Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (315 page)

HEMOLYTIC DISEASE OF THE NEWBORN

   Definition

Hemolysis occurs when fetal RBCs cross the placenta and the mother is immunized with a fetal RBC antigen that is not present on her RBCs. Some women may be immunized with more than one type of RBC antigen. The resulting immune response triggers the production of IgG antibodies that are then transferred to the fetus and cause hemolysis of the fetal RBCs. The most frequent cases are due to immunization against the D antigen of the Rh blood group. Next most common are due to immunization against the Kell antigen.

   Laboratory Findings

   Laboratory findings are those of hemolysis in the newborn.

   After birth, the by-products of RBC destruction occur, especially increased unconjugated bilirubin, with attended complications (bilirubin encephalopathy and kernicterus).

MECHANICAL HEMOLYSIS

   Definition

Physical trauma to RBCs damages them, resulting in RBC fragmentation and intravascular hemolysis. Mechanical hemolytic anemias can be divided into two groups:

   Microangiopathic: endothelial cell injury in small blood vessels due to fibrin strands in vessel lumens, as seen in DIC, TTP, HUS, disseminated malignancy; malignant hypertension; vasculitis; HELLP syndrome; scleroderma insertion of foreign bodies into the circulation; Kasabach-Merritt syndrome (giant hemangioma); chemotherapy; and the “catastrophic” antiphospholipid antibody syndrome.

   Macroangiopathic: RBC injury from malfunctioning valvular prosthesis, severe cardiac valve deformities, or aortic atheromata (Waring blender syndrome).

Mechanical hemolysis may also occur in hypersplenism, march hemoglobinuria (runner’s hemoglobinuria), and freshwater drowning or inadvertent infusion of water.

   Laboratory Findings

   Laboratory diagnosis: directed to the causative disease.

   Anemia: commensurate with the severity of underlying process.

   Peripheral blood smear (PBS): >5 of 500 RBCs are deformed (schistocytes) or helmet cells (a subtype of schistocytes) or are microspherocytes.

   Platelets: varying degrees of thrombocytopenia, occasionally without anemia.