What's new in diabetes: A1C assay emerges as a diagnostic test for diabetes

LIMITATIONS OF A1C

Despite the above arguments for the A1C assay as the worldwide standard for diagnosis of diabetes, its use has limitations. The cost of providing the assay may preclude its routine use in some parts of the world.³ In those circumstances, clinicians should continue to use previously recommended methodologies: an FPG value of 126 mg/dL or higher accompanied by symptoms; a casual blood glucose measurement of 200 mg/dL or higher; or a 2-hour OGTT result of 200 mg/dL or higher.⁴

Hemoglobinopathies may interfere with A1C assay methods. Although many assay methods can correct for the presence of the most common hemoglobin traits, any condition that changes red-cell turnover, such as hemolytic anemia, chronic malaria, blood loss, and blood transfusion, will produce erroneous A1C results. Changes in red-cell turnover during pregnancy make using A1C to diagnose diabetes in these patients problematic; therefore, glucose measurements should continue to be used.

A1C levels may not have caught up with an acute elevation in glucose levels in certain clinical settings, such as rapidly evolving type 1 diabetes. An A1C assay performed at this time will likely produce a nondiagnostic result. Most cases of type 1 diabetes, particularly in children and adolescents, should continue to be diagnosed by the classic symptoms of polyuria, polydipsia, polyphagia, unexplained weight loss, and a casual glucose value that is higher than 200 mg/dL.

The new recommendation has already sparked some controversy. William Bornstein, MD, Assistant Professor of Endocrinology at Emory University, Atlanta, Georgia, suggests that the long-established track record of the older methods may make some physicians reluctant to use the A1C assay.⁵ Lori Roust, MD, a board-certified endocrinologist at the Mayo Clinic, Scottsdale, Arizona, points out that not all persons with hemoglobinopathies are aware of their status, so screening these patients for diabetes using the A1C assay may yield inaccurate results. Roust also points out that the normal range for A1C values varies from institution to institution; therefore, results obtained from tests on the same patient could vary from laboratory to laboratory. Furthermore, the new method will likely result in more false negatives, as some patients screened using the new method may be anemic, have renal disease, or have recently received a blood transfusion, according to Curtiss Cook, MD, also with the Mayo Clinic. Still, doctors are likely to continue to use A1C assay in conjunction with other studies, such as the FPG test, random blood glucose testing, and OGTT.²

ELIMINATION OF SUBCLASSIFICATIONS

The committee warned against identifying an absolute dividing line between normoglycemia and diabetes. Although a level above which complications, such as retinopathy, rapidly escalate was identified, the committee was unable to identify a specific level at which risk for diabetes clearly begins. Furthermore, the clinical states of prediabetes, impaired fasting glucose, and impaired glucose tolerance fail to capture the continuum of risk. The committee thus argues for the elimination of these subclassifications.

On the other hand, the risk of developing diabetes increases as A1C values approach 6.5%. Therefore, the committee recommends that persons with A1C values close to the 6.5% threshold (ie, 6% and higher) should initiate lifestyle interventions. Other risk factors, including elevated triglyceride levels, an elevated BP, a high body mass index, and a family history of diabetes, should be used to determine which persons should be screened using the A1C assay. The level at which preventive services are provided to a population will primarily depend on the resources available, the size of the target population, and the nature of the intervention.

The International Expert Committee was comprised of representatives from the American Diabetes Association, the International Diabetes Federation, and the European Association for the Study of Diabetes. Its recommendations were published in the July 2009 issue of Diabetes Care.⁴ In December 2009, the ADA announced that its updated clinical practice recommendations, to be published in early 2010, promote A1C as a diagnostic test for diabetes.⁶ JAAPA

Karen Beer works in the Mayo Clinic Hospital, Phoenix, Arizona. The author has indicated no relationships to disclose relating to the content of this article.

REFERENCES

1. International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes. Diabetes Care. 2009;32(7):1327-1334. http://care.diabetesjournals.org/site/misc/DC09-9033.pdf. Accessed January 12, 2010.

2. Fiore K. ADA: diagnose diabetes with A1c test. MedPage Today Web site. http://www.medpagetoday.com/MeetingCoverage/ADA/14570. Published June 05, 2009. Accessed January 12, 2010.

3. Kerr M. ADA 2009: Expert Committee recommends use of hemoglobin A1C for diagnosis of diabetes. Medscape Today Web site. http://www.medscape.com/viewarticle/704021. Published June 5, 2009. Accessed January 12, 2010.

4. Executive summary: standards of medical care in diabetes—2009. Diabetes Care. 2009;32(suppl 1):S6-S12.

5. Landau E. Experts urge new screening for diabetes. CNNhealth.com Web site. http://www.cnn.com/2009/HEALTH/06/08/diabetes.screening.a1c/index.html. Updated June 8, 2009. Accessed January 12, 2010.

6. American Diabetes Association (ADA). American Diabetes Association's new clinical practice recommendations promote A1C as diagnostic test for diabetes. ADA Web site. http://www.diabetes.org/for-media/2009/cpr-2010-a1c-diagnostic-tool.html. Published December 29, 2009. Accessed January 6, 2010.