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Acquired hemolytic anemias are a group of disorders characterized by hemolysis of red blood cells (RBCs) not due to congenital or inherited disorders of hemoglobin synthesis or of the RBC membrane. Hemolysis of RBCs can take place within the intravascular space or in the extravascular spaces of the spleen and liver and can produce a spectrum of disease from mild, asymptomatic illness to severe hemodynamic compromise leading to critical ED encounters.
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Presenting symptoms and signs of hemolytic anemia include those common to anemia in general: weakness, fatigue, dizziness, shortness of breath, dyspnea on exertion, tachycardia, palpitations, chest pain, new or accentuated cardiac murmur, and pallor. RBC destruction generates free hemoglobin that is then broken down into bilirubin. When bilirubin production exceeds the liver's ability to conjugate it for biliary and fecal excretion, jaundice and darkened urine may develop. Splenic enlargement may promote the storage and extravascular breakdown of RBCs.
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The laboratory findings characteristic of acquired hemolytic anemia demonstrate hemolysis of RBCs, hemoglobin breakdown, and compensatory RBC production (Table 237-1). The peripheral blood smear displays abnormal RBC morphology consistent with hemolysis: schistocytes generated by intravascular shearing of RBCs and spherocytes produced by extravascular phagocytosis of RBCs within the liver and spleen.
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Intravascular hemolysis of RBCs releases hemoglobin into the bloodstream that then binds to haptoglobin and other serum proteins. The hemoglobin–haptoglobin complex travels to the liver for processing, thus decreasing the amount of free haptoglobin in the serum—an important laboratory finding of intravascular hemolysis. Breakdown of RBCs releases lactate dehydrogenase and potassium, leading to elevation of both in serum. With excessive hemoglobin breakdown comes increased bilirubin production that cannot be conjugated by the liver for biliary and fecal excretion. Laboratory findings associated with excess bilirubin production include elevated total bilirubin; elevated indirect or unconjugated bilirubin; and increased urinary urobilinogen, a by-product of bilirubin breakdown formed by the intestine and passed into the urine. Excess free hemoglobin may escape binding by serum haptoglobin as well as reabsorption by the renal tubules, creating hemoglobinuria and darkened urine.
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IMMUNE-MEDIATED ACQUIRED HEMOLYTIC ANEMIA
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Immune-mediated acquired hemolytic anemia encompasses three main categories: ...