a percentage, and the American Diabetes Association (ADA) has recommended that people with diabetes strive to keep their A1c below 7%. While this is still generally true, more than a decade of national and international efforts to improve and standardize the A1c test and its reporting led to the release of a consensus statement in 2007 (and an update in 2010) by the ADA, the European Association for the Study of Diabetes (EASD), the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), the International Society for Pediatric and Adolescent Diabetes, and the International Diabetes Federation. These joint statements and the completion of a study called ADAG (A1c-Derived Average Glucose) that further examined the relationship between blood glucose concentrations and A1c led to a recommendation that A1c be reported worldwide in two ways:
- As a percentage (based upon National Glycohemoglobin Standardization Program (NGSP) derived units) and
- In SI (Système International) units (mmol/mol)
An estimated Average Glucose (eAG) based upon a formula developed from the ADAG study with either mg/dL or mmol/l as units that continue to be recognized by the ADA and the American Association for Clinical Chemistry in the 2015 ADA Diabetes Care Position Statement may also be reported.
What this means for people with diabetes and healthcare practitioners in the U.S. is that A1c results will be reported as a percentage but may in addition to this be reported as mmol/mol and, in some cases, also as an eAG with the same type of units (mg/dL) as are reported by home glucose monitors and laboratory results.
Is there anything else I should know?
The A1c test will not reflect temporary, acute blood glucose increases or decreases, or good control that has been achieved in the last 3-4 weeks. The glucose swings of someone who has "brittle" diabetes will also not be reflected in the A1c.
Any condition that affects the quality and quantity of red blood cells (RBCs) and hemoglobin (e.g. iron deficiency, bleeding, hemolysis, etc.) will affect A1c test results. For example, if someone is iron-deficient, the A1c level may be increased or if a person receives erythropoietin therapy or has had a recent blood transfusion, the A1c may be inaccurate and may not accurately reflect glucose control for 2-3 months.