How is it used?
Screening and diagnosis
The hemoglobin A1c test may be used to screen for and diagnose diabetes and prediabetes in adults. Hemoglobin A1c, also glycated hemoglobin or A1c, is formed in the blood when glucose attaches to hemoglobin. The higher the level of glucose in the blood, the more glycated hemoglobin is formed. (Read more in the "What is being tested?" section.)
The A1c test, however, should not be used for screening for cystic fibrosis-related diabetes, for diagnosis of gestational diabetes in pregnant women, or for diagnosis of diabetes in children and adolescence, people who have had recent severe bleeding or blood transfusions, those with chronic kidney or liver disease, or people with blood disorders such as iron-deficiency anemia, vitamin B12 deficiency anemia, and some hemoglobin variants (e.g., patients with sickle cell disease or thalassemia). In these cases, a fasting plasma glucose or oral glucose tolerance test should be used for screening or diagnosing diabetes.
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 doctor'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).
Monitoring
The A1c test is also used to monitor the glucose control of diabetics 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. 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 diabetics and their healthcare providers know if the measures that are being taken to control their diabetes are successful or need to be adjusted.
A1c is frequently used to help newly diagnosed diabetics determine how elevated their uncontrolled blood glucose levels have been over the last 2-3 months. The test may be ordered several times while control is being achieved, and then at least twice a year to verify that good control is being maintained.
When is it ordered?
Screening and diagnosis
A1c may be ordered as part of a health checkup or when someone is suspected of having diabetes because of classical signs or symptoms of increased blood glucose levels (hyperglycemia) such as:
- Increased thirst and drinking fluids
- Increased urination
- Increased appetite
- Fatigue
- 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 profile (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.
People who are not diagnosed with diabetes but are determined to be at increased risk for diabetes (prediabetes) should have A1c testing yearly.
Monitoring
Depending on the type of diabetes that a person has, how well that person's diabetes is controlled, and on the healthcare provider's recommendations, the A1c test may be measured 2 to 4 times each year. The ADA recommends A1c testing for diabetics 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 control is not good, A1c may be ordered quarterly.
What does the test result mean?
In screening and diagnosis, some results that may be seen include:
- A nondiabetic person will have an 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: 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 diabetics, it is recommended that they aim to keep their hemoglobin A1c below 7%. The closer diabetics 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.
An individual with type 2 diabetes, however, may have an A1c goal selected by the person and his or her healthcare provider. 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 provider, 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 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.
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 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.
If an individual has a hemoglobin variant, such as sickle cell hemoglobin (hemoglobin S), that person will have a decreased amount of hemoglobin A. This may limit the usefulness of the A1c test in diagnosing and/or monitoring this person's diabetes, depending on the method used.
If a person has anemia, hemolysis, or heavy bleeding, A1c test results may be falsely low. If someone is iron-deficient, the A1c level may be increased.
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.
