Red blood cell hemoglobin is used to transport oxygen from the lungs to the tissues. Normal adult hemoglobin contains 95% to 98% hemoglobin A (two alpha chains and two beta chains). It also contains 2% to 3% Hb A2 (normal variant with two alpha chains and two delta chains) and 1% to 2% fetal hemoglobin consisting of two alpha and two gamma chains. Blood also contains traces of minor hemoglobins (Hb A1, etc.). Hb A levels are reduced in many conditions, including thalassemia (failure to produce alpha or beta chains). In these cases, minor Hb forms such as Hb A1, Hb A2 and Hb F, or abnormal forms such as Hb H, replace normal Hb A, though not as effectively.
Abnormally low levels of Hb A and abnormally high levels of Hb A1, Hb A2 or Hb F are correlated with the presence of abnormal forms of hemoglobin (alpha or beta thalassemia, etc.).