Haptoglobin is the plasma protein responsible for binding free hemoglobin during episodes of hemolysis. Because of its role, haptoglobin would normally demonstrate decreased levels during a hemolytic crisis since free hemoglobin is spilled into the bloodstream from lysed red blood cells.
The normal level of haptoglobin is 40-330mg/dL. Individuals who are in hemolytic crisis demonstrate greatly reduced levels to a complete absence of haptoglobin.
In alpha thalassemia, however, haptoglobin levels remain normal or only slightly decreased, even during hemolytic events.
The reason for this is that haptoglobin functions by binding the alpha chain portion of hemoglobin. With the absence of these chains in alpha thalassemia major and intermedia, haptoglobin cannot bind free hemoglobin. Therefore it is not consumed as it would be in other types of hemolytic anemia.