A hemoglobin test reveals how much hemoglobin is in a person's blood, helping to diagnose and monitor anemia and polycythemia vera. Hemoglobin is a protein inside red blood cells that carries oxygen throughout the body. A hemoglobin test is done when a person is ill or during a general physical examination. Good health requires an adequate amount of hemoglobin. The amount of oxygen in the body tissues depends on how much hemoglobin is in the red cells. Without enough hemoglobin, the tissues lack oxygen and the heart and lungs must work harder to try to compensate. If the test indicates a "less than" or "greater than" normal amount of hemoglobin, the cause of the decrease or increase must be discovered. A low hemoglobin usually means the person has anemia. Anemia results from conditions that decrease the number or size of red cells, such as excessive bleeding, a dietary deficiency, destruction of cells because of a transfusion reaction or mechanical heart valve, or an abnormally formed hemoglobin.A high hemoglobin may be caused by polycythemia vera, a disease in which too many red blood cells are made.