21.2 Haemolytic and Autoimmune Haemolytic Anaemias

3. Haemolytic Anaemias: Extravascular vs Intravascular Breakdown

Haemolysis represents premature destruction of circulating erythrocytes (normal lifespan ~120 days). The hallmark biochemical features common to all haemolytic states include:

1. Elevated serum unconjugated (indirect) bilirubin (from haem catabolism);
2. Elevated serum lactate dehydrogenase (LDH; released from lysed red cell cytoplasm); and
3. Reticulocytosis with polychromasia on peripheral blood film.

Extravascular vs Intravascular Destruction

• Extravascular Haemolysis: Erythrocytes are recognized as abnormal and phagocytosed by macrophages within the reticuloendothelial system, predominantly in the splenic cords and hepatic sinusoids. Clinically characterized by splenomegaly, unconjugated jaundice, and elevated urine urobilinogen. Because hemoglobin is broken down intracellularly, free hemoglobin is NOT released directly into plasma; therefore, serum haptoglobin is normal or only mildly decreased, and there is no haemoglobinuria or haemosiderinuria.
• Intravascular Haemolysis: Erythrocytes undergo direct mechanical shear, complement-mediated pore formation, or toxin-induced lysis within the circulating blood vessels. Free tetrameric oxyhaemoglobin is released directly into the circulation, binding irreversibly to circulating haptoglobin. The Hb-haptoglobin complex is rapidly cleared by hepatic CD163 scavenger receptors, resulting in profoundly depleted or undetectable serum haptoglobin. Once haptoglobin is completely saturated, free dimeric haemoglobin filters across the renal glomeruli into tubular fluid: excess haemoglobin causes haemoglobinuria (dark brown/black "coca-cola" urine; dipstick positive for blood in the absence of intact red cells on microscopy). Renal proximal tubular cells reabsorb and degrade hemoglobin, storing the iron as haemosiderin; sloughed tubular cells cause haemosiderinuria (detected via Prussian blue staining of urine sediment 5–7 days following acute intravascular haemolysis).

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4. Autoimmune Haemolytic Anaemias (AIHA)

Autoimmune haemolytic anaemia is diagnosed by demonstrating autoantibodies and/or complement components deposited on patient red cells via the Direct Antiglobulin Test (DAT / Coombs test).

Warm AIHA vs Cold Agglutinin Disease (CAD)

                    Autoimmune Haemolytic Anaemia (AIHA)
                                     |
     +-------------------------------+-------------------------------+
     |                                                               |
Warm AIHA (80%)                                                 Cold Agglutinin Disease (20%)
- IgG autoantibodies (bind at 37°C)                            - IgM autoantibodies (bind at 4°C)
- Splenic extravascular destruction                             - Intravascular complement-mediated lysis
- Peripheral microspherocytes                                   - Acrocyanosis triggered by cold exposure
- DAT: IgG positive (± C3d)                                     - DAT: C3d positive, IgG negative
- Causes: Idiopathic, SLE, CLL, lymphoma                        - Causes: Mycoplasma, EBV, lymphoma
- First-Line: Oral prednisolone (1 mg/kg)                       - First-Line: Keep warm, Rituximab
- Refractory: Rituximab, splenectomy                            - STEROIDS & SPLENECTOMY INEFFECTIVE!

• Warm AIHA (80% of cases):

  • Immunoglobulin & Mechanism: Mediated by IgG autoantibodies that react maximally at body temperature (37°C). The Fc portion of IgG-coated erythrocytes is recognized by splenic macrophage Fc-gamma receptors, which partially phagocytose the red cell membrane, transforming them into rigid microspherocytes that undergo extravascular splenic destruction.
  • DAT Profile: Strongly positive for IgG (and occasionally C3d).
  • Aetiology: Idiopathic (50%), systemic lupus erythematosus (SLE), chronic lymphocytic leukaemia (CLL), non-Hodgkin lymphoma, and drugs (methyldopa, high-dose penicillin, fludarabine).
  • Management: First-line therapy is high-dose oral prednisolone (1 mg/kg/day), achieving remission in 70–80%. Second-line therapy for corticosteroid-dependent or refractory disease is rituximab (anti-CD20 monoclonal antibody) or laparoscopic splenectomy.

• Cold Agglutinin Disease (CAD / Cold AIHA, 15–20% of cases):

  • Immunoglobulin & Mechanism: Mediated by IgM autoantibodies that bind to the I/i carbohydrate antigens on red cells at cold temperatures (optimal at 0–4°C in the cool acral microcirculation of the fingers, toes, nose, and ears). IgM pentamers crosslink red cells, producing gross intravascular agglutination and activating the classical complement cascade through C3b deposition. Upon warming to 37°C in the core circulation, IgM rapidly dissociates, leaving C3b fixed to the cell surface, leading to terminal complement-mediated intravascular lysis (MAC C5b-9) and hepatic Kupffer cell clearance.
  • DAT Profile: Characteristically positive for complement C3d only and strictly negative for IgG.
  • Clinical Picture: Acrocyanosis (painful mottled cyanosis of extremities precipitated by cold exposure, resolving with rewarming, distinct from Raynaud's phenomenon as it lacks reactive hyperaemic rubor), mild jaundice, and dark urine.
  • Aetiology: Primary chronic CAD (clonal B-cell lymphoproliferative disorder) or secondary reactive cold agglutinin syndrome following Mycoplasma pneumoniae infection (polyclonal anti-I antibodies), Epstein-Barr Virus (EBV / infectious mononucleosis) (polyclonal anti-i antibodies), or low-grade B-cell lymphoma.
  • CRITICAL EXAM PEARL: In cold agglutinin disease, corticosteroids and splenectomy are completely INEFFECTIVE (because IgM does not depend on splenic Fc receptors for clearance). Management consists of strict avoidance of cold environments, warm clothing, and systemic therapy with rituximab (with or without bendamustine) or the complement C1s inhibitor sutimlimab.

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Non-Immune and Microangiopathic Haemolysis

A negative direct antiglobulin test with unequivocal haemolysis shifts the differential to mechanical, infective, toxic and inherited causes.

Schistocytes plus thrombocytopenia is a medical emergency until TTP has been excluded, because plasma exchange is time-critical and platelet transfusion is harmful.

Drug-Induced Immune Haemolysis

Three mechanisms are distinguished, and MRCP questions test the drug rather than the mechanism name:

1. Drug adsorption (hapten) — high-dose penicillins and cephalosporins; the antibody is directed at drug bound to the red cell membrane.
2. Immune complex ("innocent bystander") — quinine, quinidine, rifampicin; often abrupt intravascular haemolysis.
3. True autoantibody induction — methyldopa, fludarabine, and checkpoint inhibitors; the direct antiglobulin test remains positive for months after the drug is stopped.

Interpreting the Direct Antiglobulin Test

The pattern of the DAT narrows the diagnosis further. IgG-only positivity indicates warm autoimmune haemolysis; complement (C3d)-only positivity indicates cold agglutinin disease or paroxysmal cold haemoglobinuria; both IgG and C3d occur in mixed-type disease and in lupus. Remember that a positive DAT is not synonymous with haemolysis — it is found in a small percentage of healthy blood donors and in patients on intravenous immunoglobulin — so it must always be interpreted alongside reticulocytes, haptoglobin, bilirubin and LDH.

Transfusion Pitfalls

In warm autoimmune haemolytic anaemia the autoantibody is pan-reactive, so it reacts with all donor units and can mask a clinically important alloantibody. Cross-matching therefore takes longer, and the laboratory must be warned early. Transfusion should never be withheld from a patient with critical anaemia while compatibility testing continues, but the transfusion laboratory needs the diagnosis to select the least incompatible units safely. In cold agglutinin disease, blood must be given through a warmer, and the patient kept warm throughout.