A popular test may not be as accurate as doctors thought.
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An increasingly popular test used to diagnose and monitor type 2 diabetes may not be as accurate as hoped, a new study published Tuesday in PLOS Medicine suggests, particularly in African Americans who carry a common genetic quirk.
A large team of researchers—more than 200—studied the blood of nearly 160,000 people scattered across the globe believed to not have the chronic disease. They were looking for genes that, if tweaked slightly, would influence the results of a HbA1c test, which measures how much of our hemoglobin (the protein that carries oxygen in our red blood cells) is loaded with glucose—the higher the percentage, the less control we have over our overall blood sugar levels, which can indicate diabetes. They identified 60 genetic variations a person could have that mucked with the HbA1C test, including 42 discovered for the first time.
One of these variations stuck out, involving the G6PD gene, both because it was relatively influential and almost only seen in people of recent African ancestry.
"About 11 percent of people of African American ancestry carry at least one copy of this G6PD variant, while almost no one of any other ancestry does," the researchers wrote. "We estimated that if we tested all Americans for diabetes using HbA1c, about 650,000 African Americans would be missed because of these genetically lowered HbA1c levels."
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The HbA1C test has existed since the 1970s, but it's only recently that doctors have strongly relied on it to diagnose diabetes, a trend bolstered by recommendations from major organizations like the American Diabetes Association (ADA). Compared to earlier developed methods, the HbA1C test can be less of a hassle to use. The fasting plasma glucose test, for instance, requires patients to avoid food for at least 8 hours overnight and can't be taken anytime other than the morning.
The current study isn't the first to suggest that the HbA1C test could be less reliable with certain populations like African Americans, though it's one of the first to offer a concrete reason as to why.
"The G6PD genetic variant shortens the three-month lifecycle of red blood cells, explained Elaine Wheeler, one of the study's lead authors and scientist with the Wellcome Trust Sanger Institute, a UK-based nonprofit that conducts genetic research, in a statement. "So in African Americans who have this variant, their red blood cells don't live long enough to bind to the glucose in the blood. Therefore these people will have a lower level of HbA1c, which won't show as a positive result for type 2 diabetes."
Important as this study is, touted by the authors as the largest of its kind, Wheeler and her team don't want to oversell their findings. Most of the variants they found have little effect on the test, and even the G6PD genetic variant's effect alone would only amount to about two percent, mostly men, of the 29.9 million African Americans currently living in the US being under diagnosed. "[T]he HbA1c test remains a suitable test for diagnosing and monitoring diabetes for the majority of people," said Dr. Inês Barroso, another lead author and Wellcome Trust Sanger scientist.
Small as that added drop may be, African Americans are still more than twice as likely to develop type 2 diabetes than the general population, making any missed opportunity that much harder felt. An estimated 5 million black adults are believed to have diabetes, according to the ADA. African Americans are more likely to suffer the worst consequences of having diabetes, which can include blindness, numbness of the limbs, and irreversible kidney damage.
The researchers are hoping that future studies will tease out how to best tweak the HbA1C test for people with the G6PD variant, since it's so common. In Africa alone, up to twenty percent of the native population may carry it, a figure even more relevant given people in the country of South Africa are starting to die from diabetes more often than they are of HIV. They also advise it might be worthwhile to offer genetic screening alongside the HbA1c test, or rely on other diagnostic tests, when screening anyone of African descent for diabetes.
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