Historically, tests based on measurement of plasma glucose have been used to detect diabetes in people without symptoms. These tests include a fasting plasma glucose (FPG) test or an oral glucose tolerance test (OGTT). Now A1c provides an additional option. Only A1c tests that have been referenced to an accepted laboratory method (standardized) should be utilized for diagnostic or screening purposes. (See the links below for a list of standardized A1c tests). At present 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 or screening but can be utilized to judge the effectiveness of diabetes treatments (lifestyle and drug therapies). It is also important to realize that an A1c test to diagnose diabetes may not be appropriate in certain situations, e.g., heavy bleeding, pregnancy, and certain anemias (conditions in which the rate of red blood cell turnover is higher than normal).
New A1c Guidelines
International Panel Ok'd in 2009
Last year, an international panel (the International Expert Committee) recommended the A1c test as a diagnostic tool. The panel concluded that A1c tests are accurate and precise and offer several advantages over glucose tests in diagnosing diabetes, including appreciably superior technical attributes and greater clinical convenience. (See article: Experts Recommend that A1c Test also be used to Diagnose Diabetes)
More Convenient, Practical, Stable
Convenience is a benefit of the A1c test. Patients don't have to fast for 8 hours (as for a fasting plasma glucose test) or endure multiple blood samples being taken over several hours (as for an OGTT). The A1c test reflects the average amount of glucose in the blood over the last two to three months. The A1c test is not affected by some of the conditions which may affect a glucose test such as whether the patient is ill, in pain, or stressed on the day of the test. Another advantage of the A1c test is that the sample is stable and can be stored at room temperature for much longer times than samples for glucose testing.
A1c Not an Option for Everyone
As mentioned above, this test should not be used for diagnosing diabetes in people with certain conditions that can affect the results of the A1c test. These include pregnant women, people with chronic kidney or liver disease, and those with blood disorders such as iron-deficiency anemia, vitamin B12 anemia and hemoglobin variants such as thalassemia. Recent severe bleeding and recent blood transfusion also lead to differences in A1c and therefore A1c tests should not be used for diagnosis. The conventional tests for FPG or OGTT may be used for diagnosis instead.
In a statement supporting the use of A1c as an option, the Endocrine Society nevertheless pointed out the limitations of the test in groups with the conditions listed above and also noted that more studies need to be done in populations other than those of European ancestry. They also stated that fewer people will be diagnosed with diabetes using A1c criteria and that more people will fall into the pre-diabetic category compared to using glucose tests and the corresponding cut-off points.
The World Health Organization (WHO) also noted that there are some limitations of this test in various parts of the world. For example, the relatively higher prevalence of certain anemias such as sickle cell anemia or hemoglobin variants such as thalassemia in certain countries and populations may limit the value of the test. WHO also pointed out that limited availability of standardized testing facilities and higher costs may pose barriers to adoption of this test in developing nations.
An estimated 25 percent of people in the United States who have diabetes are undiagnosed. Many people don't realize they have this chronic, potentially life-threatening disease. About 57 million people in the United States are pre-diabetic. Their higher-than-normal blood glucose levels put them at high risk for developing type 2 diabetes in the future. Long-term damage, especially to the heart and circulatory system, may occur silently in this state. The availability of this additional test for diagnosis of diabetes and pre-diabetes may help identify more people at risk. Knowing they are at risk can enable them to take steps to delay progression of disease - steps like changing their diets and becoming more physically active.
On this site
Screening: Diabetes — Children (2-12), Teens (13-18), Young Adults (19-29), Adults (30-49), Adults (50 and up)
In The News: Experts Recommend that Hemoglobin A1c also be used to Diagnose Diabetes
Elsewhere on the web
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.
American Diabetes Association. Standards of medical care in diabetes—2010 (position statement). Jan 2010. Diabetes Care 33: S11-S61.
American Diabetes Association. Summary of revisions for the 2010 Clinical Practice Recommendations. Jan 2010. Diabetes Care 33: S3.
American Diabetes Association. Pre-Diabetes. Available online at http://www.diabetes.org/diabetes-basics/prevention/pre-diabetes through http://www.diabetes.org. Accessed 13 Jan 2010.
International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes. Jul 2009. Diabetes Care 32(7): 1327-1334.
World Health Organization. Update on WHO recommendations for diagnostic criteria for diabetes mellitus (statement). Jan 7, 2010. PDF available for download at http://www.who.int/entity/dietphysicalactivity/diabetes_statement_20100107_en.pdf through http://www.who.int. Accessed 12 Jan 2010.
(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.