Picture: ISTOCK
Picture: ISTOCK

People of African descent may mistakenly get the all-clear from a widely used type 2 diabetes test called HbA1c, according to an international study published in PLOS Medicine on September 12.

Scientists found 42 new genetic variants that influence a person’s HbA1c measurements, including one that could lead to Africans and African-Americans being under-diagnosed with type 2 diabetes.

The findings have important implications as they suggest that using this test as the sole means to diagnose diabetes would do harm, said University of the Witwatersrand endocrinologist Alisha Wade.

"Recent World Health Organisation recommendations cite HbA1c as a method for the diagnosis of diabetes, in addition to more conventional methods of testing blood sugar in a fasted state or after an oral glucose challenge," she said. "There is substantial debate aboutthe use of HbA1c for diagnosis as there are known ethnic variations in the relationship between HbA1c and blood sugar, and the majority of studies to date have been done in populations of European origin. [This] study gives us a genetic explanation for why we should continue to be cautious about a wholesale shift to HbA1c for diagnosing diabetes."

The Society for Endocrinology, Metabolism and Diabetes of SA’s guidelines recommend HbA1c for diagnosing type 2 diabetes only if there are no haemoglobin abnormalities, but screening for these abnormalities is not routinely done in SA, she said.

The HbA1c test is used to monitor type 2 diabetes and assess blood sugar control in patients who have already been diagnosed. A person’s level of HbA1c, or glycated haemoglobin, depends on both blood glucose levels and characteristics of their red blood cells.

The scientists found that a variant in the G6PD gene, which encodes an enzyme related to the life-span of red blood cells, lowers HbA1c levels no matter what the blood glucose level is. Their analysis included almost 160,000 individuals of European, African, East Asian and South Asian ancestry, 33,000 of which went on to develop type 2 diabetes. They found about 11% of African-Americans carried at least one copy of this G6PD variant, while almost no one of any other ancestry did.

This means the test would miss about 650,000 African-Americans due to their genetically lowered HbA1c levels if it were given to all Americans, said the study’s lead authors James Meigs of Harvard Medical School and Inês Barroso of the Wellcome Trust Sanger Institute in the UK.

The research, which identified a total of 60 genetic variants that influence a person’s HbA1c measurements, found people who had more genetic variants that change HbA1c levels through effects on blood glucose levels were at increased risk of developing diabetes over time, but people who had more genetic variants that affected HbA1c through changes to red blood cells did not have an increased diabetes risk.

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