The Hemoglobin A1c Test

Common A1c test misses many cases of diabetes

by Barbara Hewitt on April 1, 2019

Using the haemoglobin A1c blood test to diagnose diabetes tends to underestimate the prevalence of the disease, according to a new study.

Lead researcher Maria Mercedes Chang Villacreses from the Diabetes and Metabolism Research Institute in Duarte, California, explained that the A1c test should be used in conjunction with the oral glucose test for increased accuracy.


The haemoglobin A1c is a test that shows the average level of blood sugar over the past two to three months. People who have diabetes usually have this test to see whether their blood sugar levels have been staying within a target range.

The test is also used to diagnose type 1 and type 2 diabetes. It is often used to diagnose diabetes because no fasting or any preparation is required.

A glucose tolerance test, also known as the oral glucose tolerance test, measures the body’s response to sugar (glucose). In this test, a person’s blood is taken after an overnight fast, and then again two hours after they drink a sugary drink. The glucose tolerance test can be used to screen for type 2 diabetes.

The new study included 9,000 adults without a diabetes diagnosis. The participants got both an A1c test and an oral tolerance glucose test, and the researchers compared the results.

The researchers found the A1c test didn’t catch 73% of diabetes cases that were detected by the oral glucose test. ‘The A1c test said these people had normal glucose levels when they didn’t,’ Chang Villacreses said.

The researchers also found race and ethnicity had a significant impact on the accuracy of A1c. It was more likely to detect abnormal glucose levels in non-Hispanic whites than in non-Hispanic blacks or Hispanics.

‘Our results indicated that the prevalence of diabetes and normal glucose tolerance defined solely by A1c is highly unreliable, with a significant tendency for underestimation of the prevalence of diabetes and overestimation of normal glucose tolerance,’ Chang Villacreses added.

The meeting also heard from scientists from the Lewis Katz School of Medicine at Temple University in Philadelphia that Caucasians and Hispanics with diabetes have a greater risk of fractures compared to those without diabetes, while African Americans with diabetes have little to no additional fracture risk.

‘Diabetes has been associated with additional risk of fracture, but it had not been well studied in African Americans or Hispanics, the two racial-ethnic groups with the highest rates of diabetes in the United States,’ said lead researcher Rajesh Jai.

The researchers evaluated data from 19,153 people with diabetes of which 7,618 were Caucasian, 7,456 African American and 4,079 Hispanic, and 26,217 people with hypertension of which 15,138 were Caucasian, 8,301 African American, and 2,778 Hispanic, all at least 40 years of age.

When controlling for other important factors, the risk of fracture in white and Hispanic people with diabetes was 23% higher than those without diabetes. However, the risk of fractures in African Americans with diabetes was not significantly different than those without diabetes.

‘This is a novel finding and has not been previously reported. African Americans, regardless of whether they had diabetes, had more than a 10 fold risk of a fracture if they had a fracture in the past, compared with about a twofold increased risk in white and Hispanic people. This suggests risk factors for fracture may differ in African Americans,’ Jain said.


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