So convinced of the strength of their data, the authors discouraged use of the A1C test alone to rule out diabetes, especially in people with prediabetes or those who are at known risk for the disease. The test has a “significant tendency” for underestimating the prevalence of type 2 diabetes and overestimating normal blood glucose levels, says lead researcher Maria Mercedes Chang Villacreses, MD, a postdoctoral fellow at the City of Hope Diabetes and Metabolism Research Institute in Duarte, California. “The A1C is the more-used method of screening,” compared to traditional oral glucose testing, Dr. Villacreses says. “But we recommend not relying solely on the A1C.” Instead, the A1C test should be used in conjunction with an oral glucose test for diagnosing the disease, she says. A1C can be used to diagnose prediabetes, the precursor to type 2 diabetes. It’s also well-known to patients as a test that provides information about average blood glucose levels over three months, according to the NIDDK. Doctors typically order A1C tests for people with diabetes on a regular basis to monitor the effectiveness of treatment. The study did not address the accuracy of the test in the regular monitoring of patients. RELATED: American Diabetes Association Releases 2019 Standards of Care
How Researchers Studied the Usefulness of A1C for Diabetes Diagnosis
The study included 9,000 adults, ages 20 and older, who were part of the 2005–2014 National Health and Nutrition Examination Survey. These individuals did not have a diabetes diagnosis. The participants received both an A1C test and an oral tolerance glucose test (a test that measures the body’s response to sugar, according to the NIDDK). Researchers observed that the A1C test failed to detect 73 percent of diabetes cases that were detected by the oral glucose test. The oral glucose test requires a patient to fast overnight and then consume a sugary drink. The blood is tested after the fast and again after consuming the drink. A1C does not require fasting. Doctors have stepped up their efforts to diagnosis prediabetes and diabetes earlier in the course of the disease, before it causes other health problems, such as an increased risk for heart disease and nerve damage, and when it is more easily treated or even reversed. The A1C test is a convenient way for doctors to screen their patients and help identify the large number of Americans who remain undiagnosed, says Washington DC–based Robert Lash, MD, the chief professional and clinical affairs officer for the Endocrine Society. “What this abstract shows is the test has many limitations,” Dr. Lash says. “It’s not as sensitive. But the test you get, even if it’s not great, is better than the test you don’t get.” Professionals’ guidelines for diabetes screening and diagnosis allow for the sole use of the A1C test, he says, adding, “The guidelines say you can use it, but you should recognize that it’s not perfect.” The majority of endocrinologists are likely aware of the limitations of the test, Villacreses says. But the large gap the study found in the test’s accuracy “is important,” she says, adding that “clinicians shouldn’t rely solely on the A1C. The majority of endocrinology practices are aware of that. But primary care practitioners now manage a lot of diabetes. It’s important to bring this up as an alert for them, too.” RELATED: 8 Surprising Type 2 Diabetes Risk Factors
The Success of A1C in Diagnosing Diabetes May Differ Depending on Ethnicity
The study also showed racial and ethnic variations. The A1C test was more accurate in detecting abnormal glucose levels in white people compared to black and Hispanic people, Villacreses says. The test’s ability to correctly detect normal glucose levels was the lowest for black people, at 71.7 percent, followed by 85.8 percent for non-Mexican Hispanic people and 86.6 percent in Mexican-American people. It was highest in white people, at 89.4 percent. It’s unclear why the test varies widely in accuracy among various racial and ethnic groups, Villacreses said. “There are racial and ethnic differences,” Lash says. “We’re not 100 percent sure what to do with those differences right now. We need to know more about what A1C levels mean in different populations.” RELATED: Gene Variant May Skew A1C Results for Some African-Americans
