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PA16.1 | Haemolytic Anaemia — Definition, Classification & Lab Markers — SDL Guide (Part 3)

The Haemolysis Laboratory Marker Map — An Integrated View

Four-step diagnostic flowchart for hemolytic anemia showing laboratory markers, morphology assessment, and classification into intravascular versus extravascular types. — **Panel A:** Step 1 hemolysis triad with arrows showing ↑reticulocytes, ↑unconjugated bilirubin, ↑LDH converging to positive diagnosis. **Panel B:** Step 2 intravascular vs extravascular pathways with specific laboratory markers and associated conditions. **Panel C:** Step 3 red blood cell morphology illustrations showing spheres, schistocytes, sickles, and bite cells with disease correlations. **Panel D:** Step 4 hereditary versus acquired classification decision tree with clinical distinguishing features.

Integrating all markers into a decision framework helps you distinguish haemolysis from other causes of anaemia and characterise its type:

Step 1 — Is there haemolysis?
Look for the haemolysis triad: ↑reticulocytes + ↑unconjugated bilirubin + ↑LDH. If all three are present, haemolysis is likely regardless of the specific cause.

Step 2 — What type?
• ↓haptoglobin + haemoglobinuria + urinary haemosiderin → intravascular haemolysis → think PNH, MAHA, G6PD crisis, ABO mismatch.
• Normal/mildly ↓haptoglobin + splenomegaly + no haemoglobinuria → extravascular haemolysis → think hereditary spherocytosis, warm AIHA, thalassaemia.

Step 3 — What is the red cell morphology telling you?
Spheres → hereditary spherocytosis or AIHA. Schistocytes → MAHA. Sickles → sickle-cell disease. Bite cells → G6PD. This step often gives the diagnosis directly.

Step 4 — Hereditary or acquired?
• Family history + lifelong symptoms + intrinsic red cell morphology → hereditary.
• Recent onset in previously well adult + autoimmune markers or microangiopathic picture → acquired.

IMPORTANT: A normal haptoglobin in a jaundiced patient with reticulocytosis should not reassure you — haptoglobin is an acute-phase reactant that can be falsely elevated during infection, masking haemolysis. Always interpret it in context.

Three-panel medical diagram showing hemolysis laboratory markers flow chart, compensated hemolytic state, and crisis types in hemolytic anemias. — **Panel A:** Hemolysis laboratory marker flow diagram showing intravascular pathway (free Hb, haptoglobin depletion, hemoglobinuria, urinary hemosiderin), extravascular pathway (macrophage phagocytosis, bilirubin, urobilinogen), and shared markers (reticulocytes, LDH, unconjugated bilirubin) with directional change arrows. **Panel B:** Compensated hemolytic state comparison showing normal RBC lifespan (120 days) versus shortened lifespan (20-40 days) with bone marrow compensation maintaining hemoglobin levels, clinical signs including mild jaundice, splenomegaly, elevated reticulocytes, and gallstone risk. **Panel C:** Three types of hemolytic crises: hemolytic crisis (acute acceleration of hemolysis), aplastic crisis (parvovirus B19-induced marrow suppression), and megaloblastic crisis (folate deficiency from high turnover).

The Compensated Haemolytic State in Detail

Diagram showing how increased marrow reticulocyte production compensates for chronic haemolysis, producing mild jaundice, splenomegaly, gallstones, and vulnerability to haemolytic, aplastic, and megaloblastic crises. — **Panel A:** Compensated haemolysis overview showing shortened RBC lifespan, splenic RBC removal, increased bone marrow erythropoiesis, reticulocyte release, mild scleral icterus, mild splenomegaly, and pigment gallstones.. **Panel B:** Balance between increased RBC destruction and 3-5x marrow red cell output, with haemoglobin maintained around 11-13 g/dL.. **Panel C:** Clinical recognition pattern: mild jaundice, mild splenomegaly, reticulocytes 5-10%, and no or mild anaemia.. **Panel D:** Three decompensation mechanisms: haemolytic crisis, aplastic crisis due to parvovirus B19, and megaloblastic crisis due to folate depletion..

The concept of a compensated haemolytic state is clinically vital because it explains why many patients with significant haemolytic disorders do not present with anaemia.

Consider hereditary spherocytosis: RBC lifespan may be 20–40 days (vs normal 120 days). Yet the patient may have a haemoglobin of 11–13 g/dL because the marrow is producing 3–5 times the normal red cell output. The patient is well, but has:
• Mild jaundice (scleral icterus, often unnoticed)
• Mild splenomegaly
• Reticulocyte count 5–10% (chronically elevated)
• Pigment gallstones by age 30–40

This state is vulnerable to acute decompensation by three main mechanisms:
1. Haemolytic crisis: Acute acceleration of haemolysis (e.g., oxidant stress, infection) → haemoglobin drops acutely.
2. Aplastic crisis (parvovirus B19): Transient cessation of erythropoiesis → haemoglobin crashes over 1–2 weeks. Most dramatic in sickle-cell disease and hereditary spherocytosis.
3. Megaloblastic crisis: Chronic high-turnover erythropoiesis → folate depletion → megaloblastic change → superimposed ineffective erythropoiesis → haemoglobin falls.

Recognising the compensated state — jaundice + splenomegaly + elevated reticulocytes but no or mild anaemia — is an important clinical pattern that prevents under-diagnosis.

SELF-CHECK

Which combination of laboratory findings most specifically points to intravascular rather than extravascular haemolysis?

A. ↑Reticulocyte count + ↑unconjugated bilirubin + splenomegaly

B. ↓Haptoglobin + haemoglobinuria + urinary haemosiderin

C. ↑LDH + ↑unconjugated bilirubin + polychromasia on smear

D. Spherocytes on smear + positive direct Coombs test

Reveal Answer

Answer: B. ↓Haptoglobin + haemoglobinuria + urinary haemosiderin

Markedly ↓haptoglobin (from free Hb binding and clearance), haemoglobinuria (free Hb crossing the renal threshold), and urinary haemosiderin (iron accumulation in renal tubular cells from chronic free Hb filtration) are the triad specific to intravascular haemolysis. Options A and C represent general markers of haemolysis shared by both types. Option D (spherocytes + positive DAT) points to warm AIHA — predominantly extravascular haemolysis.

Looking Ahead — SDL2 and SDL3 Categories

A three-panel classification diagram shows haemolytic anaemias branching into hereditary SDL2 categories and acquired SDL3 categories with representative examples and key laboratory or smear clues. — **Panel A:** Central haemolytic anaemia framework branching into SDL2 hereditary haemolytic anaemias and SDL3 acquired haemolytic anaemias, linked back to SDL1 classification axes and laboratory markers.. **Panel B:** SDL2 hereditary categories: membrane defects including hereditary spherocytosis and elliptocytosis; enzyme defects including G6PD deficiency and pyruvate kinase deficiency; haemoglobin defects including sickle-cell disease and alpha/beta thalassaemia.. **Panel C:** SDL3 acquired categories: immune haemolytic anaemias including warm AIHA, cold AIHA, and drug-induced haemolysis; microangiopathic haemolytic anaemias with schistocytes; other acquired causes including PNH, malaria, hypersplenism, and prosthetic valve destruction..

The classification frameworks and laboratory markers established in this SDL apply to all haemolytic anaemias. The next two SDLs in this cluster zoom into specific categories:

SDL2 — Hereditary Haemolytic Anaemias (PA16.2–PA16.5)
• Membrane defects: Hereditary spherocytosis (spectrin/ankyrin/band 3 mutations) — most common hereditary haemolytic anaemia in North Indians; hereditary elliptocytosis.
• Enzyme defects: G6PD deficiency (X-linked, most common enzyme deficiency worldwide; especially prevalent in Indians); pyruvate kinase deficiency.
• Haemoglobin defects: Sickle-cell disease (HbS/HbS) — vasoocclusion + haemolysis; the thalassaemia syndromes (alpha and beta) — microcytic anaemia + variable haemolysis.

SDL3 — Acquired Haemolytic Anaemias (PA16.6–PA16.8)
• Immune haemolytic anaemias: Warm AIHA (IgG, 37°C, extravascular), cold AIHA / cold agglutinin disease (IgM, <4°C, intravascular/agglutination), drug-induced (three mechanisms).
• Non-immune (microangiopathic) haemolytic anaemias (MAHA): DIC, TTP, HUS, pre-eclampsia/HELLP — schistocytes are the smear hallmark.
• Other acquired causes: PNH (GPI-anchor defect, CD55/CD59 loss), malaria, hypersplenism, mechanical destruction (prosthetic valves).

As you move through SDL2 and SDL3, revisit this SDL's classification tables — every specific disorder fits neatly into the axes you have just learned.