August 2009 Clinical Laboratory News: Expert Committee Endorses HbA1c Test for Diagnosing Diabetes

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August 2009: Volume 35, Number 8

Expert Committee Endorses HbA1c Test for Diagnosing Diabetes
ADA Considering Official Recommendation
By Genna Rollins

An international expert committee convened by the American Diabetes Association (ADA), International Diabetes Federation, and European Association for the Study of Diabetes recently recommended that the HbA1c test be used for the diagnosis of diabetes. The recommendation, which has not yet been officially endorsed by the three organizations, represents a fundamental shift in diagnosis of the disease. Existing guidelines call for screening and diagnosing diabetes by one of three methods, including fasting plasma glucose test (FPG), oral glucose tolerance test (OGTT), or symptoms of hyperglycemia combined with a casual plasma glucose (CPG) test, with a repeat test on another day in the absence of unequivocal hyperglycemia. FPG, OGTT, and CPG have screening and diagnostic cut-offs that are the same for each respective test. A person is considered to have diabetes if his FPG level is ≥126 mg/dL (7.0 mmol/L), OGTT level is at least 200 mg/dL (11.1 mmol/L) or if his CPG level is ≥200 mg/dl (11.1 mmol/L) and he has symptoms of hyperglycemia.

In making the recommendation to use HbA1c levels for diagnosis, the committee noted that with advances in instrumentation and standardization “the accuracy and precision of A1c assays at least match those of glucose assays,” and that recent introduction of a new reference method to calibrate all A1c assay instruments should further improve standardization world-wide. HbA1c also has “substantially less” biologic variability and preanalytic instability than FPG. In addition, the committee noted that HbA1c is a better index of overall glycemic exposure and risk for long-term complications, is relatively unaffected by acute fluctuations in glucose levels due, for example, to stress or illness, and that the test is currently used to guide management and adjust therapy for diabetics.

“A1c values vary less than FPG values and the assay for A1c has technical advantages compared with the glucose assay,” said expert committee chair David Nathan, MD, director of the Diabetes Center at Massachusetts General hospital and professor of medicine at Harvard Medical School. “Also, testing for diabetes using A1c is more convenient and easier for patients who will no longer be required to perform a fasting or oral glucose tolerance test.” Nathan also spoke in favor of labs reporting both A1c and estimated average glucose at a debate about the pros and cons of doing so which took place at AACC’s annual meeting in July.

Based on cumulative evidence about the prevalence of moderate retinopathy in diverse populations, the committee suggested that a cut point ≥6.5% would be appropriate for the diagnosis of diabetes. The report acknowledged that “this cut point should not be construed as an absolute dividing line between normal glycemia and diabetes; however, the A1c level of 6.5% is sufficiently sensitive and specific to identify individuals who are at risk for developing retinopathy and who should be diagnosed as diabetic.” Nathan elaborated on this point. “Glucose impairment runs on a continuum, making selection of a specific value where diabetes risk begins somewhat arbitrary. However, those persons whose A1c levels are close to the 6.5% diagnostic level are clearly at higher risk,” he said. The diagnosis should be confirmed with a repeat HbA1c test, unless the patient is symptomatic with plasma glucose levels >200 mg/dL (>11.1 mmol/L).

The committee acknowledged that in certain parts of the world where the HbA1c assay is not in common use, clinicians should continue to use FPG or OGTT to diagnose the disease. The HbA1c test also may not be appropriate for certain populations, including patients with hemoglobin traits like HbS and HbC, and those with conditions that impact red cell turnover, such as hemolytic anemia, chronic malaria, or blood transfusions.

The committee emphasized that HbA1c tests to diagnose diabetes should be performed on clinical laboratory equipment, as point-of-care technologies have not yet been proven to be sufficiently accurate or precise for this purpose.

The committee hopes the report will spur the diabetes community and professional organizations to consider use of the HbA1c in diagnosing diabetes. The ADA endorsed the principle of using HbA1c as a diagnostic test and plans to establish a task force to explore the report’s implications, including how best to implement the recommendations.

CLN reported on the controversy surrounding the concept of using A1c for diagnosing diabetes in December at this link. The new report is available on the ADA website.

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