A1C Test Recommended for Diagnosis of Diabetes
The A1C test, long used as a diabetes management tool, is now recommended by the American Diabetes Association (ADA) for use in the diagnosis of diabetes and prediabetes. The test is an alternative to standard glucose testing with the fasting plasma glucose (FPG) test and the oral glucose tolerance test (OGTT). The recommendation was made in the ADA's "Standards of Medical Care in Diabetes–2010," published in the January 2010 issue of Diabetes Care.
Also called the hemoglobin A1C test or the glycated hemoglobin test, the A1C test provides an estimate of average blood glucose levels over the preceding 2 to 3 months. A normal A1C level is around 5 percent. An A1C level of 6.5 percent or above indicates diabetes, and an A1C level of 5.7 to 6.4 percent is considered prediabetes.
Although the A1C test is not more accurate than the FPG test and the OGTT, it does not require fasting and can be measured at any time of day. Experts hope the convenience of the test will result in more people who are at risk of diabetes or prediabetes being tested, thus reducing the number of people with undiagnosed diabetes in the United States.
The ADA had not previously recommended the A1C test for diagnostic purposes due to the lack of standardization of the assay. However, laboratory-based A1C tests are now highly standardized due to efforts by the National Glycohemoglobin Standardization Program (NGSP). The ADA recommends that when used for diagnosis, the A1C test should be performed in a laboratory using a method certified by the NGSP. A1C tests done in doctors' offices—point of care tests—are useful in guiding therapy but are not sufficiently accurate to be used for diagnosis of diabetes.
The A1C test can be misleading in people with less common forms of red blood cell hemoglobin, or hemoglobinopathies, such as sickle cell trait. The NGSP provides a list of A1C assays that are accurate in people with hemoglobinopathies. The A1C test should not be used to diagnose diabetes in people with conditions that shorten red blood cell survival, including certain types of anemia and pregnancy. More information about the A1C test and factors that interfere with test results is available at www.ngsp.org .
ADA Definitions of Prediabetes and Diabetes
The following table presents the ADA definitions of prediabetes and diabetes using the three recommended diagnostic tests. When a test result indicates diabetes, the diagnosis should be confirmed by repeating the same test on a different day or by using the results of a different test. Diabetes can also be diagnosed using a random plasma glucose test in a person with severe, classic symptoms of diabetes.
|A1C||5.7 to 6.4 percent||6.5 percent or above|
|FPG||100 to 125 milligrams per deciliter (mg/dL) or 5.6 to 6.9 millimoles per liter (mmol/L)—impaired fasting glucose||126 mg/dL or 7.0 mmol/L and above|
|OGIT||140 to 199 mg/dL or 7.8 to 11.0 mmol/L—impaired glucose tolerance||200 mg/dL or 11.1 mmol/L and above|
The ADA noted in diagnosing prediabetes, risk extends below the lower limit of the range and is disproportionately greater at the higher end of the range for all three tests. Thus, a person with an A1C above 6.0 percent should be considered at very high risk, and a person with an A1C of below 5.7 percent may still be at risk depending on individual factors.
The National Diabetes Information Clearinghouse, an information dissemination service of the National Institute of Diabetes and Digestive and Kidney Diseases, has fact sheets and easy-to-read booklets about diabetes testing and management. For more information and to obtain copies, visit www.diabetes.niddk.nih.gov
NIH Publication No. 10-4562
Page last updated: December 5, 2011