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A1c and eAG

Also known as: Hemoglobin A1c; HbA1c; Glycohemoglobin; Glycated hemoglobin; Glycosylated hemoglobin
Formal name: A1c and estimated Average Glucose
Related tests: Glucose; Microalbumin; Microalbumin/creatinine ratio; Fructosamine

At a Glance

Why Get Tested?

To monitor a person's diabetes and to aid in treatment decisions; to screen for and/or diagnose diabetes and prediabetes

When to Get Tested?

When first diagnosed with diabetes and then 2 to 4 times per year; as part of a health checkup or if you have symptoms of diabetes

Sample Required?

A blood sample drawn from a vein in the arm or from a fingerstick

Test Preparation Needed?

None

The Test Sample

What is being tested?

The A1c test evaluates the average amount of glucose in the blood over the last 2 to 3 months. It does this by measuring the concentration of glycated (also often called glycosylated) hemoglobin A1c. Hemoglobin is an oxygen-transporting protein found inside red blood cells (RBCs). There are several types of normal hemoglobin and many identified hemoglobin variants, but the predominant form – about 95-98% – is hemoglobin A. Hemoglobin A can be further subdivided, with one of the subcomponents known as hemoglobin A1c. As glucose circulates in the blood, some of it spontaneously binds to hemoglobin A. The glucose-hemoglobin molecules formed are said to be glycated. The higher the concentration 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 combination of glucose and hemoglobin A is referred to as HbA1c or 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.

This test is used to monitor diabetes treatment in someone who has been diagnosed with diabetes. It helps to evaluate how well their glucose levels have been controlled by treatment over time. For certain people, this test may be used to screen for and diagnose diabetes and prediabetes. In 2010, new clinical practice guidelines from the American Diabetes Association (ADA) stated that A1c may be added to fasting plasma glucose (FPG) and oral glucose tolerance test (OGTT) as an option for diabetes screening and diagnosis.

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 ADA has recommended that people with diabetes strive to keep their A1c below 7%. While this is still 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 by the ADA, the European Association for the Study of Diabetes, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC), and the International Diabetes Federation.

This joint statement and the recent completion of a study called ADAG (A1c-Derived Average Glucose) that further examined the relationship between blood glucose concentrations and A1c have led to a recommendation that A1c be reported worldwide three ways:

  • as a percentage (based upon National Glycohemoglobin Standardization Program (NGSP) derived units)
  • in IFCC units (mmol/mol) and
  • as an estimated Average Glucose (eAG) based upon a formula developed from the ADAG study with either mg/dl or mmol/l as units.

What this means for the diabetic patient and their doctor in the U.S. is that the person’s A1c results will likely be reported both in the percentage that they are used to seeing AND as an eAG, with the same type of values or units (mg/dl) as reported by home glucose monitors and laboratory results.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or a drop of blood is taken from your finger by pricking it with a small, pointed lancet.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

The A1c test and eAG calculation are 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 and eAG result give a picture of the average amount of glucose in the blood over the last few months. They can help you and your doctor know if the measures you are taking to control your 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. It may be ordered several times while control is being achieved, and then several times a year to verify that good control is being maintained.

The A1c test may be used to screen for and diagnose diabetes. However, A1c should not be used for diagnosis in pregnant women, people who have had recent severe bleeding or blood transfusions, those with chronic kidney or liver disease, and people with blood disorders such as iron-deficiency anemia, vitamin B12 anemia, and hemoglobin variants. Also, only A1c tests that have been referenced to an accepted laboratory method (standardized) should be used for diagnostic or screening purposes. In these cases, a fasting plasma glucose or oral glucose tolerance test may be used for screening or diagnosis. Currently, point-of-care tests, such as those that may be used at a doctor's office or a patient's bedside, are too variable for use in diagnosis but can be used to monitor treatment (lifestyle and drug therapies).

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When is it ordered?

Depending on the type of diabetes that you have, how well your diabetes is controlled, and your doctor, your A1c may be measured 2 to 4 times each year. The American Diabetes Association recommends testing your A1c at least twice a year. When someone is first diagnosed with diabetes or if control is not good, A1c may be ordered more frequently.

For diagnostic and screening purposes, A1c may be ordered as part of a health checkup or when someone is suspected of having diabetes because they have signs or symptoms of increased blood glucose levels (hyperglycemia) such as:

  • Increased thirst
  • Increased urination
  • Fatigue
  • Blurred vision
  • Slow-healing infections

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What does the test result mean?

For monitoring glucose control, A1c is currently reported as a percentage, and it is recommended that diabetics aim to keep their A1c below 7%. The report for your A1c test also may include an estimated Average Glucose (eAG), which is a calculated result based on your A1c levels. The purpose of reporting eAG is to help you relate your A1c results to your everyday glucose monitoring levels. The formula for eAG converts percentage A1c to units of mg/dL or mmol/L so that you can compare it to your glucose levels from home monitoring systems or laboratory tests.

It should be noted that the eAG is still an evaluation of your glucose over the last couple of months. It will not match up exactly to any one daily glucose test result. The American Diabetes Association has adopted this calculation and provides a calculator and information on the eAG on their web site.

The closer a diabetic can keep their A1c to 6% without experiencing excessive hypoglycemia, the better their diabetes is in control. As the A1c and eAG increase, so does the risk of complications.

In screening and diagnosis, some results that may be seen include:

  • A nondiabetic person will have an A1c result between 4% and 6%.
  • Diabetes: A1c level is 6.5% (47 mmol/mol) or higher.
  • Pre-diabetes (increased risk of developing diabetes in the future): A1c is 5.7% - 6.4% (39 - 46 mmol/mol)

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Is there anything else I should know?

The A1c test will not reflect temporary, acute blood glucose increases or decreases. The glucose swings of someone who has "brittle" diabetes will not be reflected in the A1c.

If you have a hemoglobin variant, such as sickle cell hemoglobin (hemoglobin S), you will have a decreased amount of hemoglobin A. This may limit the usefulness of the A1c test in diagnosing and/or monitoring your diabetes. If you have anemia, hemolysis, or heavy bleeding, your test results may be falsely low. If you are iron deficient, you may have an increased A1c measurement. If you have had a recent transfusion, then your A1c will be falsely increased (blood preservative solutions contain high glucose levels) and not accurately reflect your glucose control for 2 to 3 months.

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Common Questions

1.  How is estimated Average Glucose (eAG) calculated?

The ADAG formula that is used to calculate the eAG from your A1c result is:

    28.7 X A1c – 46.7 = eAG

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.

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2.  Is there a home test for 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 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 doctor.

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3.  Are all A1c tests the same?

Not yet, but great strides have been made in standardizing the A1c test. For more than a decade, a variety of national and international organizations have been working together as part of the National Glycohemoglobin Standardization Program and/or as part of the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC). Included in the recent consensus statement of the ADA, IFCC, European Association for the Study of Diabetes, and the International Diabetes Federation were statements regarding a new way of reporting (see What does the test result mean?) and a statement recommending that a new improved reference system developed by the IFCC be adopted as the only valid “anchor” for measurement standardization. In the past, A1c tests have used different methods that have measured one or more of the glycated subcomponents of hemoglobin A. The result of this was an A1c test result that could vary significantly depending upon which method was used. The IFCC method measures only hemoglobin A1c.

The process of using the IFCC method will be invisible for people having an A1c test performed. The NGSP is in the process of certifying that manufacturers’ are capable of producing the same results as the IFCC system. Practically, as this is adopted over the next few years, it means that a person would get the same or very similar A1c results regardless of to which laboratory their test was sent.

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby’s Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 503-505.

Little, R., Nathan, D. and Sacks, D. (2008 December 9). Hemoglobin A1c: New Uses for Today and Possibilities for Tomorrow. AACC [Audioconference].

Holt, E. (2008 June 17, Updated). HbA1c. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003640.htm. Accessed on 12/13/08.

American Diabetes Association. A1C test. [On-line information]. Available online at http://www.diabetes.org/type-1-diabetes/a1c-test.jsp through http://www.diabetes.org. Accessed on 12/13/08.

(2008). But What Does My A1C Level Really Mean? American Diabetes Association [On-line information]. Available online at http://www.diabetes.org//diabetes-research/summaries/nathan-what-a1c-level-means.jsp through http://www.diabetes.org. Accessed on 12/13/08.

Nathan, D. et. al. (2008 June 7). Translating the A1C Assay Into Estimated Average Glucose Values. Diabetes Care 31:1473-1478, 2008 DOI: 10.2337/dc08-0545 [On-line journal]. Available online at http://care.diabetesjournals.org/cgi/content/abstract/31/8/1473 through http://care.diabetesjournals.org. Accessed on 12/14/08.

Crandall, J. (2007 May, Revision). Diabetes Mellitus. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec12/ch158/ch158b.html?qt=diabetes&alt=sh#sec12-ch158-ch158b-1159 through http://www.merck.com. Accessed on 12/14/08.

Consensus Committee (2007 September). Consensus Statement on the Worldwide Standardization of the Hemoglobin A1c Measurement. Diabetes Care v30 (9). [On-line journal]. PDF available for download at http://www.ifcc.org/4dc9925consensus.pdf through http://www.ifcc.org.

Kahn, R. and Fonseca, V. (2008 August). Translating the A1c Assay. DiabetesCare v 31(8) [On-line information]. PDF available for download at http://professional.diabetes.org/Content/Editorial.pdf through http://professional.diabetes.org. Accessed on 12/14/08.

(2007 November). Sickle Cell Trait and Other Hemoglobinopathies and Diabetes: Important Information for Physicians. National Diabetes Information Clearinghouse [On-line information]. Available online at http://diabetes.niddk.nih.gov/dm/pubs/hemovari-A1C/index.htm through http://diabetes.niddk.nih.gov. Accessed on 12/14/08.

S11 (2008). Q&A Estimated Average Glucose: an alternative to A1C. American Diabetes Association [On-line information]. Available online at http://professional.diabetes.org/GlucoseCalculator.aspx through http://professional.diabetes.org. Accessed on 12/21/08.

American Diabetes Association. Executive summary: standards of medical care in diabetes—2010. Jan 2010. Diabetes Care 33: S4-S10.

(January 2010) The Endocrine Society Statement on the use of A1c for Diabetes Diagnosis and Risk Estimation. PDF available for download at http://www.endo-society.org/advocacy/upload/TES-Statement-on-A1C-Use.pdf through http://www.endo-society.org. Accessed January 2010.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(2002 May 01, Updated). Other Diabetes Management Tests, Glycosylated Hemoglobin. Diabetes Information, U.S Food and Drug Administration [On-line information]. Available online at http://www.fda.gov/diabetes/glucose.html#18 through http://www.fda.gov.

(1995-2004). Diabetes Mellitus. The Merck Manual of Medical Information-Second Home Edition [On-line information]. Available online at http://www.merck.com/mmhe/sec13/ch165/ch165a.html?qt=Diabetes&alt=sh through http://www.merck.com.

Mbanya, J. et. al. Working Group Members (2004 January 20). Report of the ADA/EASD/IDF Working Group of the HbA1c Assay. EASD All News [On-line Report]. Available online at http://www.easd.org/. (Click "All News" and "2004/05").

A.D.A.M., Updated (2003 October 10, Updated). HbA1c. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003640.htm.

(2004 June). 2004 NGSP Clinical Advisory Committee Meeting. American Diabetes Association 64th Annual Scientific Sessions, June 2004. NGSP, What’s New [On-line information]. Available online at http://www.missouri.edu/~diabetes/ngsp/indexwn.html through http://www.missouri.edu.