A1c blood test found to be unreliable in diabetes diagnoses

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Research has found that many cases of diabetes are missed when the hemoglobin A1c blood test is used solely to diagnose the disease.

A1c is used to determine blood sugar levels at a specific point in timeKateryna Novikova | Shutterstock

Statistics published by Diabetes UK state that more than five million people in the UK will be living with diabetes by 2025, and in the last 20 years, the number of people diagnosed with diabetes has more than doubled. Author of the study Maria Mercedes Chang Villacreses describes diabetes as a “global epidemic.”

There are two types of diabetes: type 1, and type 2. Type 1 is caused by the immune system destroying insulin-producing cells in the pancreas. There are a number of causes that contribute to the development of type 2 diabetes. A very common risk factor for type 2 diabetes is a family history of the condition, but other risk factors include obesity, poor diet, a sedentary lifestyle, and increased age.

There are three types of blood test used to diagnose diabetes. Fasting and random blood tests are used to measure the blood sugar levels at a particular point in time.

The A1c test provides the average blood glucose levels over two to three months but does not include a period of fasting before the test. There is also an oral glucose tolerance test that is carried out after an eight-hour fasting period, which is commonly used to diagnose gestational diabetes.

Now, a new study carried out by lead researcher Maria Mercedes Chang Villacreses, M.D., of City of Hope’s Diabetes and Metabolism Research Institute in Duarte, California, has found that the A1c test missed the diabetes diagnosis in 73 percent of the 9,000 people included in her study.

Participants in the study were all over the age of 20 and did not have a diabetes diagnosis. They were given an A1c test and an oral glucose tolerance test so the researchers could then compare the results.

Chang Villacreses found that the “A1c test said these people had normal glucose levels when they didn’t.”

Since the incorporation of A1c in 2010, it’s been relied on more and more for its convenience because patients don’t have to fast. But it’s not a perfect test.”

Maria Mercedes Chang Villacreses, First Author

The A1c test is known to produce differing results in black and Hispanic communities, and anemia and other blood disorders can also affect the results produced by the A1c test.

However, researchers of this new study have not identified the specific reason for the stark difference in diagnostic results between the different tests.

If you have a patient where you have risk factors for diabetes or you have a high suspicion of diabetes, you should go ahead and do the [glucose tolerance test]. If you rely only on A1c, you may miss an opportunity to intervene earlier.”

Maria Mercedes Chang Villacreses, First Author

She also stated that while the oral glucose test is more “cumbersome” to carry out, it does give “more of an idea of the physiology of the patient.”

The findings of the study were presented at ENDO 2019, the Endocrine Society’s annual meeting, and Chang Villacreses explained that the “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.”

She advises that: “Based on our findings, A1c should not be solely used to determine the prevalence of diabetes,” and, “It should be used in conjunction with the oral glucose test for increased accuracy.”

However, the American Diabetes Association believes that the fasting blood sugar test, A1c and the oral glucose tolerance test (OGTT) are “appropriate for diagnostic testing.”

Dr. William Cefalu, the chief scientific, medical, and mission officer of the American Diabetes Association elaborated.

“While the issue of using the A1c test vs. OGTT as the primary diagnostic tool for type 2 diabetes has also been the subject of professional debate for a number of years, this retrospective analysis does not change the American Diabetes Association’s recommendations and confirms multiple prior observations and studies.”

Additionally, Dr. Joel Zonszein, Director of the Clinical Diabetes Center at Montefiore Medical Center in New York City said, “A1c has been approved for the diagnosis and management of diabetes. It’s a very practical test, but it’s not perfect. A random A1c test gives us enough information to diagnose diabetes. It’s a good tool.”

He also said that no test for diabetes is perfect, emphasizing that it isn’t about which test is better than the other. He explained that each test gives medical professionals “different information”.


The results of the study were presented at ENDO 2019: A1c test misses many cases of diabetes.

Lois Zoppi

Written by

Lois Zoppi

Lois is a freelance copywriter based in the UK. She graduated from the University of Sussex with a BA in Media Practice, having specialized in screenwriting. She maintains a focus on anxiety disorders and depression and aims to explore other areas of mental health including dissociative disorders such as maladaptive daydreaming.


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