• The mature RBC is a biconcave disc (~6-8 μm diameter, MCV 80-100 fL).
• The membrane is a lipid bilayer composed of:
  • Phospholipids (inner and outer leaflets)
  • Cholesterol (intercalated between phospholipids, modulates fluidity)
  • Glycolipids (outer leaflet, carry blood group antigens)
• The cytoskeleton (spectrin, ankyrin, band 3, protein 4.1, actin) tethers to the inner leaflet, maintaining the biconcave shape and deformability.

• The RBC membrane lipid composition is in dynamic equilibrium with plasma lipoproteins.
• In liver disease or alcoholism, there is dysregulated lipoprotein metabolism → abnormal ratios of cholesterol and phospholipids in plasma → excess free cholesterol is deposited into the RBC membrane.
• This increases the membrane surface area without increasing the cell volume proportionally → the cell becomes flatter and larger → target cells (codocytes) and round macrocytes appear on the smear.
• In severe liver disease, an abnormal lipoprotein called Lipoprotein-X (Lp-X) accumulates, which is particularly cholesterol-rich and drives membrane expansion.
• Additionally, in alcoholism, acetaldehyde (ethanol metabolite) directly damages the erythroid membrane and interferes with lipid turnover.

• Reticulocytes are immature RBCs that have just extruded their nucleus but still contain residual RNA and organelles.
• They are 20-25% larger than mature RBCs (MCV of a reticulocyte ≈ 110-120 fL).
• When there is brisk haemolysis or acute blood loss → compensatory reticulocytosis → the influx of large reticulocytes raises the overall MCV → "pseudomacrocytosis."
• This is why haemolytic anaemia causes macrocytic indices through reticulocytosis ^[1].

Why is this the most common cause?

Alcohol causes macrocytosis through multiple independent mechanisms, and it does so even without concomitant folate deficiency or liver disease:

1. Direct toxic effect on erythropoiesis: Ethanol and its metabolite acetaldehyde directly inhibit erythroid precursor proliferation and maturation in the bone marrow. This leads to vacuolization of erythroid and myeloid precursors (visible on bone marrow biopsy). The developing red cells undergo abnormal endomitosis (the nucleus may replicate without the cell dividing properly), producing larger cells.

2. Membrane lipid abnormalities: Alcohol alters hepatic lipid metabolism, increasing free cholesterol in plasma → excess cholesterol incorporation into RBC membranes → increased surface area → round macrocytes.

3. Spur cell formation (acanthocytes): In advanced alcoholic liver disease, severely abnormal lipoproteins cause irregular cholesterol deposition, creating spiculated "spur cells."

4. Associated folate deficiency: Alcoholics often have poor dietary intake + alcohol inhibits folate absorption in the jejunum + alcohol increases urinary folate excretion. However, the macrocytosis from alcohol can occur independently of folate status — this is a crucial exam point.

5. Associated liver disease: Adds another layer of membrane lipid abnormality (see below).

Key clinical pearl: An MCV of 100–110 fL in a known heavy drinker, with round macrocytes on PBS and no hypersegmented neutrophils, is classic for alcoholic macrocytosis. The MCV may take 2–4 months to normalize after alcohol cessation (because the lifespan of an RBC is ~120 days).

Pathophysiology:

• The liver is the central organ for lipoprotein metabolism. In chronic liver disease (cirrhosis from any cause — HBV, HCV, alcoholic, NAFLD/MASLD, autoimmune hepatitis), there is:

  • Impaired lecithin-cholesterol acyltransferase (LCAT) activity → cannot esterify free cholesterol → free cholesterol accumulates in plasma
  • Accumulation of Lipoprotein-X → an abnormal LDL particle rich in free cholesterol and phospholipids
  • Free cholesterol passively transfers into RBC membranes → increased membrane surface area → round macrocytes and target cells

• The degree of macrocytosis correlates loosely with the severity of liver disease.

• Additional mechanisms in liver disease:

  • Splenomegaly/hypersplenism (from portal hypertension) → increased RBC sequestration and destruction → reticulocytosis → further raises MCV
  • Impaired folate metabolism (liver stores folate) → can contribute a megaloblastic component

In Hong Kong: Given the high prevalence of chronic HBV, liver disease-related macrocytosis is commonly seen. Always check for concomitant B12/folate deficiency before attributing macrocytosis solely to liver disease.

Pathophysiology:

• Thyroid hormones (T3/T4) stimulate erythropoiesis by:

  • Upregulating EPO production in the kidney
  • Directly stimulating erythroid progenitor proliferation in the bone marrow
  • Maintaining normal cellular metabolic rate including lipid metabolism

• In hypothyroidism:

  • Decreased EPO → reduced erythropoiesis → mild anaemia
  • Decreased metabolic rate → reduced oxygen demand → less physiological drive for RBC production (a compensatory "appropriate" reduction)
  • Altered lipid metabolism → hypercholesterolaemia is a hallmark of hypothyroidism → excess cholesterol deposits into RBC membranes → round macrocytes
  • Reduced GI motility → may impair iron and folate absorption (contributory)

• The anaemia is typically mild (Hb 9–12 g/dL), with MCV 100–110 fL.

• Corrects with thyroid hormone replacement (levothyroxine).

Pathophysiology:

• In any form of haemolysis (intravascular or extravascular), the bone marrow mounts a compensatory response by increasing erythropoiesis, releasing reticulocytes prematurely into the peripheral blood.
• Reticulocytes are larger than mature RBCs (MCV ~110-120 fL) → if the reticulocyte count is sufficiently high (e.g., > 5-10%), the overall MCV rises above 100 fL.
• The macrocytosis here is "pseudomacrocytosis" — it's not that the mature RBCs are inherently large; it's that the population is diluted by large immature cells.
• On PBS, you see polychromasia (bluish-grey reticulocytes on Wright stain due to residual RNA), not oval macrocytes or hypersegmented neutrophils.

Common causes of haemolytic anaemia in HK:

• G6PD deficiency (X-linked; ~4.5% of males in HK are affected) → episodic haemolysis triggered by fava beans, infections, drugs (primaquine, dapsone, sulfonamides) → brisk reticulocytosis
• Autoimmune haemolytic anaemia (AIHA) → warm (IgG) or cold (IgM) antibodies
• Hereditary spherocytosis → spectrin/ankyrin defects → extravascular haemolysis
• Thalassaemia intermedia/major → ineffective erythropoiesis + peripheral haemolysis ^[6]
• Haemoglobin Köln (unstable haemoglobin) → precipitated Hb forms Heinz bodies → extravascular haemolysis ^[6]

Definition: Pancytopenia resulting from bone marrow hypoplasia or aplasia ^[5].

Pathophysiology of macrocytosis in aplastic anaemia:

• The few remaining haematopoietic stem cells in the hypocellular marrow undergo stress erythropoiesis.
• Under stress, EPO levels are very high → the marrow attempts to compensate by releasing red cell precursors earlier → these stress erythroid cells are larger.
• Additionally, fetal haemoglobin (HbF) production is upregulated under stress erythropoiesis → HbF-containing cells tend to be larger.
• The result is a normocytic to macrocytic anaemia with very low reticulocyte count (because the problem is marrow failure, not peripheral destruction).

Aetiology ^[5][7]:

• Congenital: Fanconi anaemia (AR), dyskeratosis congenita, Shwachman-Diamond syndrome
• Acquired:
  • Idiopathic (70–80%) — T-cell mediated autoimmune destruction of haematopoietic stem cells
  • Secondary:
    • Drugs: chloramphenicol, gold, NSAIDs (diclofenac, indomethacin), anticonvulsants, cytotoxic drugs, sulfonamides
    • Infection: non-A, B, C hepatitis (seronegative hepatitis) — a well-known association where hepatitis precedes aplastic anaemia ^[5][8]
    • Environmental: benzene, pesticides
    • Radiation
    • Pregnancy

Key pathophysiology: Infection of the haematopoietic stem cell, T-cell mediated destruction of the HSC ^[8].

Lab findings ^[7]:

• CBC: pancytopenia with normocytic/macrocytic anaemia, decreased reticulocytes
• PBS: normal morphology (just reduced cells)
• BM biopsy: hypocellular with fat infiltration (> 90% fat), no malignant infiltration

Severity classification ^[7]:

• Severe AA: BM cellularity < 25% + ≥ 2 of: absolute reticulocyte count < 20 × 10⁹/L, platelet < 20 × 10⁹/L, ANC < 0.5 × 10⁹/L
• Very severe AA: severe + ANC < 0.2 × 10⁹/L
• Non-severe AA: not fulfilling severe criteria

Definition: Heterogeneous group of clonal diseases characterised by ineffective/dysmorphic haematopoiesis ^[7].

Pathophysiology of macrocytosis in MDS:

• The myelodysplastic clone produces morphologically abnormal (dysplastic) cells across one or more lineages.
• Erythroid dysplasia manifests as disordered nuclear maturation — the nuclei may show abnormal chromatin condensation, nuclear budding, internuclear bridging, and megaloblastoid changes (resembling megaloblastic anaemia but NOT due to B12/folate deficiency).
• Ring sideroblasts may form: iron-laden mitochondria encircle the nucleus of erythroid precursors (≥ 5 iron granules encircling ≥ 1/3 of the nucleus) → these are pathognomonic for certain MDS subtypes (MDS with ring sideroblasts, MDS-RS).
• The dysplastic erythroid precursors undergo intramedullary apoptosis (ineffective erythropoiesis) → the few cells that escape into the peripheral blood are often macrocytic and morphologically abnormal.
• Importantly, MDS can show pseudo-Pelger-Huët anomaly (hypolobulated neutrophils with bilobed "pince-nez" nuclei) — the opposite of the hypersegmentation seen in megaloblastic anaemia.

Key features ^[7]:

• Increased incidence with age (median ~65y); M > F
• Trilineage failure: progressive decline in cell counts
• Pre-leukaemia: may transform to acute leukaemia (AML)
• NO hepatosplenomegaly (helps distinguish from myeloproliferative neoplasms)

Clinical relevance in HK: MDS is increasingly recognised in the elderly population. In any elderly patient with unexplained macrocytic anaemia and cytopenias, MDS must be considered and a bone marrow biopsy is indicated.

• A unique form of aplastic anaemia that only involves the erythroid series — white cells and platelets are normal.
• Differential diagnosis for PRCA ^[5]:
  • Congenital: Diamond-Blackfan syndrome, congenital dyserythropoietic anaemia
  • Acquired: Lymphoproliferative diseases, thymoma, Parvovirus B19
• Parvovirus B19 has tropism for erythroid progenitors (binds P antigen/globoside on erythroid precursors) → causes transient aplastic crisis in patients with chronic haemolytic anaemias (e.g., sickle cell disease, hereditary spherocytosis) or persistent PRCA in immunocompromised patients.

• While typically microcytic or dimorphic, some sideroblastic anaemias (especially MDS with ring sideroblasts) present with macrocytic indices.
• The ring sideroblasts (erythroid precursor with ≥ 5 iron granules encircling ≥ 1/3 of nucleus) ^[7] are visible on Prussian blue staining of the bone marrow.
• Pathophysiology: defective mitochondrial haem synthesis → iron accumulates in mitochondria around the nucleus → forms the "ring."

• Luspatercept (Reblozyl) is a specific activin receptor fusion protein that acts as a ligand trap to neutralize negative regulators of late-stage erythropoiesis ^[9].
• In MDS, the SMAD2/3 signalling pathway (part of the TGF-β superfamily) is constitutively activated and overexpressed in CD34+ cells → inhibits terminal erythroid differentiation → anaemia ^[9].
• Luspatercept binds these TGF-β superfamily ligands before they can activate SMAD2/3, thereby releasing the brake on late-stage erythroid maturation and reducing transfusion dependence.
• Approved for transfusion-dependent anaemia in MDS with ring sideroblasts.

Why does it cause macrocytosis? Multiple mechanisms — direct toxic effect of ethanol/acetaldehyde on erythroid precursors, altered hepatic lipid metabolism causing excess free cholesterol deposition in RBC membranes, and potentially associated folate deficiency (though macrocytosis occurs independently of folate status).

Clinical clues pointing towards this diagnosis:

• History of heavy alcohol intake (often underreported — need collateral history)
• Stigmata of chronic alcohol use: Dupuytren's contracture, spider naevi, parotid enlargement, gynaecomastia, tremor
• May or may not have overt liver disease
• MCV typically 100-110 fL; rarely > 115 fL unless concurrent folate deficiency
• MCV takes 2-4 months to normalize after cessation (RBC lifespan ~120 days)

Distinguishing features:

• LFT may be normal or show raised GGT (most sensitive marker of alcohol use) + raised AST:ALT ratio > 2 (characteristic of alcoholic liver disease)
• B12 and folate levels are normal (if folate is also low, may have a mixed megaloblastic + non-megaloblastic picture)
• PBS: round macrocytes, target cells, possibly stomatocytes; NO hypersegmented neutrophils

Why does it cause macrocytosis? The liver produces and metabolises lipoproteins. In cirrhosis, impaired LCAT activity and accumulation of Lipoprotein-X → excess free cholesterol → deposits into RBC membrane → increased surface area → round macrocytes and target cells.

Clinical clues:

• Known chronic liver disease (HBV/HCV carrier — very common in HK — alcoholic liver disease, NAFLD/MASLD, autoimmune hepatitis)
• Signs of chronic liver disease: jaundice, spider naevi, palmar erythema, ascites, splenomegaly, caput medusae
• Signs of portal hypertension: splenomegaly → hypersplenism adds another element of cytopenias

Distinguishing features:

• LFT abnormal: elevated bilirubin, low albumin, raised INR (synthetic dysfunction)
• PBS: round macrocytes, target cells (very characteristic of liver disease) ^[13], acanthocytes (spur cells in severe disease)
• Coagulopathy from impaired clotting factor synthesis (not present in other non-megaloblastic causes)
• May have concurrent iron deficiency (e.g., variceal bleeding) creating a dimorphic picture

Why does it cause macrocytosis? Decreased metabolic rate → hypercholesterolaemia → excess cholesterol deposited in RBC membranes. Additionally, reduced EPO production and decreased bone marrow activity.

Clinical clues:

• Symptoms of hypothyroidism: cold intolerance, weight gain, constipation, dry skin, lethargy, menorrhagia, cognitive slowing
• Signs: bradycardia, non-pitting oedema (myxoedema), periorbital puffiness, delayed relaxation of ankle jerks, goitre
• Anaemia is typically mild (Hb 9-12 g/dL), MCV 100-110 fL

Distinguishing features:

• TSH elevated, free T4 low — definitive
• Normal LFT, normal reticulocyte count
• Anaemia corrects with levothyroxine replacement
• May have associated iron deficiency (from menorrhagia in women) or associated pernicious anaemia (autoimmune clustering — Hashimoto's + pernicious anaemia)

Why does it cause macrocytosis? Compensatory reticulocytosis — reticulocytes are 20-25% larger than mature RBCs, so a high reticulocyte count raises the MCV.

Clinical clues:

• Pallor, jaundice (without tea-coloured urine in extravascular haemolysis), splenomegaly ^[14]
• Hemoglobinuria (coca-cola urine) suggests intravascular haemolysis — top 2 differentials: paroxysmal nocturnal haemoglobinuria (PNH) and paroxysmal cold haemoglobinuria ^[14]
• History of gallstones (pigmented stones from chronic unconjugated hyperbilirubinaemia) ^[14]
• History to elicit: family history, ethnicity, infection, drugs, transfusion history, co-existing autoimmune/lymphoproliferative diseases ^[14]

Distinguishing features:

• Reticulocyte count is elevated (> 2%) — this is THE distinguishing feature from other non-megaloblastic causes where reticulocytes are low/normal
• Haemolytic screen: elevated LDH, elevated unconjugated bilirubin, reduced/absent haptoglobin ^[1][15]
• PBS: polychromasia (bluish-grey reticulocytes), spherocytes (hereditary spherocytosis, warm AIHA), schistocytes/fragmented RBCs (microangiopathic haemolysis/DIC), RBC agglutination (cold agglutinin disease), bite cells/Heinz bodies (G6PD deficiency) ^[11][15]
• Direct antiglobulin test (DAT/Coombs) positive in immune haemolytic anaemia ^[15]

Laboratory features of haemolytic anaemia: anaemia (mildly macrocytic usually) with reticulocytosis, increase in unconjugated bilirubin, LDH, reduced serum haptoglobin, increased methaemalbumin ^[15]

Sub-classification of haemolytic anaemia as a differential ^[11]:

Why does it cause macrocytosis? Stress erythropoiesis with high EPO drive → premature release of larger erythroid precursors, HbF upregulation (HbF-containing cells are larger).

Clinical clues:

• Pancytopenia: anaemia + neutropenia + thrombocytopenia ^[16]
• Symptoms/signs of all three lineages: fatigue (anaemia), recurrent infections (neutropenia), mucosal bleeding/petechiae/ecchymoses (thrombocytopenia)
• NO splenomegaly, NO hepatomegaly — the marrow is empty, there is no extramedullary haematopoiesis (unlike MPN)
• Reticulocyte count is LOW (reticulocytopenia) ^[16] — critical distinguishing feature from haemolytic anaemia
• May have history of preceding hepatitis (non-A, B, C seronegative hepatitis is a well-known trigger) ^[5][8], drug exposure (chloramphenicol, NSAIDs, anticonvulsants), or chemical exposure (benzene)

Distinguishing features:

• PBS: macrocytosis and anisopoikilocytosis, but NO blasts, NO dysplastic cells ^[16]
• BM biopsy: hypocellular marrow with prominent fat cells (> 90% fat), residual haematopoietic cells are morphologically normal, NO malignant infiltration or fibrosis ^[16]
• Lymphocyte count typically normal; monocytopenia may be present ^[16]
• Must exclude PNH (related condition — GPI-anchored protein deficiency) by flow cytometry for CD55/CD59 ^[16]
• Must exclude hypocellular MDS — perform cytogenetics (karyotyping/FISH) and molecular genetics ^[16]

Why does it cause macrocytosis? Clonal dysplastic erythropoiesis with disordered nuclear maturation, intramedullary apoptosis (ineffective erythropoiesis), and release of morphologically abnormal, larger cells into the peripheral blood.

Clinical clues:

• Elderly patient (median age ~65) ^[7]
• Insidious cytopenias: progressive decline in cell counts ^[7]
• NO hepatosplenomegaly ^[7] — important negative finding
• May have history of prior chemotherapy/radiation (therapy-related MDS)
• Pre-leukaemia: may transform to AML ^[7]

Distinguishing features:

• PBS: may show pseudo-Pelger-Huët anomaly (bilobed neutrophils — hypolobulated, the OPPOSITE of hypersegmentation in megaloblastic anaemia), hypogranular neutrophils, Howell-Jolly bodies ^[11][13]
• Reticulocyte count is LOW (ineffective erythropoiesis)
• BM biopsy: dysplastic morphology in ≥ 1 lineage, may show ring sideroblasts (≥ 5 iron granules encircling ≥ 1/3 of nucleus on Prussian blue stain) ^[7], blast percentage must be < 20% (otherwise it is AML by definition) ^[7][13]
• Cytogenetics: characteristic abnormalities like del(5q) (good prognosis), del(7q), monosomy 7 ^[7][13]
• Must check B12 and folate in all patients with suspected MDS to exclude megaloblastic anaemia mimicking MDS ^[13]

From Ryan Ho Haematology notes: differential diagnoses of MDS include AML (≥ 20% blasts), MDS/MPN (e.g., CMML with monocyte count > 1 × 10⁹/L), aplastic anaemia (hypocellular but morphologically normal cells), PMF (myelofibrosis with splenomegaly and JAK2 mutation), HIV infection (dysplastic haematopoiesis), and megaloblastic anaemia ^[13].

Why does it cause macrocytosis? Depends on the drug mechanism:

• Drugs that impair DNA synthesis (methotrexate, azathioprine, 6-MP, hydroxyurea, zidovudine, cytarabine) → cause a picture closer to megaloblastic (oval macrocytes, possible hypersegmented neutrophils) — these are sometimes classified UNDER megaloblastic ^[1]
• Other drugs that cause macrocytosis via unknown/non-DNA mechanisms → round macrocytes

Clinical clues:

• Temporal relationship between starting the drug and developing macrocytosis
• Common culprits: antiretrovirals (zidovudine/AZT), cytoreductive agents (hydroxyurea for MPN), immunosuppressants, anticonvulsants (phenytoin, valproate)

Distinguishing features:

• Drug history is the key — always take a thorough drug history in any macrocytic anaemia
• MCV typically returns to normal after drug discontinuation (timing depends on RBC lifespan)
• If the drug inhibits DNA synthesis, PBS may show hypersegmented neutrophils (overlap with megaloblastic)

There is no single formal "diagnostic criterion set" published by a guideline body. Instead, diagnosis is established by a combination of findings:

The CBC is the first-line investigation for any anaemia. For non-megaloblastic macrocytic anaemia, focus on:

The PBS is the single most important investigation for classifying macrocytic anaemia. It provides morphological information that automated analysers cannot.

These are mandatory to exclude megaloblastic anaemia before diagnosing a non-megaloblastic cause.

Why check both B12 AND folate? Because B12 deficiency can falsely elevate serum folate levels (B12 normally "traps" folate into cells — the methyl-folate trap; without B12, folate accumulates extracellularly) ^[1]. Checking only serum folate in the presence of B12 deficiency may give a false sense of reassurance.

If B12/folate levels are equivocal ^[10]:

• Methylmalonic acid (MMA) and homocysteine levels:
  • B12 deficiency: ↑MMA, ↑homocysteine
  • Folate deficiency: normal MMA, ↑homocysteine
  • Both normal: no B12/folate deficiency ^[1]

Why "inappropriately normal"? If a patient is anaemic, EPO should be high → marrow should be producing reticulocytes briskly. A "normal" reticulocyte count in the face of significant anaemia is actually inappropriately low and suggests the marrow is failing.

Ordered when reticulocytes are elevated to confirm haemolysis:

According to the Haematology Introduction lecture slides: haemolytic anaemia laboratory features include anaemia (mildly macrocytic usually) with reticulocytosis, increase in unconjugated bilirubin, LDH, reduced serum haptoglobin, increased methemalbumin. Blood film shows polychromasia, spherocytes (hereditary spherocytosis, immune haemolytic anaemia), RBC fragmentation (microangiopathic haemolysis), RBC agglutination (cold agglutinin disease). Direct antiglobulin test is positive in immune haemolytic anaemia. ^[15]

After confirming haemolysis, the next step depends on DAT result ^[14]:

• DAT positive → immune-mediated → classify as warm (IgG) vs cold (IgM/complement), investigate underlying cause (autoimmune, lymphoproliferative, drug-induced)
• DAT negative → intrinsic RBC defect → further workup:
  • G6PD assay
  • Haemoglobin electrophoresis/HPLC
  • Osmotic fragility test (hereditary spherocytosis)
  • Flow cytometry for GPI-anchored proteins (PNH)

After confirming a patient has haemolysis, the next step to determine whether the patient is undergoing autoimmune-mediated destruction is the direct antiglobulin test. It will be positive in immune haemolytic anaemias. However, some proportion of the healthy population will have positive DAT but no haemolysis → pre-test probability must be established first ^[14].

Simple and definitive. If TSH is raised and fT4 is low, hypothyroidism explains the macrocytosis. Treat with levothyroxine and monitor Hb/MCV response.

While primarily used for microcytic anaemia, iron studies are important in the non-megaloblastic workup to:

• Exclude concurrent iron deficiency (creating a dimorphic picture where the MCV may be "normal" — averaged between micro and macro populations)
• Assess for iron overload in aplastic anaemia/MDS (from repeated transfusions)

Serum ferritin: most sensitive and specific marker for iron deficiency. Low serum ferritin is diagnostic of iron deficiency. Clinical decision cutoffs: Adults < 34 pmol/L (15 μg/L); Elderly hospitalized < 100 pmol/L (45 μg/L); Community-based elderly < 49 pmol/L (22 μg/L) ^[23].

Serum Fe CANNOT be used alone. Transferrin Fe saturation CANNOT be used alone ^[23]. Always interpret the full iron profile together.

When to order: Unexplained pancytopenia, suspected aplastic anaemia or MDS, failure to respond to treatment, uncertain diagnosis after initial workup ^[12][13][16].

BM aspirate and trephine biopsy can be performed at different sites 1-2cm apart. Absolute contraindication: severe bleeding disorders (DIC, severe haemophilia) — but thrombocytopenia of any severity is NOT a contraindication; just transfuse to > 20 × 10⁹/L beforehand ^[25].

Key BM findings by diagnosis:

When to transfuse?

Why not transfuse liberally?

• Each unit of packed RBCs contains ~250 mg of elemental iron ^[1] → repeated transfusions cause iron overload (haemochromatosis), damaging liver, heart, and endocrine organs
• Transfusion reactions (febrile, allergic, haemolytic)
• Alloimmunisation → makes future transfusions and HSCT more difficult
• In aplastic anaemia patients being considered for HSCT, minimise transfusions to reduce alloimmunisation risk

Indicated in transfusion-dependent patients (aplastic anaemia, MDS, thalassaemia) to prevent transfusional iron overload.

When to start chelation? When ferritin > 1000 μg/L or positive MRI findings of iron deposition in liver/heart ^[1].

Why does iron overload matter?

• Excess iron saturates transferrin → non-transferrin-bound iron (NTBI) appears, bound loosely to albumin/citrate → NTBI catalyses the Haber-Weiss reaction (iron + H₂O₂ → reactive oxygen species) → oxidative tissue damage ^[1]
• Target organs: liver (cirrhosis), heart (dilated cardiomyopathy → heart failure), endocrine (DM, hypogonadism, hypothyroidism), skin (hyperpigmentation → "bronze diabetes")

• Indicated in all patients with chronic haemolysis (e.g., haemolytic anaemia, thalassaemia intermedia/major) ^[10][27][28]
• Why? Chronic haemolysis → ↑RBC turnover → ↑folate demand for DNA synthesis in erythroid precursors → risk of developing superimposed megaloblastic anaemia on top of the non-megaloblastic picture
• Dose: 1-2 mg/day for moderate/severe haemolysis; 5 mg/day if pregnant ^[28][29]
• Body stores of folate are sufficient for only ~3 months ^[1] → depletion occurs much faster than B12

Relevant mainly for aplastic anaemia and MDS (where neutropenia is present):

• Antifungal prophylaxis in patients with prolonged and profound neutropenia ^[30]
• Proper nursing care: reverse isolation, face mask, hand hygiene, low bacteria diet ^[30]
• Immediate empirical broad-spectrum antibiotics for neutropenic fever ^[30]

Management is straightforward: alcohol cessation.

The macrocytosis improves when the liver disease is controlled, though in advanced cirrhosis it may persist.

While renal anaemia is typically normochromic normocytic ^[33], it is included here because CKD patients may have concurrent macrocytic causes (drugs, folate deficiency, etc.), and the treatment of renal anaemia is highly examinable.

Renal anaemia guideline (UK): Hb target between 10 to 11 g/dL, but not greater than 12 g/dL. Iron saturation > 20%. Serum ferritin at least 100 in pre-dialysis patients, 200 in hemodialysis patients ^[34].

Complications of anaemia include: cardiac ischaemia, increased thrombocytopenic bleeding, and increased mortality ^[10][36].

Why does high-output failure occur? Let's trace it from first principles:

• Anaemia → ↓Hb → ↓O₂ content per unit of blood → tissues sense hypoxia
• Compensatory mechanisms: ↑2,3-BPG (shifts O₂-dissociation curve right → easier unloading), ↑cardiac output (↑HR via sympathetic drive, ↑SV via Frank-Starling), peripheral vasodilation (↓afterload)
• These mechanisms maintain tissue oxygenation in mild-moderate anaemia
• But sustained ↑CO → eccentric ventricular hypertrophy → eventually systolic dysfunction → decompensated heart failure

Severe anaemia is independently associated with increased mortality, particularly in elderly patients and those with comorbidities. Anaemia increases mortality ^[10][36] — this is a simple but crucial point. Even "asymptomatic" chronic anaemia increases perioperative risk, cardiovascular events, and all-cause mortality.

Relevant to haemolytic anaemia as a non-megaloblastic cause of macrocytosis.

Aplastic anaemia has unique complications beyond the anaemia itself, because all three lineages are affected (pancytopenia):

From GC 047 lecture slides: when taking history for inherited causes of anaemia, ask about complications (including those due to long-term red cell transfusion) ^[38].

This is one of the most important long-term complications affecting chronically transfusion-dependent patients (aplastic anaemia, MDS, thalassaemia major). It deserves detailed attention.

Why does iron overload occur?

• Each unit of packed RBCs contains approximately 200-250 mg of iron [35a]^[1]
• Normal daily iron excretion is only ~1 mg/day [35a] — the body has no active mechanism for excreting excess iron
• Normal daily iron absorption is also ~1 mg/day to maintain balance [35a]
• Therefore, a patient receiving regular transfusions accumulates iron at a rate far exceeding the body's capacity to excrete it

Target organ damage from iron overload:

Transfusion haemosiderosis causes iron accumulation within: [35a]

Monitoring:

• Serum ferritin (target < 1000 μg/L in transfusion-dependent patients)
• Liver and cardiac MRI (T2) to directly assess organ iron deposition* ^[1] — ferritin alone can be unreliable (it is a positive acute phase reactant)

Prevention and treatment:

• Iron chelation therapy (deferoxamine, deferasirox, deferiprone) — see Management section
• Venesection if the patient can tolerate it (not usually feasible in aplastic anaemia/MDS where the marrow is failing)

The macrocytosis itself has minimal direct complications. The complications are from the alcohol use and associated liver disease:

The macrocytic anaemia from hypothyroidism is generally mild and has few complications on its own. However:

HSCT is potentially curative but carries significant morbidity. The complications are divided into early and late ^[37]:

Early complications ( < 1 year):

Late complications ( > 1 year):

Overall prognosis of HSCT ^[37]:

• Allogeneic HSCT: 75% long-term survival; 80% at 15y for those surviving ≥ 2 years; 9.9× mortality of general population (highest years 2-5 post-HSCT); 29% relapse, 22% chronic GvHD
• Autologous HSCT: 80% overall survival at 20y for those surviving ≥ 5 years; 4-9× mortality of general population

Relevant for patients with hereditary spherocytosis, refractory AIHA, or thalassaemia major who undergo splenectomy.

Post-splenectomy patients are at risk of life-threatening infection from encapsulated bacteria → overwhelming post-splenectomy infection (OPSI) ^[31].