(A) The Ham test incubates PNH red cells with acidified serum, which lyses cells that lack glycosylphosphatidylinositol (GPI)-linked complement inhibitors such as CD55 and CD59. (B) The indirect Ham test incubates control or aHUS serum with TF-1 cells that lack these complement regulators. Complement-induced cell death prevents the conversion of WST-1 into formazan, a red dye. RBC, red blood cell. Professional illustration by Ken Probst, XavierStudio.