The metabisulfite test is used to detect the presence of hemoglobin S, but it does not differentiate the heterozygous sickle cell trait from the homozygous sickle cell disease. The test is based on the fact that erythrocytes with a large propoion of hemoglobin S sickle in solutions of low oxygen content. Metabisulfite is a reducing substance that enhances the process of deoxygenation. The osmotic fragility test is a diagnostic test for hereditary spherocytosis. Spherocytes lyse at a higher concentration of salt than do normal cells, thus causing an increased osmotic fragility. The direct antiglobulin test (DAT), or Coombs' test, is used to differentiate autoimmune hemolytic anemia (AIHA) due to the presence of anti-red cell antibodies from other forms of hemolytic anemia. In this test, antibodies to human immunoglobulin cause the agglutination (clotting) of red cells if these anti-red cell antibodies are present on the surface of the red cells. In patients with paroxysmal nocturnal hemoglobinuria, the erythrocytes are excessively sensitive to complement-mediated lysis in low ionic environments (the basis for the sucrose hemolysis test) or in acidotic conditions, such as sleep, exercise, or the Ham's acid hemolysis test. The Schilling test, which measures intestinal absorption of vitamin B12 with and without intrinsic factor, is used to diagnose decreased vitamin B12 caused by pernicious anemia, which is characterized by a lack of intrinsic factor. Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition.