Hemoglobin (Hb) is a protein found in red blood cells that carries oxygen from the lungs to the bodys tissues and returns carbon dioxide from the tissues back to the lungs. It is composed of two pairs of dissimilar chains, α and β, each defined by a specific amino acid sequence and incorporating an iron-containing heme group. Two α–β dimers combine to form a hemoglobin tetramer, which allows for the "heme–heme" interaction necessary for effective oxygen uptake (deoxyhemoglobin → oxyhemoglobin) and delivery (oxyhemoglobin → deoxyhemoglobin) . The amount of hemoglobin in whole blood is expressed in grams per deciliter (g/dl), and the normal Hb level for males is 14 to 18 g/dl, while that for females is 12 to 16 g/dl. When the hemoglobin level is low, the patient has anemia, which can be caused by a variety of factors such as iron deficiency, blood loss, or certain medical conditions. The hemoglobin test is a common test and is almost always done as part of a complete blood count (CBC) .