A 49-year-old female presents with signs of anemia and states that every morning her urine is dark. Workup reveals that her red blood cells lyse in vitro with acid (positive Ham's test). What is the best diagnosis for this patient?

Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal stem cell disorder that is characterized by abnormal red cells, granulocytes, and platelets. The red blood cells (RBCs) are abnormally sensitive to the lytic activity of complement due to a deficiency of glycosyl phosphatidyl inositol (GPI) linked proteins, namely decay-accelerating factor (DAF, or CD 55), membrane inhibitor of reactive lysis (CD55), or CD59 (a C8 binding protein). Complement is normally activated by acidotic states, such as occur with exercise or sleep. In patients with PNH, the acidotic condition that develops during sleep (which is usually at night) causes hemolysis of red blood cells and results in red urine in the morning. The erythrocytes of these patients lyse in vitro with acid (Ham's test) or sucrose (sucrose lysis test). Complications of PNH include the development of frequent thromboses, paicularly of the hepatic, poal, or cerebral veins. Since PNH is a clonal stem cell disorder, patients are at an increased risk of developing aplastic anemia or acute leukemia. The autoimmune hemolytic anemias are impoant causes of acute anemia in a wide variety of clinical states and can be separated into two main types: those secondary to "warm" antibodies and those reactive at cold temperatures. Warm-antibody autoimmune hemolytic anemias react at 37degC in vitro, are composed of IgG, and do not fix complement. They are found in patients with malignant tumors, especially leukemia-lymphoma; with use of such drugs as a-methyldopa; and in the autoimmune diseases, especially lupus erythematosus. Cold-antibody autoimmune hemolytic anemia reacts at 4 to 6degC, fixes complement, is of the IgM type, and is classically associated with Mycoplasma pneumonitis (pleuropneumonia-like organisms). These antibodies are termed cold agglutinins and may reach extremely high titers and cause intravascular red cell agglutination. Reference: Robbins & Cotran Pathologic Basis of Disease, 9edition.