This Blood Sugar Test May Be Less Accurate in African-American Patients
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This Blood Sugar Test May Be Less Accurate in African-American Patients

The test was standardized based on white patients

February 7, 2017

A new study has found that a widely-used blood test for measuring blood sugar may be less accurate in people with the sickle cell trait, which is carried by eight to 10 percent of African-Americans.

Doctors have been warned before that the A1C blood test can give imprecise results based on the patient's race and other factors. These results can often lead to a misdiagnosis or mismanagement of diabetes. The new study emphasizes the test's flaw as it applies to people carrying the sickle cell trait.

The levels of glucose (sugar) in the blood rise and fall constantly, so it is not always easy to diagnose diabetes or manage it using a single exam. The A1C test gets around this problem.

The test measures sugar that binds to molecules of hemoglobin in red blood cells. It gives an average of blood sugar levels over the past three months, "so this has turned out to be an incredibly powerful test, both for the diagnosis and treatment of diabetes," according to Dr. Anthony Bleyer, who works as a kidney specialist at the Wake Forest School of Medicine. He was not involved in the study.

The problem? The results of the test can vary based on circumstance. For instance, it can provide inaccurate readings for people with anemia as well as for those carrying unusual kinds of hemoglobin, the most well-known of which is sickle cell trait.

Eight to 10 percent of African-Americans carry this trait, but only people who inherit two copies of the trait—one from each parent—actually develop the disease.

Scientists realized a few years ago that the readings for African-Americans resulting from this test usually do not match readings for whites. Usually, they are higher.

"The test was really standardized based on white individuals, and there were just a small number of African-American individuals in that study," Bleyer said.

Although the gap is not big, it can be very important, particularly for those who are hovering on the edge of a diagnosis of diabetes. A person who seems to be just below the defining line and who has been diagnosed with prediabetes may, in reality, have a higher A1C level. This would put them over the line and lead to a diabetes diagnosis.

Variations in these readings can also be inaccurate—even harmful—for patients who are treated according to the test. Doctors may be too aggressive in controlling their blood sugar, even to the point where their blood sugar levels become seriously low.

Scientists from Brown University and the National Institutes of Health reviewed data from two big studies to compare the test results of African-Americans with and without the sickle cell trait. The studies had used standard A1C tests that had previously been shown to provide low readings for patients with the trait. The researchers were surprised at how big a difference there was.

Roughly four percent of study participants carrying the trait were diagnosed with diabetes. However, the scientists expect that a test corrected for bias would have identified around seven percent, almost two times as many people.

"We were really shocked by that, honestly," said Mary Lacy, lead author and a graduate student at Brown University. "That's huge!"

They also discovered 40 percent fewer cases of prediabetes than they had expected among participants with the trait.

Though the test results are off by only a few tenths of one percentage point, that was still enough to place many patients below the lines indicating diabetes or prediabetes.

Researchers believe that this observation provides one reason why doctors should be careful when interpreting A1C test results.

"Doctors generally take the test fairly literally," said Tamara Darsow, senior vice president for research and community programs at the American Diabetes Association (ADA). "How much this impacts care and the interpretation of A1C results I think is variable."

The ADA's guidance document warns doctors that the tests can be off among patients with unusual hemoglobin traits by plus or minus seven percent, a figure that—as underscored by the new study—represents more than enough variability to have an impact on a diagnosis.

Instead of diagnosing such patients immediately upon receiving the affected results—particularly African-American patients—the doctors could then run more traditional tests for blood sugar. Those tests are not influenced either by race or sickle cell states, though they can vary for other reasons.

"Information together from all of these tests can be much more powerful than those taken in isolation," said Darsow.

Ultimately, the ADA would like to develop more concrete guidelines for treating patients, but for now it wants doctors to be aware of this issue and to use their best judgment accordingly.