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[HbA.sub.1c] gains legitimacy for diagnosis of diabetes.

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 ([HbA.sub.1c]) an accepted method of diagnosing diabetes, according to staffers from the ADA. Although the decision is not yet finalized, "the group will likely recommend [[HbA.sub.1c]] as the preferred test," placing it above the current diagnostic standard (the fasting blood glucose level) and also above the historic criterion for diabetes diagnosis (the glucose tolerance test), said Dr. Sue Kirkman, the ADA's vice president for clinical affairs.

The report by the ADA's Expert Committee on the Diagnosis and Classification of Diabetes will also set the [HbA.sub.1c] cut point for diagnosing diabetes, but the value has not yet been finalized.

This shift on the use of [HbA.sub.1c] 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 "[HbA.sub.1c] is a more valid way to look at what is going on with glucose," compared with glycemia levels.

Adoption of [HbA.sub.1c] as the primary diagnostic method also stands to simplify the diagnosis of diabetes, meaning that more people will probably be tested and identified as having the disease. "Since the [HbA.sub.1c] 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, noting that an estimated 25% of people in the United States who have diabetes are undiagnosed.

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 committee, which began its deliberations last year, includes members picked by the ADA, along with representatives from the European Association for the Study of Diabetes and the International Diabetes Federation. "Eventually it is hoped that all three organizations will adopt the recommendations so that there is a worldwide standard." ADA officials think the report may released before or during the ADA's annual scientific sessions in June.

Making [HbA.sub.1c] an accepted diagnostic test--let alone the preferred test--has been on the table for years. At a recent meeting sponsored by the ADA in New York, Dr. William C. Knowler spelled out the case in favor of using glycosylated hemoglobin, as well as the shortcomings of this approach. The strengths of [HbA.sub.1c] as a diagnostic tool include the following:

* A more standardized assay with 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, such as those caused by illness.

Another attraction of [HbA.sub.1c] is that a level above 7.0% is strongly correlated with the development of microvascular complications, Dr. Davidson noted in an interview. "There is no absolute way to diagnose" diabetes; "where we draw the line is somewhat arbitrary." Basing diagnosis on a test that reliably predicts the risk for microvascular complications is attractive because these complications "are fairly specific to diabetes," he said.

But relying on [HbA.sub.1c] for diagnosis also has limitations. A person's [HbA.sub.1c] 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 might 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 following an oral glucose challenge to a fasting blood glucose level. That led to a sudden spike in the number of patients diagnosed with diabetes. At that time, the ADA decided against adopting [HbA.sub.1c] as its diagnostic standard because tests for it were not sufficiently standardized, Dr. Knowler said.

The fact that an [HbA.sub.1c] cut point for diagnosis has still not been set highlights the controversy this issue generates. Having thoroughly reviewed the evidence, Dr. Knowler still wasn't re-pared 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 that it's unlikely the diagnositc threshold will be set so high.
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Author:Zoler, Mitchel L.
Publication:Internal Medicine News
Geographic Code:1USA
Date:Mar 15, 2009
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