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NEW YORK (Reuters Health) - A new proposed diabetes test could miss millions of cases of diabetes and pre-diabetes and also over-diagnose black Americans if it was used as a screening tool, suggests a new study.Over 23 million Americans have diabetes - about 8 percent of the population - and another 57 million are at risk for developing diabetes, according to the American Diabetes Association (ADA). The annual cost of diabetes in the U.S. recently topped $200 billion, and the disease is linked to heart disease, high blood pressure, and kidney disease. Cases of diabetes have been on the rise, leading doctors to hunt for a quick and easy screening test that can help them catch the disease early as well as identify who is at risk for getting it in the future."A third of Americans with diabetes are believed to not know they have the disease," Dr. Darin Olson, the study's lead author from Emory University in Atlanta, told Reuters Health. "In order to intervene with useful medications, we have to be able to diagnose them."In the study, published in Diabetes Care, researchers compared scores on two different tests for diabetes in about 5,000 Americans who had not been diagnosed with the disease. Those tests included an oral glucose (sugar) tolerance test, the "gold standard" for diabetes screening and diagnosis, and the new proposed test that measures hemoglobin A1c, which forms when extra glucose in the blood clings to oxygen-carrying molecules in red blood cells.In an oral glucose test, patients drink a glucose-filled beverage after not eating for at least 8 hours. Blood is drawn right before they drink and 2 hours later, and doctors compare the amount of glucose in the blood at both times. A high glucose level after drinking suggests that the body is having trouble using glucose and means that a person is at risk for diabetes.But that test has its drawbacks - namely how long it takes, along with the hassle of having to fast beforehand. The A1c test is a simple blood test, and patients can eat or drink right up until they have their blood drawn. The A1c test also has proven success at helping people already diagnosed with diabetes keep track of how well they are managing the disease and determine how at risk they are for diabetes-related complications. So some wondered if it might be useful for diabetes screening as well. "I think we need to find something that (makes it) easy to detect more people," Dr. Carlos Lorenzo, of University of Texas Health Science Center in San Antonio, told Reuters Health. "If we find a test that is easier to perform and more reliable, it would be good. We were thinking that this was going to be the A1c (test)," said Lorenzo, who was not involved in the current study, but who has studied the A1c test for diabetes diagnosis and screening. Using databases that had scores on both tests for a diverse group of American adults, Olson and his colleagues diagnosed each person as diabetic or pre-diabetic based on the established guidelines for the oral glucose test and recommendations proposed by the ADA and an international committee for the A1c test. According to the oral glucose screening standards, 36 percent of people had pre-diabetes and 6 percent had diabetes. Using the ADA's A1c guidelines, 20 percent were at a high risk for diabetes, while only 2 percent actually had the disease.That means that nearly 6 million Americans with diabetes and more than 40 million with pre-diabetes who would test positive on the oral glucose test could be missed if all adults were screened using the A1c test.There were also differences in A1c readings based on race. Even when they had similar glucose levels in the blood, black Americans had higher hemoglobin A1c levels than whites, leading them to be over-diagnosed with diabetes when the A1c test was used.That confirms a similar result released earlier this year (see Reuters Health story of June 14, 2010: Use of proposed diabetes test may depend on race). There were also fewer false negatives - when the test showed a person didn't have pre-diabetes or diabetes, but they actually did - in black Americans than white Americans.Olson thinks the A1c test could be a useful add-on to current diabetes testing methods, but shouldn't stand on its own. "The A1c is a good test for diagnosing diabetes," he said, "but it is not a good screening test and it will miss a lot of people with early diabetes. If our plan going forward will be to take care of people early and identify people earlier in the disease and miss less people, this ... probably shouldn't be the first test in screening."