Hemoglobin A1 measurement is a measure of the quantity of the metallo protein hemoglobin A. Hemoglobin A (HbA), also known as adult hemoglobin or a2b2, is the most common human hemoglobin tetramer, comprising over 97% of the total red blood cell hemoglobin. It consists of two alpha chains and two beta chains. The trait is Experimental Factor Ontology entry EFO_0007629 (hemoglobin A1 measurement). Also known as: HbA levels, HbA1 levels, hemoglobin A measurement, hemoglobin A1 levels.