Hemolytic anemias are characterized by reduced RBC survival due to premature destruction, with the bone marrow attempting to compensate by increasing erythropoiesis. The laboratory hallmark is a hyperproliferative anemia with elevated reticulocyte count and evidence of increased RBC breakdown. Hemolytic anemias are classified by site of hemolysis (intravascular vs extravascular) and by the underlying cause (intrinsic RBC defect vs extrinsic factor).
Intravascular vs Extravascular Hemolysis
Intravascular hemolysis occurs within the circulation, releasing hemoglobin directly into plasma. Laboratory findings: hemoglobinemia (pink to red plasma), hemoglobinuria (red-brown urine, positive dipstick without RBCs on microscopy), decreased haptoglobin (consumed by hemoglobin binding), elevated LDH, and methemalbumin formation. Causes include ABO-incompatible transfusion, microangiopathic hemolytic anemia (TTP, HUS, DIC), paroxysmal nocturnal hemoglobinuria (PNH), glucose-6-phosphate dehydrogenase (G6PD) deficiency during oxidative stress, mechanical hemolysis (prosthetic heart valves, ECMO), and severe burns.
Extravascular hemolysis occurs in the reticuloendothelial system (spleen, liver, bone marrow), where macrophages phagocytose antibody-coated or abnormally shaped RBCs. Laboratory findings: indirect (unconjugated) hyperbilirubinemia, elevated LDH, decreased haptoglobin (less pronounced than intravascular), and increased urobilinogen in urine. The peripheral blood smear may show spherocytes. Causes include autoimmune hemolytic anemia (AIHA), hereditary spherocytosis, hemoglobinopathies (sickle cell disease), RBC enzyme deficiencies, and delayed hemolytic transfusion reactions.
Autoimmune Hemolytic Anemia (AIHA)
AIHA is caused by autoantibodies directed against RBC antigens. Warm AIHA (75–80% of cases) is mediated by IgG antibodies (optimal reactivity at 37°C) and commonly caused by drugs (methyldopa, penicillin, cephalosporins, NSAIDs), lymphoproliferative disorders (CLL, lymphoma), and systemic lupus erythematosus, or idiopathic. The direct antiglobulin test (DAT, Coombs test) is positive for IgG alone or IgG + C3. The peripheral blood smear shows spherocytosis and polychromasia (reticulocytes). Treatment includes corticosteroids, rituximab, splenectomy, and immunosuppressive agents. Cold AIHA (15–20%) is mediated by IgM antibodies (optimal reactivity at 4°C), causing RBC agglutination in the cold and complement-mediated hemolysis. It is commonly post-infectious (Mycoplasma pneumoniae, EBV) or associated with lymphoproliferative disorders. DAT is positive for C3 only. The blood smear may show RBC agglutinates. Treatment includes cold avoidance and rituximab. Mixed-type AIHA has both warm IgG and cold IgM components.
Hereditary Spherocytosis
Hereditary spherocytosis (HS) is the most common hereditary hemolytic anemia in Northern Europeans (1:2000), caused by defects in RBC membrane skeleton proteins (spectrin, ankyrin, band 3, protein 4.2). The loss of membrane surface area reduces RBC deformability, leading to splenic trapping and extravascular hemolysis. Laboratory findings: mild to moderate hemolytic anemia (Hb 9–12 g/dL in most), elevated reticulocyte count, spherocytes on peripheral blood smear (small, dense, lacking central pallor), elevated MCHC (> 36 g/dL on automated analyzers), and increased RDW. Diagnosis is confirmed by the incubated osmotic fragility test, which shows increased hemolysis at decreasing saline concentrations. The eosin-5-maleimide (EMA) binding test by flow cytometry measures band 3 deficiency and has largely replaced the osmotic fragility test. Splenectomy is curative for severe HS.
G6PD Deficiency
Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked disorder affecting the pentose phosphate pathway, reducing NADPH production and the RBC’s ability to handle oxidative stress. It is the most common RBC enzyme deficiency (affecting 400 million people worldwide), with higher prevalence in Africa, the Mediterranean, and Southeast Asia. Hemolysis is episodic, triggered by oxidative stress: certain drugs (sulfonamides, dapsone, nitrofurantoin, primaquine, aspirin in high doses), fava beans (broad beans, hence the historical name favism), and infections. Laboratory findings during an acute hemolytic episode: hemoglobinuria, elevated LDH, decreased haptoglobin, and elevated bilirubin. The peripheral blood smear shows bite cells (blister cells), Heinz bodies (denatured hemoglobin, visible with supravital stain), and hemighosts. Between episodes, the CBC and smear are normal. Diagnosis is confirmed by quantitative G6PD enzyme assay, which may be falsely normal during acute hemolysis (young RBCs have higher enzyme activity) and should be repeated after recovery.
Paroxysmal Nocturnal Hemoglobinuria (PNH)
PNH is an acquired clonal stem cell disorder caused by a PIGA mutation, leading to deficiency of glycosylphosphatidylinositol (GPI)-anchored complement regulatory proteins (CD55, CD59) on RBCs. This renders RBCs susceptible to complement-mediated intravascular hemolysis. Laboratory findings: evidence of intravascular hemolysis (hemoglobinuria, low haptoglobin, elevated LDH), negative DAT, and iron deficiency from chronic hemoglobinuria. Venous thrombosis (especially hepatic, mesenteric, cerebral) is a major complication. Pancytopenia may be present (PNH often occurs in the setting of aplastic anemia). Diagnosis is confirmed by flow cytometry detection of CD55/CD59-deficient RBCs and WBCs (high-sensitivity, GPI-anchor detection using FLAER reagent).
Microangiopathic Hemolytic Anemias (MAHA)
MAHA results from RBC fragmentation in the microcirculation due to endothelial injury and microthrombi. The peripheral blood smear shows schistocytes (RBC fragments, helmet cells, triangular cells) and thrombocytopenia. Thrombotic thrombocytopenic purpura (TTP) is caused by ADAMTS13 deficiency (autoantibody), presenting with MAHA, severe thrombocytopenia, fever, neurologic symptoms, and renal impairment. Treatment: plasma exchange. Hemolytic uremic syndrome (HUS) is triggered by Shiga toxin-producing E. coli (STEC, O157:H7), presenting with MAHA, thrombocytopenia, and acute kidney injury. Treatment is supportive. DIC causes MAHA with abnormal coagulation parameters.
