Hemoglobin A1c (HbA1c) testing refers to laboratory procedures to detect and measure the presence of HbA1c in red blood cells. HbA1c forms in RBC when glucose combines with hemoglobin. HbA1c testing uses laboratory devices and reagents, chemicals that form known reactions with HbA1c present in RBCs. Hemoglobin A1c testing is performed to screen patients and to assess the adequacy of treatment in patients with diabetes.