Historically, diabetes screening and diagnosis in people without symptoms has hinged on results of a fasting plasma glucose (FPG) test or an oral glucose tolerance test (OGTT). The A1c test, which shows the average glucose level in the blood over the last two to three months, had been used to monitor glucose control in people already diagnosed with diabetes, but not for diagnosis. This was because of previous concerns about the test's variability. Endorsement of A1c as a diagnostic tool by ADA, the Endocrine Society, and the American Association of Clinical Chemistry follows huge improvements in standardization and analytical performance of the test methods.
Now, recent studies have been undertaken to take a closer look at A1c's performance as a diagnostic test.
One study concludes that A1c may miss some cases of diabetes. Published in the January 2011 issue of Diabetic Medicine, the study analyzed data from the Finnish Diabetes Prevention Study, which involved 522 overweight men and women with confirmed impaired glucose tolerance. Of those with diabetes diagnoses based on two OGTTs, 60% would have remained undiagnosed if diagnosis had been based on A1c using ADA criteria, the researchers found.
Another study, published in the Dec. 1, 2010 Journal of Clinical Endocrinology and Metabolism, reported "considerable discordance" between FPG and A1c-based diagnosis of diabetes and pre-diabetes in elderly people. Using the ADA criteria for diabetes and pre-diabetes, Kasia Lipska of Yale University School of Medicine and colleagues measured A1c and FPG in 1,865 adults aged 70 to 79 who had no known diabetes. Of these, 4.3% met either A1c or FPG criteria for diabetes. About one third of this group had only high A1c levels, while roughly one third had only high FPG. Another third had both elevated A1c and FPG.
Other researchers found the diagnostic performance of A1c to be lacking in sensitivity for another age group, adolescents, compared to that of adults. Joyce M. Lee and colleagues reported their findings in the January issue of Journal of Pediatrics. Using the same A1c cut points as those established for adults, the researchers tested 1,156 overweight and obese adolescents ages 12 to 18 using A1c, FPG and 2-hour plasma glucose and compared them with the results from 6,751 adults ages 19 to 70. In detecting diabetes and pre-diabetes, A1c performed poorly for testing adolescents as compared to adults. Tests for FPG and 2-hour PG were more sensitive for this younger age group.
Genetic differences may explain why correlations between A1c and glucose levels differ across populations, writes Allan S. Brett, MD in the February 1 Journal Watch General Medicine. Genetics can produce variation in how sugars bind to proteins and lipids and in red blood cell life span, he explains. Prior to these studies, the Endocrine Society had called for more study of A1c as a diagnostic test in populations of non-European ancestry because previous research had found that, based on A1c, more people of non-European ancestry fall into the pre-diabetic category compared to fasting or oral glucose tolerance testing.
Although A1c is an attractive diagnostic test because patients do not need to fast for the test or endure multiple blood draws, doctors and laboratories have long known that the A1c test has limitations for monitoring glucose control in some people. These include patients with chronic kidney or liver disease, blood disorders such as iron-deficiency anemia, vitamin B12 anemia, and thalassemia, or people who have recently had severe bleeding or blood transfusions.
Patients should expect doctors to continue to use the A1c test but be aware that like any other test, it is not perfect. "No test is 100% sensitive or specific," explains William E. Winter, MD, of the University of Florida. If A1c results are normal but concerns about possible diabetes continue, he recommends that doctors order the FPG test. "If this result is not clearly abnormal and the question of diabetes persists, then an oral glucose tolerance test can be performed," he suggests.
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In the News: ADA recommends A1c test to diagnose diabetes and pre-diabetes (2010)
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.
Lipska KJ et al. Identifying dysglycemic states in older adults: Implications of the emerging use of hemoglobin A1c. J Clin Endocrinol Metab. Available online at http://dx.doi.org/10.1210/jc.2010-1171 through http://dx.doi.org. Published December 2010. Accessed February 15, 2011.
Allan S. Brett. Use of Hemoglobin A1c to Diagnose Diabetes. Journal Watch General Medicine. Available at http://general-medicine.jwatch.org/cgi/content/citation/2011/201/3 through http://general-medicine.jwatch.org. Published February 1, 2011. Accessed February 14, 2011.
American Diabetes Association. How to Tell if You Have Prediabetes. Available online at http://www.diabetes.org/diabetes-basics/prevention/pre-diabetes/how-to-tell-if-you-have.html#%22 through http://www.diabetes.org. Accessed February 17, 2011.
(January 26, 2011) Lee JM, et al., Diagnosis of Diabetes using Hemoglobin A1c: Should Recommendations in Adults Be Extrapolated to Adolescents? Journal of Pediatrics. Abstract available online at http://www.jpeds.com/article/PIIS0022347610009972/abstract?rss=yes through http://www.jpeds.com. Accessed March 2011.
(February 25, 2011) Mann, Denise. WebMD Health News, What’s the Best Test for Children’s Diabetes? Available online at http://diabetes.webmd.com/news/20110225/whats-the-best-test-for-childrens-diabetes through http://diabetes.webmd.com. Accessed March 2011.
William E. Winter, MD. Lab Tests Online adjunct board member.