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A study indicated that a common blood sugar level test missed 73 percent of diabetes cases. Getty Images

A commonly used blood test may not be the best way to diagnose diabetes.

Researchers say testing blood sugar levels for type 1 or type 2 diabetes without also testing for glucose tolerance can miss nearly three-quarters of all cases of these chronic diseases.

The hemoglobin A1C blood test “should not be solely used to determine the prevalence of diabetes,” said Dr. Maria Mercedes Chang Villacreses, a postdoctorate fellow at the City of Hope’s Diabetes and Metabolism Research Institute in California. “It should be used in conjunction with the oral glucose test for increased accuracy.”

Chang Villacreses led a study on the accuracy of diabetes testing that was presented at the recent annual meeting of the Endocrine Society in New Orleans.

The hemoglobin A1C test shows the average level of blood sugar over a two- to three-month period. It is used to monitor the blood sugar of people known to have diabetes, but it is also a common test for diagnosing type 1 and type 2 diabetes because it doesn’t require any patient preparation, such as fasting.

The glucose tolerance test, on the other hand, is given after an overnight fast and again two hours later after the person consumes a sugary drink. People also must limit their carbohydrate consumption for several days prior to testing. The test measures the body’s response to glucose.

The study led by Chang Villacreses found that the A1C test missed 73 percent of the diabetes cases later detected by the glucose tolerance test.

“The A1C test said these people had normal glucose levels when they didn’t,” she said. “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.”

The study included 9,000 adults who didn’t have a diabetes diagnosis at the time of testing. Researchers first administered the A1C test, then the oral glucose tolerance test.

The A1C test was especially deficient at detecting diabetes in Hispanic and non-Hispanic black participants, researchers reported.

Chang Villacreses told Healthline that researchers suspect that these study subjects may have underreported anemia and other blood diseases that are more prevalent among these racial groups.

However, she said more research is needed on this issue.

The A1C is considered the frontline test for diabetes, having first been recommended by the American Diabetes Association (ADA) in 2010, according to Dr. David B. Sacks, a member of the College of American Pathologists’ Clinical Chemistry Resources Committee.

The test is useful for long-term detection of blood sugar because glucose binds with red blood cells and remains affixed for up to 120 days, he said.

Chang Villacreses said that the A1C test has become the standard analysis because it’s much easier to administer than the glucose tolerance test, which requires people to spend a minimum of two hours in the lab.

“Not everyone has that kind of time,” she said.

“The A1C test is much easier but not as accurate,” said Chang Villacreses. “We suggest that care of each patient has to be individualized.”

For example, she said, follow up with the glucose tolerance test when people at high risk of diabetes because of age, weight, diet, inactivity, family history, or other risk factors test negative on an A1C test.

“That’s a very practical solution and I’d definitely support that,” said Sacks.

He also suggested that doing a fasting glucose test — which requires only a single blood test and a shorter fasting window — in conjunction with the A1C tests could yield more accurate results.

People with an A1C blood sugar level of 6.5 percent or higher on two separate tests are considered to have diabetes. An A1C blood sugar of between 5.7 and 6.4 percent indicates prediabetes. Below 5.7 is considered normal, according to ADA guidelines.

With the glucose tolerance test, a blood sugar level of less than 140 mg/dL is considered normal. Between 140 and 199 mg/dL is considered prediabetes, and more than 200 indicates diabetes.

Chang Villacreses stressed that the A1C remains a “great test” for monitoring blood-sugar levels among people already diagnosed with diabetes.

Sacks noted that some research suggests that the A1C test also may be useful for identifying individuals whose health is most at risk if they develop diabetes.

“The real reason we test for diabetes is to prevent complications, many of which are irreversible,” he said. “If we detect diabetes early enough, we can prevent these complications or at least slow them down.”

Some studies have suggested that “people who are above the cutoff for A1C are more likely to develop complications than those who exceed the cutoff for glucose tolerance,” Sacks continued.

Therefore, the relatively higher accuracy of the more cumbersome glucose tolerance test “may not really matter for a lot of people because they may not develop complications.”

“It is clear that the glucose test picks up more people,” said Sacks. “The question is whether it matters from a practical standpoint.”

The most commonly used blood test for diabetes is far less accurate than glucose tolerance testing, researchers are reporting.

In a recent study, researchers reported that the A1C test missed 73 percent of cases of type 1 and type 2 diabetes that were later picked up by a glucose monitoring test.

Experts say people at high risk of diabetes should get confirmatory tests even if A1C results are negative.