Haemolytic Anaemias — Intravascular vs Extravascular, Hereditary Spherocytosis, G6PD Deficiency, AIHA, PNH and Sickle Cell Disease

Red cells normally survive about 120 days. Haemolytic anaemia is anaemia from their premature destruction. It can be classified as acute or chronic, inherited or acquired, immune or non-immune, intracorpuscular (defect inside the red cell) or extracorpuscular, and intravascular or extravascular by where the cells are destroyed.

Four tests confirm haemolysis: raised reticulocyte count, raised LDH, raised unconjugated (indirect) bilirubin and low haptoglobin. Haptoglobin falls because it binds the free haemoglobin released from lysed cells and is then cleared. The peripheral smear and the direct antiglobulin (Coombs) test then point to the cause. See anaemia classification and approach for where haemolysis fits in the wider work-up.

Hereditary spherocytosis (HS) is the commonest inherited haemolytic anaemia due to a red-cell membrane defect (about 1 in 2,000). Deficiency of a cytoskeletal protein — spectrin (most often), ankyrin, band 3 or band 4.2 — weakens the link between the cytoskeleton and the lipid bilayer. The cell loses membrane, becomes a small sphere, cannot deform and is destroyed in the spleen. Ankyrin mutations cause both dominant and recessive forms.

• Clinical: anaemia, jaundice, splenomegaly, family history; gallstones (cholelithiasis) are a known complication.
• Lab clue: Coombs-negative haemolysis with raised MCHC and spherocytes on the smear.
• Confirmatory tests: eosin-5-maleimide (EMA) binding by flow cytometry (validated by the BCSH guideline), osmotic fragility (increased) and acidified glycerol lysis test.
• Crises: haemolytic (commonest, triggered by viral infection), aplastic (parvovirus B19 — worse, may cause heart failure) and megaloblastic.
• Severity by haemoglobin: severe < 8 g/dL, moderate 8–10 g/dL, mild 10–11.5 g/dL (women) or 10–13.5 g/dL (men); clinical response to splenectomy tracks the degree of spectrin deficiency.
• Treatment: splenectomy improves most patients; afterwards Howell-Jolly bodies appear and lifelong risk of sepsis from encapsulated organisms (S. pneumoniae, N. meningitidis, H. influenzae) needs vaccination.

G6PD catalyses the first, rate-limiting step of the pentose phosphate pathway, making NADPH. NADPH keeps glutathione reduced so it can detoxify hydrogen peroxide. Red cells have no other source of NADPH, so in G6PD deficiency oxidant stress denatures haemoglobin into Heinz bodies and damages the membrane. The gene is X-linked, so males are mainly affected, though heterozygous females can be affected because of lyonisation. It is common across the malaria belt.

• Smear: bite cells and blister cells; Heinz bodies need a supravital stain.
• Episodic: haemolysis is acute and self-limited once the trigger is removed; chronic haemolysis is rare.
• Testing trap: enzyme assay during an attack can be falsely normal — repeat the quantitative assay after recovery.
• Newborns: G6PD deficiency causes severe neonatal jaundice; in one US registry about 20% of infants with kernicterus had it.

Pyruvate kinase deficiency is the other enzyme defect to know. Red cells depend on glycolysis for energy, so the defect starves them of ATP. Increased 2,3-DPG improves oxygen unloading in the tissues. The smear shows polychromasia with echinocytes; PKLR gene mutations confirm it, and severe cases benefit from splenectomy in late childhood with vaccination against encapsulated bacteria.

The direct antiglobulin test (DAT, direct Coombs) detects antibody or complement already bound to the patient's red cells — the key test for immune haemolysis. The indirect antiglobulin test detects free antibody in the serum that could react with donor red cells, as in cross-matching. About 5–10% of AIHA is DAT-negative, so a negative DAT does not fully exclude it.

PNH is an acquired stem-cell disorder caused by a somatic mutation of the X-linked PIGA gene. Without PIGA the cell cannot make the glycosylphosphatidylinositol (GPI) anchor, so GPI-anchored complement regulators CD55 and CD59 are missing. Red cells are then lysed by complement inside vessels.

• Triad: intravascular haemolysis (haemoglobinuria), thrombosis and bone-marrow failure (cytopenias; overlaps with aplastic anaemia).
• Thrombosis — often in unusual sites such as hepatic veins (Budd-Chiari) — is the commonest cause of death.
• Diagnosis: flow cytometry for absent GPI-anchored proteins, including FLAER — the gold standard.
• Treatment: complement C5 inhibitors eculizumab and ravulizumab have brought survival close to normal; they raise the risk of meningococcal infection, so vaccinate first.

Sickle cell disease is autosomal recessive. A point mutation in the beta-globin gene on chromosome 11 replaces glutamate with valine at position 6, forming HbS, which polymerises when deoxygenated and distorts red cells into sickles. Rigid cells block small vessels (vaso-occlusion) and are destroyed early. Sickle cell trait (HbAS) rarely causes symptoms.

• Diagnosis: haemoglobin electrophoresis, HPLC or isoelectric focusing.
• Spleen: splenic sequestration in children aged 1–4 years while the spleen is still intact; later loss of splenic function → invasive bacterial infections and Howell-Jolly bodies.
• Aplastic crisis: parvovirus B19 — haemoglobin drops at least 3–6 g/dL below baseline with low reticulocytes.
• Stroke: the most devastating complication; without prevention about 10% of children have an overt stroke.
• Acute chest syndrome: antibiotics (cephalosporin plus macrolide), oxygen, analgesia, transfusion and incentive spirometry.
• Hydroxyurea: ribonucleotide reductase inhibitor that raises HbF; MCV rises on treatment. Crizanlizumab blocks P-selectin to reduce pain crises.

• Haptoglobin is the most sensitive marker of haemolysis; LDH and bilirubin can also rise with other causes.
• Normal G6PD level during an acute attack does not exclude deficiency — repeat after recovery.
• PNH is acquired, not inherited, even though PIGA is on the X chromosome.
• Cold agglutinin disease is IgM and complement-mediated; paroxysmal cold haemoglobinuria is IgG (Donath-Landsteiner).
• Aplastic crisis in any chronic haemolytic anaemia (HS, sickle cell) = parvovirus B19, with a low reticulocyte count.

Related: kernicterus and neonatal jaundice for haemolysis in newborns, megaloblastic anaemia and hypersensitivity reactions — AIHA is a classic type II reaction.