Hemoglobin is the protein in red blood cells that makes them red. Its principal function is to carry oxygen from the lungs to other parts of the body. When hemoglobin comes into contact with sugar, they combine to form hemoglobin A1c. The more sugar in the blood, the higher the hemoglobin A1c level. Normal values range from 4.0% to 5.9%. Clinicians use hemoglobin A1c levels to estimate the average blood sugar during the previous 4 weeks to 3 months. Because patients with untreated diabetes have elevated levels of blood sugar, clinicians can use the hemoglobin A1c level to determine whether a patient has diabetes and whether patients with diabetes are being treated adequately. Although black persons have higher hemoglobin A1c levels than white persons, most observers thought that was because blood sugar levels were higher in black persons than in white persons. Recently, 3 studies have found that in patients with diabetes or a condition that leads to diabetes, black persons have higher hemoglobin A1c levels than white persons with the same blood sugar levels.