Ham test is used for diagnosis of:

The definitive diagnosis of paroxysmal nocturnal hemoglobinuria (PNH) must be based on the demonstration that a substantial propoion of the patient's red cells have an increased susceptibility to complement (C), due to the deficiency on their surface of proteins (paicularly CD59 and CD55) that normally protect the red cells from activated C. The sucrose hemolysis test is unreliable, and the acidified serum (Ham) test is carried out in few labs. The gold standard today is flow cytometry, which can be carried out on granulocytes as well as on red cells. A bimodal distribution of cells, with a discrete population that is CD59-, CD55-, is diagnostic of PNH. Ref: Harrison's principle of internal medicine 17th edition, chapter 101.