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The hemoglobin A1c test—also known as glycated hemoglobin, glycosylated hemoglobin, HbA1c, or simply A1c—is used to measure an individual's glucose control levels. The test shows average blood sugar levels over the past 90 days, expressed as a percentage. In addition, it can be used to diagnose diabetes mellitus.[1] Hemoglobin is a protein found exclusively in red blood cells, giving blood its bright red color. The primary role of hemoglobin is to carry oxygen from the lungs to all the cells in the body. Hemoglobin becomes glycated or coated with glucose from the bloodstream. As blood glucose levels increase, more glucose attaches to the hemoglobin protein, resulting in a higher A1c value.[2] Since red blood cells have an average lifespan of about 3 months, the A1c test measures hemoglobin levels in the bloodstream over this period, making it a reliable indicator of blood sugar control.

Regular A1c testing is essential for individuals with diabetes mellitus to ensure their average blood glucose levels are within the target range. The American Diabetes Association (ADA) recommends that individuals with diabetes mellitus who have stable blood sugar control and are meeting treatment goals should have their HbA1c (A1C) checked at least twice a year.[2] For individuals whose therapy has changed or who are not meeting glycemic targets, testing should be performed every 3 months. This approach helps optimize diabetes control and reduce the risk of complications.[7]

All clinicians caring for patients with diabetes mellitus need a thorough understanding of HbA1c and its clinical significance. In general, HbA1c reflects the average glucose concentration over 3 months. HbA1c is often used as an outcome measure to determine whether an intervention in a population is successful by demonstrating a decrease in HbA1c by a specific percentage. There is a movement within the medical community to shift away from using HbA1c as an exclusive standard of care test to assess patient response to treatment. The newest proposed methods include the estimated average glucose and the glucose time in range.[43] These methods use data obtained from continuous glucose monitors that record blood glucose levels around the clock. In addition, these methods can offer healthcare professionals a more accurate picture of the blood sugar average and fluctuations. However, these methods are not available to all patients on a widespread basis. According to ADA guidelines, HbA1c levels should be measured 2 times annually in stable patients and at least 4 times in patients with glucose fluctuations or those who have changed their diabetes treatment. HbA1c is one of the preferred diabetes diagnostic tests today. The blood draw can occur anytime, and no special handling requirements exist. However, to ensure that the A1c value is correct, clinicians must be aware of the causes of false-positive and false-negative results. As many patients with diabetes mellitus are treated in outpatient settings, diabetes care nurses must understand HbA1c results and know when to refer patients to an endocrinologist for further evaluation. Pharmacists must also fully understand and interpret this test, as they are involved in glycemic management, medication, agent selection, dosing, and monitoring. The nurse and pharmacist must inform the treating clinician regarding any changes in HbA1c and verify patient medication compliance. HbA1c is a valuable tool for managing diabetes mellitus and other glycemic control disorders, but its effectiveness is maximized within an interprofessional healthcare team environment.