What Are the A1C and GTT Tests?


A study recently published in the International Journal of Obesity found that  HbA1c is more sensitive than OGTT for identifying prediabetes 

HbA1c and oral glucose tolerance tests (OGTT) used to identify prediabetes yielded conflicting results in both Hispanic and non-Hispanic white populations in New Mexico, study data show.

“Diagnostic tests for prediabetes will often provide different answers depending upon which test is ordered,” Mark R. Burge, MD, professor of medicine, endocrinology, and metabolism at the University of New Mexico Health Sciences Center in Albuquerque, told Endocrine Today. “HbA1c appears to provide a more sensitive test for prediabetes in a population heavily enriched in Hispanic individuals.”

The HbA1c test is used to evaluate glucose levels in the blood over the 2 to 3 months previous to the test. This test is very important in managing one’s diabetes. Scientists believe that keeping the blood sugar in the body within normal range can help individuals with diabetes to avoid many of the risks and side effects that people with diabetes often face. The benefit of the hemoglobin A1c blood test is that is provides information on overall glycemic health over a several month period. Other blood tests of glucose level are highly sensitive to determining glucose levels at the time the test is taken, but they do not give information on average glucose blood levels.

The test works by measuring the hemoglobin A1c level. Hemoglobin is stored in the red blood cells. When glucose levels are high, the sugar starts to combine with the hemoglobin. It takes the body 8 to 12 weeks to bring hemoglobin A1c levels back to normal. Therefore, if hemoglobin A1c levels are high, that means that there has been a high level of glucose in the blood over the last 2 to 3 months. 

A1c is measured primarily to determine the three-month average blood sugar level and can be used as a diagnostic test for diabetes mellitus and as an assessment test for glycemic control in people with diabetes. The test is limited to a three-month average because the average lifespan of a red blood cell is four months. Since individual red blood cells have varying lifespans, the test is used as a limited measure of three months. Normal levels of glucose produce a normal amount of glycated hemoglobin. As the average amount of plasma glucose increases, the fraction of glycated hemoglobin increases in a predictable way. In diabetes, higher amounts of glycated hemoglobin, indicating poorer control of blood glucose levels, have been associated with cardiovascular disease, nephropathy, neuropathy, and retinopathy.

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Thienel M, et al. Int J Obes. 2016;doi:10.1038/ijo.2016.149