Hemolytic Anemias

Intrinsic (usually inherited, Coombs-negative)

• Membrane defects: hereditary spherocytosis (ankyrin, spectrin, band 3, protein 4.2 mutations; autosomal dominant, Northern European ancestry) and hereditary elliptocytosis — loss of vertical membrane–cytoskeleton anchoring causes membrane vesiculation and splenic trapping
• Enzymopathies: G6PD deficiency (X-linked recessive; hemizygous males of African, Mediterranean, or Southeast Asian ancestry) fails to regenerate NADPH/glutathione under oxidant stress; pyruvate kinase deficiency (autosomal recessive) depletes ATP and stiffens the membrane
• Hemoglobinopathies: sickle cell disease and unstable hemoglobins; HbS polymerizes when deoxygenated, producing both hemolysis and vaso-occlusion
• PNH: acquired PIGA mutation in a hematopoietic stem cell → loss of GPI-anchored CD55/CD59 → complement-mediated intravascular lysis

Extrinsic (acquired)

• Immune: warm AIHA (IgG; idiopathic, SLE, CLL/lymphoma, methyldopa, cephalosporins, beta-lactams), cold agglutinin disease (IgM; Mycoplasma pneumoniae, EBV, Waldenström), paroxysmal cold hemoglobinuria (Donath–Landsteiner biphasic IgG, post-viral in children), and alloimmune hemolysis (ABO/Rh incompatibility, hemolytic disease of the fetus and newborn)
• Microangiopathic: TTP (ADAMTS13 deficiency), Shiga toxin–mediated and complement-mediated HUS, DIC, malignant hypertension, HELLP, and mechanical shear from a prosthetic valve or LVAD
• Infectious/toxic: malaria, babesiosis, Clostridium perfringens alpha-toxin, snake venom, copper (Wilson disease)
• Hypersplenism from portal hypertension

• Increased RBC destruction exceeding marrow replacement: RBCs are destroyed by splenic macrophages (extravascular), complement-mediated lysis (intravascular), or mechanical trauma; the reticulocyte count rises appropriately but cannot match the rate of destruction, resulting in net anemia

• Intravascular hemolysis mechanisms: Direct lysis of RBCs in circulation releases free hemoglobin into plasma, which binds haptoglobin; when haptoglobin is saturated, hemoglobin is oxidized to methemoglobin, filtered by kidneys (hemoglobinuria), and can cause acute kidney injury via tubular precipitation; also releases LDH and indirect bilirubin into circulation

• Extravascular hemolysis mechanisms: Opsonized RBCs (antibody or complement-coated) are recognized by splenic macrophages via Fc and complement receptors; RBCs are phagocytosed in the spleen (or liver in severe cases), and hemoglobin is catabolized to unconjugated bilirubin and iron; accounts for ~80% of hemolytic anemias

• Bone marrow response: Inappropriate erythropoietin (EPO) drive stimulates erythropoiesis with reticulocytosis and marrow hyperplasia; ineffective response suggests bone marrow disease superimposed on hemolysis (e.g., SLE with bone marrow involvement)

• Pathogenic RBC defects: Membrane abnormalities (spherocytosis, elliptocytosis), enzymatic deficiencies (G6PD, pyruvate kinase), abnormal hemoglobin (sickle cell, thalassemia), or immune-mediated destruction (warm/cold AIHA, drug-induced) all increase RBC fragility or targeting

• Constitutional symptoms: Fatigue, dyspnea, syncope, and palpitations from anemia; severity correlates with hemoglobin level and acuity of onset; chronic hemolytic anemias may present with mild symptoms despite significant anemia due to compensatory mechanisms

• Jaundice and dark urine: Unconjugated hyperbilirubinemia from extravascular hemolysis produces clinical jaundice (usually >3 mg/dL); dark urine (cola-colored or tea-colored) suggests intravascular hemolysis with hemoglobinuria or myoglobinuria; elevated indirect bilirubin with normal conjugated bilirubin is the classic pattern

• Splenomegaly: Marked splenomegaly occurs in chronic hemolytic anemias from extramedullary hematopoiesis and sequestration of RBCs; hepatomegaly may also occur; left upper quadrant pain may develop from splenic infarction

• Acute hemolytic crisis: Fever, severe back/flank pain, hemoglobinuria, and rapid drop in hemoglobin; may be triggered by infection (viral, bacterial), transfusion incompatibility, medication exposure, or oxidative stress in G6PD deficiency

• Pallor, tachycardia, and tachypnea: Signs of anemia and compensatory increases in cardiac output; may see high-output cardiac failure in severe chronic hemolysis

• Cholelithiasis: Unconjugated bilirubin increases risk of pigmented gallstones; may present with biliary colic or acute cholecystitis

• Classic presentations by type: Warm AIHA (elderly, SLE), cold agglutinin disease (Raynaud's phenomena, acrocyanosis in cold), hereditary spherocytosis (family history, neonatal jaundice), G6PD (hemolytic crisis with fava beans/sulfonamides), sickle cell (dactylitis, acute chest, vaso-occlusive pain)

• CBC and reticulocyte count: Hemoglobin <11.5 g/dL with elevated reticulocyte count (>2%, ideally corrected reticulocyte count >2%) is hallmark of hemolysis; spherocytes or schistocytes may appear on blood smear; MCV variable (macro in reticulocytosis, micro in thalassemia)

• Indirect hyperbilirubinemia with elevated LDH and low haptoglobin: Unconjugated (indirect) bilirubin >3 mg/dL, LDH >200 IU/L (often >500), and haptoglobin <25 mg/dL is the "classic triad" of hemolysis; haptoglobin is most specific but least sensitive; LDH elevation exceeding bilirubin suggests intravascular hemolysis with myoglobinuria or hepatic injury

• Direct antiglobulin test (Coombs test): IgG or C3 antibodies binding RBC surface detected by direct Coombs; positive in immune hemolysis (warm AIHA, cold agglutinin disease, drug-induced); negative Coombs narrows differential to hereditary membrane defects, enzymopathies, or intravascular hemolysis (microangiopathic, hypersplenism)

• Peripheral blood smear interpretation: Spherocytes (round, dense) in hereditary spherocytosis and warm AIHA; schistocytes (helmet cells) in mechanical hemolysis (TTP, HUS, DIC); bite cells and blister cells in G6PD deficiency (oxidative damage); polychromasia (blue-staining immature RBCs) reflects reticulocytosis; cold agglutinin disease shows RBC agglutination in clumps

• Intravascular vs. extravascular hemolysis differentiation:
• Intravascular markers: Hemoglobinuria (dark urine, positive dipstick without RBCs), elevated free plasma hemoglobin (>5 mg/dL), very low haptoglobin, elevated LDH disproportionate to bilirubin, schistocytes on smear
• Extravascular markers: Jaundice, indirect hyperbilirubinemia, splenomegaly, spherocytes on smear, positive Coombs

• Specialized testing by category:
• Immune hemolysis: Flow cytometry for IgG/C3, direct Coombs (warm), cold agglutinin titer, direct antiglobulin test with polyspecific reagent
• Membranopathy: Osmotic fragility test (hereditary spherocytosis), eosin-5-maleimide binding, spectrin quantitation
• Enzymopathy: G6PD level (false negatives during acute hemolysis from reticulocytosis), pyruvate kinase level, sugar water test (alternative for PNH)
• Hemoglobinopathy: Hemoglobin electrophoresis, HPLC, genetic testing for sickle cell, thalassemia
• PNH: Flow cytometry for CD55/CD59 deficiency on RBCs and neutrophils (HAM test and sucrose lysis test are outdated)

• Key diagnostic pearl: A positive reticulocyte count with elevated LDH/low haptoglobin/indirect hyperbilirubinemia confirms hemolysis; negative Coombs rules out immune causes; peripheral smear morphology guides further classification

• Supportive care and transfusion: Folic acid 1 mg daily supplementation (bone marrow increased demand); avoid triggers (cold exposure in cold agglutinin disease, oxidative stress in G6PD); transfusion only for symptomatic anemia (Hgb <7 g/dL or high-output cardiac failure) as it can worsen hemolysis in immune-mediated disease via alloimmunization; ensure transfused RBCs are antigen

Emergencies — recognize immediately

• Thrombotic thrombocytopenic purpura: ADAMTS13 deficiency → VWF multimer–driven platelet microthrombi; schistocytes plus thrombocytopenia with normal PT/PTT. Untreated mortality is very high; per ISTH 2020 guidance start therapeutic plasma exchange plus corticosteroids on clinical suspicion, adding caplacizumab and rituximab, without waiting for the ADAMTS13 result
• Aplastic crisis: parvovirus B19 infects erythroid progenitors; signaled by a sudden hemoglobin drop with an inappropriately low reticulocyte count in a patient with chronic hemolysis
• Splenic sequestration crisis: rapidly enlarging spleen, falling hemoglobin, and hypovolemic shock in young children with sickle cell disease
• Acute chest syndrome and stroke in sickle cell disease: new infiltrate with fever/hypoxemia, or focal deficit; NHLBI and ASH guidance supports urgent exchange transfusion for severe disease
• Overwhelming post-splenectomy infection: fulminant sepsis with encapsulated organisms (S. pneumoniae, H. influenzae type b, N. meningitidis); ACIP recommends pneumococcal, meningococcal, and Hib vaccination before elective splenectomy
• Acute hemolytic transfusion reaction: fever, flank pain, hemoglobinuria — stop the transfusion, support the circulation, notify the blood bank

Chronic disease complications

• Pigment gallstones and cholecystitis: chronic unconjugated bilirubin load
• Iron overload: from repeated transfusion — cardiomyopathy, cirrhosis, endocrinopathy; monitored by ferritin and hepatic/cardiac MRI
• Pulmonary hypertension and leg ulcers: free plasma hemoglobin scavenges nitric oxide in chronic intravascular hemolysis; an elevated tricuspid regurgitant jet velocity flags it
• Thrombosis in PNH, classically hepatic vein (Budd–Chiari) or cerebral sinus
• Folate depletion from sustained erythropoiesis; kernicterus in neonates with G6PD deficiency or ABO incompatibility

Treatment-related

• Corticosteroids: hyperglycemia, osteoporosis, infection
• Rituximab: hepatitis B reactivation (screen before use), hypogammaglobulinemia, rare PML
• Eculizumab/ravulizumab: terminal complement blockade causes meningococcal disease — vaccination plus prophylaxis is required under the FDA REMS
• Hydroxyurea: myelosuppression requiring serial counts

• **Bite cells and *Heinz bodies*** (seen only on supravital stain such as crystal violet) after dapsone, primaquine, nitrofurantoin, or fava beans = G6PD deficiency. The trap: a G6PD assay drawn during the acute crisis can be falsely normal because surviving young RBCs and reticulocytes are enzyme-replete — repeat testing several weeks to months later
• Spherocytes are not diagnostic of hereditary spherocytosis. Warm AIHA also produces spherocytes; the single best next step is the direct antiglobulin (Coombs) test. Coombs-positive → immune; Coombs-negative with increased MCHC and a family history → hereditary spherocytosis, confirmed by eosin-5-maleimide flow cytometry
• Schistocytes + thrombocytopenia + normal coagulation studies = thrombotic microangiopathy. Start plasma exchange for suspected TTP before ADAMTS13 returns (ISTH 2020). The classic distractor is platelet transfusion, which is withheld absent life-threatening bleeding
• Warm vs cold: warm AIHA is IgG-mediated, extravascular in the spleen, and steroid-responsive; cold agglutinin disease is IgM/C3-mediated, driven by Mycoplasma or EBV, and responds poorly to steroids — cold avoidance and rituximab-based therapy are the answer
• Hemoglobinuria on waking plus unexplained thrombosis in an atypical site in a Coombs-negative patient = PNH; diagnose with flow cytometry for CD55/CD59, not with the obsolete sucrose lysis or Ham test
• Sudden anemia with a low reticulocyte count in chronic hemolysis is a parvovirus B19 aplastic crisis, not a hemolytic exacerbation — reticulocytes distinguish the two instantly
• Sickle cell one-liners: dactylitis in an infant, Howell–Jolly bodies signaling functional asplenia, and penicillin prophylaxis until age five per NHLBI/ASH guidance; hydroxyurea works by raising fetal hemoglobin
• Never attribute low haptoglobin alone to hemolysis — it is sensitive but also falls in liver disease; pair it with a high LDH, high indirect bilirubin, and reticulocytosis