The way that diabetes is diagnosed in the United States is about to change.
Later this year, an expert panel organized by the American Diabetes Association will issue a report making a person's blood level of glycosylated hemoglobin (HbA1c) an accepted method for diagnosing diabetes, according to staffers from the ADA. Although the decision is not yet finalized, “the group will likely recommend HbA1c as the preferred test,” placing it above the current diagnostic standard (fasting blood glucose level) and also above the historic criterion, the glucose tolerance test, said Dr. Sue Kirkman, the ADA's vice president for clinical affairs.
The report from the ADA's Expert Committee on the Diagnosis and Classification of Diabetes will also set the HbA1c cut point for diagnosing diabetes, but this value has not yet been finalized.
This shift on the use of HbA1c for diagnosis stands to legitimize the method that is already commonly used by many primary care physicians, said Dr. Mayer B. Davidson, an endocrinologist at Charles R. Drew University of Medicine and Science in Los Angeles and professor of medicine at the University of California, Los Angeles. He applauded the decision, noting that “HbA1c is a more valid way to look at what is going on with glucose,” compared with glycemia levels.
And, “since the HbA1c test doesn't require fasting, the hope is that it will be more convenient and that more people will get tested and diagnosed early,” Dr. Kirkman said in an interview. “In the United States, the percentage of people with diabetes who are undiagnosed is now estimated to be 25%. There is hope that making the diagnosis easier will further decrease the number of people who are undiagnosed,” she added.
The Expert Committee on the Diagnosis and Classification of Diabetes is an ad hoc group that the ADA convenes when it “feels there is a need to revisit some area related to diagnosis or classification,” Dr. Kirkman said. The current round of deliberations began last year, and the group was constituted not only with members picked by the ADA, but also with representatives from the European Association for the Study of Diabetes and the International Diabetes Federation. “It is hoped that all three organizations will adopt the recommendations so that there is a worldwide standard.” ADA officials think the report may be ready for release before or during the ADA's annual scientific sessions in June.
Making HbA1c an accepted diagnostic test—let alone the preferred test—has been on the table for years. In a recent talk at a meeting sponsored by the ADA in New York, Dr. William C. Knowler spelled out the case in favor of using HbA1c, as well as some shortcomings. The strengths of HbA1c as a diagnostic tool include:
▸ A more standardized assay and much less interlaboratory variability, compared with blood glucose measurements.
▸ A better index of overall glycemia.
▸ Consistency in using the same assay for diagnosis that's also routinely used to monitor patient treatment and to predict the risk for long-term complications.
▸ No need for fasting before the specimen is drawn.
▸ No effect from acute changes in blood glucose levels, e.g., those caused by illness.
Another draw of HbA1c is that when above 7.0%, it correlates strongly with the development of microvascular complications, noted Dr. Davidson. “There is no absolute way to diagnose” diabetes. “Where we draw the line is somewhat arbitrary.” Basing diagnosis on a test that can reliably predict the risk for microvascular complications is attractive because these complications “are fairly specific to diabetes,” he said in an interview.
But relying on HbA1c for diagnosis also has limitations. The test is not universally available around the world. And a person's HbA1c level can be affected by hemoglobinopathies, variations in red cell turnover, and unexplained racial differences, said Dr. Knowler, chief of the Diabetes Epidemiology and Clinical Research Section of the National Institute of Diabetes and Digestive and Kidney Diseases in Phoenix and a member of the Expert Committee.
Perhaps most importantly, switching the diagnostic criterion will create a break from the past that may make it hard to reconcile old epidemiologic observations with new ones. A similar break occurred in 1997, when the ADA switched its diagnostic standard from the blood glucose level 2 hours after an oral glucose challenge to a fasting blood glucose level. That switch resulted in a sudden spike in the number of patients diagnosed with diabetes, Dr. Knowler said.
The fact that an HbA1c cut point for diagnosis has still not been set highlights the controversy this issue generates. Despite thoroughly reviewing the evidence, Dr. Knowler said in mid-February that he was still unable to answer that question. A cut point of 6.5% has “some useful properties,” he acknowledged, but 5.5% is “a level to raise concern” that a person is at risk for eventually developing diabetes. Choosing a cut point “is a complicated issue that depends on how harmful are missed diagnoses and overdiagnosis,” he said.
In contrast, Dr. Davidson, who is not a member of the current Expert Committee although he served on it in the past, leans toward a cut point of 7.0% because of its significance for microvascular disease, but adds that he is in the minority in the endocrinology community and it's unlikely the diagnostic threshold will be set so high.
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