Guide. A sickle cell test is a blood test done to check for sickle cell trait or sickle cell disease. Sickle cell disease is an inherited blood disease that causes red blood cells to be deformed (sickle-shaped). These people have normal red blood cells, unless they have some other disease. 2  One gene that makes hemoglobin A and one that makes hemoglobin S. These people carry the sickle cell trait (and are called a carrier ), but they do not have sickle cell disease. 3  Sickle cell trait is usually a harmless condition.