Glycated hemoglobin (HbA1c) and blood glucose are two indicators commonly measured in blood tests. Both are related to glucose (sugar) levels in the blood, but they provide different, although complementary, information: Blood glucose refers to the concentration of sugar (glucose) in the blood. A blood glucose test reflects the glucose level at the exact moment the blood sample is taken. The result is expressed as the amount of glucose per volume of blood, for example, in mg/dL. Glycated hemoglobin (HbA1c) is the proportion of hemoglobin that is bound to glucose at the time the blood sample is collected. It reflects the exposure of red blood cells to glucose over the previous 2 to 3 months rather than at that specific moment. The result is expressed as a percentage. How does glycated hemoglobin provide information about previous months? Hemoglobin is a protein found only in red blood cells. Some of the glucose present in the bloodstream binds spontaneously to hemoglobin. The binding of glucose to hemoglobin is called glycation and is irreversible. Once the bond is formed, it remains for the rest of the red blood cell lifespan. Glycated hemoglobin (or glycosylated hemoglobin) is hemoglobin that has undergone glycation. The more glucose there is in the blood, the more glucose binds to hemoglobin. Because red blood cells have a lifespan of approximately 120 days, a higher proportion of glycated hemoglobin indicates that blood glucose levels have been higher over the previous months. This period is usually considered to be the last 2 to 3 months. Is it possible to estimate blood glucose levels over the previous 2 to 3 months from glycated hemoglobin? Yes, it is. The estimated average glucose can be calculated from the HbA1c value. However, estimated average glucose should be interpreted differently from fasting blood glucose because the reference values are not the same. For this reason, HbA1c reference ranges are generally more useful than estimated average glucose values when assessing whether blood glucose levels have been elevated during the previous months. Are these two tests only requested when diabetes is suspected? No. They may be requested in a variety of situations. Helping diagnose diabetes and assessing blood glucose control in people with diabetes are common reasons for ordering these tests. Other situations include: Investigation of symptoms that may be related to glucose metabolism; Assessment of diabetes risk in people who are overweight, have high blood pressure, high cholesterol, or a family history of diabetes; Evaluation of changes in glucose metabolism during pregnancy (gestational diabetes is diagnosed using specific tests, such as the oral glucose tolerance test). Important The results of blood glucose and glycated hemoglobin tests, like all laboratory tests, should always be interpreted by a doctor , taking into account the person's individual circumstances and medical history. Several conditions can affect the results . For example, disorders involving red blood cells or hemoglobin, bleeding, blood transfusions, and other situations may falsely increase or decrease glycated hemoglobin levels. Is fasting required for blood glucose and glycated hemoglobin tests? For diagnostic purposes or other medical evaluations, blood glucose testing is most commonly performed after fasting. After eating, even a small meal, blood sugar levels rise, which can affect the interpretation of the results. Ideally, a fasting blood glucose test should be performed after 8 to 12 hours of fasting. Glycated hemoglobin testing does not require fasting because it does not measure the glucose concentration at the time of blood collection, and a meal consumed shortly beforehand does not affect the amount of glucose already bound to hemoglobin. However, there are few situations in which a doctor requests HbA1c alone. It is usually ordered together with other blood tests that do require fasting, and for that reason it is often performed while fasting as well. Author : Betânia Ferreira The content published in the Health Guide is intended for the general public and is provided for informational and educational purposes only. It does not replace individual medical assessment, diagnosis, or personalized clinical advice from a qualified healthcare professional. The content is produced or reviewed by physicians and other healthcare professionals at Hospital da Luz and is based on the knowledge available at the time of publication or the most recent update, as indicated at the beginning of each item. English translation generated with AI and reviewed by the editorial team.
