Hemoglobin A1C Test Explained

The A1C test is a blood test that provides information about a person’s average levels of blood glucose, also called blood sugar, over the past 3 months. The A1C test is sometimes called the hemoglobin A1c, HbA1c, or glycohemoglobin test. The A1C test is the primary test used for diabetes management and diabetes research.

How does the A1C test work?

The A1C test is based on the attachment of glucose to hemoglobin, the protein in red blood cells that carries oxygen. In the body, red blood cells are constantly forming and dying, but typically they live for about 3 months.

Thus, the A1C test reflects the average of a person’s blood glucose levels over the past 3 months. The A1C test result is reported as a percentage.

The higher the percentage, the higher a person’s blood glucose levels have been. A normal A1C level is below 5.7 percent.

What is eAG?

Your health care provider may report your A1C test result as eAG, or “average glucose,” which directly correlates to your A1C. eAG may help you understand your A1C value because eAG is a unit similar to what you see regularly through self-monitoring on your meter.

A1C is reported as a percent (7% for example) and eAG uses the same units (mg/dl) as your glucose meters.

What’s Your Number?

The American Diabetes Association suggests an A1C of 7%, which is an eAG of 154 mg/dl, but a more or less stringent glycemic goals may be appropriate for each individual.

Use the converter below to translate your A1C percentage into an eAG number, or vice versa.

Can the A1C test be used to diagnose type 2 diabetes and prediabetes?

Yes. In 2009, an international expert committee recommended the A1C test as one of the tests available to help diagnose type 2 diabetes and prediabetes.1 Previously, only the traditional blood glucose tests were used to diagnose diabetes and prediabetes.

Because the A1C test does not require fasting and blood can be drawn for the test at any time of day, experts are hoping its convenience will allow more people to get tested—thus, decreasing the number of people with undiagnosed diabetes.

However, some medical organizations continue to recommend using blood glucose tests for diagnosis.
1The International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes.

Why should a person be tested for diabetes?

Testing is especially important because early in the disease diabetes has no symptoms. Although no test is perfect, the A1C and blood glucose tests are the best tools available to diagnose diabetes—a serious and lifelong disease.

Testing enables health care providers to find and treat diabetes before complications occur and to find and treat prediabetes, which can delay or prevent type 2 diabetes from developing.

Has the A1C test improved?

Yes. A1C laboratory tests are now standardized. In the past, the A1C test was not recommended for diagnosis of type 2 diabetes and prediabetes because the many different types of A1C tests could give varied results.

The accuracy has been improved by the National Glycohemoglobin Standardization Program (NGSP), which developed standards for the A1C tests.

The NGSP certifies that manufacturers of A1C tests provide tests that are consistent with those used in a major diabetes study. The study established current A1C goals for blood glucose control that can reduce the occurrence of diabetes complications, such as blindness and blood vessel disease.

2Nathan DM, Genuth S, Lachin J, et al. The effect of intensive treatment of diabetes on the development and progression of long-term complications in insulin-dependent diabetes mellitus. The New England Journal of Medicine.

How is the A1C test used to diagnose type 2 diabetes and prediabetes?

The A1C test can be used to diagnose type 2 diabetes and prediabetes alone or in combination with other diabetes tests.

When the A1C test is used for diagnosis, the blood sample must be sent to a laboratory that uses an NGSP-certified method for analysis to ensure the results are standardized.

What is the Difference Between eAG and the Average on My Monitor?

However, eAG is not the same average glucose level as the average of your readings on your meter. Because people with diabetes are more likely to check their blood glucose more often when they are low (for example, first thing in the morning and before meals), the average of the readings on their meter is likely to be lower than their eAG.

Monitoring measures your blood glucose at a moment in time, whereas eAG/A1C represent an average of your glucose levels 24 hours a day, including times when people are less likely to check their blood glucose (for example, post-meal periods of higher blood glucose).

Is the A1C test used during pregnancy?

The A1C test may be used at the first visit to the health care provider during pregnancy to see if women with risk factors had undiagnosed diabetes before becoming pregnant.

After that, the oral glucose tolerance test (OGTT) is used to test for diabetes that develops during pregnancy—known as gestational diabetes.

After delivery, women who had gestational diabetes should be tested for persistent diabetes. Blood glucose tests, rather than the A1C test, should be used for testing within 12 weeks of delivery.

Can blood glucose tests still be used for diagnosing type 2 diabetes and prediabetes?

Yes. The standard blood glucose tests used for diagnosing type 2 diabetes and prediabetes-the fasting plasma glucose (FPG) test and the OGTT—are still recommended.

The random plasma glucose test, also called the casual glucose test, may be used for diagnosing diabetes when symptoms of diabetes are present. In some cases, the A1C test is used to help health care providers confirm the results of a blood glucose test.

Can the A1C test result in a different diagnosis than the blood glucose tests?

Yes. In some people, a blood glucose test may indicate a diagnosis of diabetes while an A1C test does not. The reverse can also occur—an A1C test may indicate a diagnosis of diabetes even though a blood glucose test does not.

Because of these variations in test results, health care providers repeat tests before making a diagnosis.

People with differing test results may be in an early stage of the disease, where blood glucose levels have not risen high enough to show on every test.

Sometimes, making simple changes in lifestyle—losing a small amount of weight and increasing physical activity—can help people in this early stage reverse diabetes or delay its onset.

Are diabetes blood test results always accurate?

All laboratory test results can vary from day to day and from test to test. Results can vary
within the person being tested.

A person’s blood glucose levels normally move up and down depending on meals, exercise, sickness, and stress.
between different tests.

Each test measures blood glucose levels in a different way. For example, the FPG test measures glucose that is floating free in the blood after fasting and only shows the blood glucose level at the time of the test. Repeated blood glucose tests, such as self-monitoring several times a day with a home meter, can record the natural variations of blood glucose levels during the day.

The A1C test represents the amount of glucose attached to hemoglobin, so it reflects an average of all the blood glucose levels a person may experience over 3 months. The A1C test will not show day-to-day changes.

The following chart shows how multiple blood glucose measurements over 4 days compare with an A1C measurement.

How is the A1C test used after diagnosis of diabetes?

Health care providers can use the A1C test to monitor blood glucose levels in people with type 1 or type 2 diabetes. The A1C test is not used to monitor gestational diabetes.

The American Diabetes Association recommends that people with diabetes who are meeting treatment goals and have stable blood glucose levels have the A1C test twice a year. Health care providers may repeat the A1C test as often as four times a year until blood glucose levels reach recommended levels.

The A1C test helps health care providers adjust medication to reduce the risk of long-term diabetes complications. Studies have demonstrated substantial reductions in long-term complications with the lowering of A1C levels.

When the A1C test is used for monitoring blood glucose levels in a person with diabetes, the blood sample can be analyzed in a health care provider’s office using a POC test to give immediate results.

However, POC tests are less reliable and not as accurate as most laboratory tests.

How does the A1C relate to estimated average glucose?

Estimated average glucose (eAG) is calculated from the A1C. Some laboratories report eAG with the A1C test results. The eAG number helps people with diabetes relate their A1C to daily glucose monitoring levels.

The eAG calculation converts the A1C percentage to the same units used by home glucose meters—milligrams per deciliter (mg/dL).

The eAG number will not match daily glucose readings because it is a long-term average rather than the blood glucose level at a single time, as measured with the home glucose meter. The following table shows the relationship between the A1C and the eAG.

Source & More Info: NIDC and Diabetes.org

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