21.1 MCV-Based Diagnostic Approach to Anaemia

Anaemia is defined by the World Health Organization (WHO) as a haemoglobin concentration <130 g/L in adult men, <120 g/L in non-pregnant women, and <110 g/L in pregnant women. In the MRCP(UK) Part 1 examination, questions on anaemia demand rigorous algorithmic problem-solving based on the Mean Corpuscular Volume (MCV), precise biochemical differentiation of iron parameters, and deep pathophysiological knowledge of haemolytic conditions and globin chain mutations.

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1. Algorithmic Diagnostic Approach Based on Mean Corpuscular Volume (MCV)

The initial diagnostic step in evaluating any anaemia is categorizing the red cell size via the MCV into microcytic (<80 fL), normocytic (80–100 fL), or macrocytic (>100 fL).

                         Anaemia Workup Flowchart
                                    |
                +-------------------+-------------------+
                |                                       |
          MCV < 80 fL                             MCV > 100 fL
      (Microcytic Anaemia)                    (Macrocytic Anaemia)
                |                                       |
    +-----------+-----------+               +-----------+-----------+
    |           |           |               |                       |
Ferritin     Ferritin     Mentzer      Blood Film              Non-Megaloblastic
< 30 ug/L   Normal/High   < 13         Hypersegmented          (Alcohol, Liver,
 (IDA)        (ACD)     (Thalassaemia)  Neutrophils            MDS, Thyroid)
                                            |
                                  +---------+---------+
                                  |                   |
                             Low Serum B12       Low Folate
                           (Pernicious Anaemia)  (Dietary/Malabsorption)

A. Microcytic Anaemias (MCV <80 fL)

Microcytosis reflects impaired haemoglobin synthesis inside maturing erythroid precursors, stemming from deficient iron availability, defective globin chain production, or impaired protoporphyrin synthesis.

1. Iron Deficiency Anaemia (IDA):

   • Biochemical Profile: Serum ferritin <30 ug/L (the single most sensitive and specific biochemical test for total body iron depletion; levels <15 ug/L are definitively diagnostic), elevated Total Iron-Binding Capacity (TIBC / transferrin), low serum iron, and transferrin saturation <16%.
   • Important Diagnostic Caveat: Ferritin is an acute-phase reactant. In the presence of systemic inflammation, infection, chronic kidney disease, or hepatic necrosis, a ferritin value up to 100 ug/L may still be compatible with coexistent iron deficiency.
   • Clinical Investigation Mandate: In the UK (NICE and British Society of Gastroenterology guidelines), unexplained iron deficiency anaemia in all men and post-menopausal women requires urgent bidirectional endoscopy (oesophagogastroduodenoscopy [OGD] and colonoscopy) to exclude occult gastrointestinal malignancy (colorectal adenocarcinoma, gastric carcinoma), even in the complete absence of gastrointestinal symptoms. Coeliac serology (anti-tissue transglutaminase IgA and total IgA) must be checked in all patients.

2. Anaemia of Chronic Disease / Inflammation (ACD):

   • Pathophysiology: Driven by pro-inflammatory cytokines, predominantly interleukin-6 (IL-6) and tumour necrosis factor-alpha (TNF-a). IL-6 stimulates hepatic production and secretion of hepcidin, the master iron-regulatory hormone. Hepcidin binds to and induces the internalization and lysosomal degradation of ferroportin-1 (the sole known cellular iron exporter on duodenal enterocytes and reticuloendothelial macrophages). This traps iron inside macrophages, prevents intestinal iron absorption, and starves erythroid precursors of iron despite abundant total body iron stores.
   • Biochemical Profile: Serum ferritin is normal or elevated (>100 ug/L), TIBC is low or normal, serum iron is low, and soluble transferrin receptor (sTfR) is normal (in contrast to IDA where sTfR is elevated).

3. Thalassaemias:

   • Characterized by intact iron stores but profound microcytosis disproportionate to the degree of anaemia (MCV often <70 fL with Hb 100–110 g/L).
   • Mentzer Index: Calculated as MCV / RBC count (in millions/uL). A Mentzer index <13 strongly points to a thalassaemia trait, whereas an index >13 points toward iron deficiency anaemia.
   • Beta-Thalassaemia Trait (Minor): Heterozygous point mutations in the HBB gene on chromosome 11. Hemoglobin electrophoresis or HPLC reveals characteristically elevated HbA2 >3.5% (typically 4.0–6.0%) and mildly elevated HbF (1–3%). Blood film shows target cells, basophilic stippling, and microcytic hypochromic red cells.
   • Alpha-Thalassaemia Trait: Gene deletions in the duplicated HBA1 and HBA2 globin genes on chromosome 16. Two-gene deletion (alpha-thalassaemia minor / trait: -a/-a or --/aa) presents with microcytosis but normal hemoglobin electrophoresis in adults (because alpha chains participate equally in HbA, HbA2, and HbF). Three-gene deletion (--/-a) results in HbH disease, producing unstable beta-globin tetramers (HbH) that precipitate as "golf-ball" inclusions on brilliant cresyl blue supravital staining, presenting with moderate-to-severe chronic haemolytic anaemia and splenomegaly. Four-gene deletion (--/--) produces Hb Barts (gamma-4 tetramers), causing fatal hydrops fetalis in utero.

4. Sideroblastic Anaemias:

   • Characterized by defective incorporation of iron into the protoporphyrin IX ring within erythroid mitochondria, resulting in pathological mitochondrial iron accumulation ringing the nucleus.
   • Diagnosis: Bone marrow aspirate stained with Prussian blue (Perls' stain) demonstrates >=5 ringed sideroblasts (erythroblasts containing >=5 siderotic granules encircling at least one-third of the nuclear circumference).
   • Aetiologies:
     • Hereditary: X-linked defect in delta-aminolevulinate synthase 2 (ALAS2), the initial and rate-limiting enzyme in erythroid haem biosynthesis (responsive to pyridoxine / vitamin B6).
     • Acquired Clonal: Myelodysplastic neoplasms with ring sideroblasts (MDS-RS), strongly associated with somatic mutations in the RNA splicing gene SF3B1.
     • Acquired Reversible: Chronic alcohol toxicity, lead poisoning (inhibits ALA dehydratase and ferrochelatase; coarse basophilic stippling on blood film), medications (isoniazid, which antagonizes pyridoxal phosphate; chloramphenicol), and severe copper deficiency (often secondary to zinc toxicity).

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B. Normocytic Anaemias (MCV 80–100 fL)

The fundamental discriminator in normocytic anaemia is the reticulocyte count (or reticulocyte production index, RPI):

1. High Reticulocyte Response (>2.0% or >100 x 10^9/L):
   • Indicates appropriate bone marrow erythroid hyperplasia responding to acute peripheral erythrocyte loss or accelerated destruction.
   • Primary Causes: Active haemolysis (immune, enzymopathy, membranopathy, microangiopathy) or acute haemorrhage/blood loss.
2. Low or Inappropriate Reticulocyte Response (<2.0% or <50 x 10^9/L):
   • Indicates defective bone marrow production or systemic suppression of erythropoiesis.
   • Primary Causes:
     • Erythropoietin (EPO) Deficiency: Chronic kidney disease (CKD stage 3b–5; reduced peritubular interstitial fibroblastic EPO production).
     • Bone Marrow Failure: Aplastic anaemia (pancytopenia with hypocellular fatty marrow), pure red cell aplasia (PRCA; isolated erythroblastopenia, strongly associated with thymoma, Parvovirus B19, and ABO-incompatible allogeneic bone marrow transplantation), or myelophthisic marrow infiltration (metastatic carcinoma, haematological malignancy, granulomas).
     • Endocrine Hypofunction: Hypothyroidism, hypoadrenalism (Addison's disease), and hypogonadism.

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C. Macrocytic Anaemias (MCV >100 fL)

Macrocytosis is divided pathophysiologically into megaloblastic and non-megaloblastic forms based on peripheral blood film morphology.

1. Megaloblastic Anaemias (Nuclear-Cytoplasmic Dyssynchrony):

   • Caused by impaired DNA synthesis with preserved RNA and protein synthesis, resulting in large, immature-appearing nuclei with fine, delicate chromatin in cells with mature, fully haemoglobinized cytoplasm.
   • Peripheral Blood Film Hallmark: Oval macrocytes and hypersegmented neutrophils (defined as the presence of >=5 nuclear lobes in >5% of neutrophils, or any single neutrophil with >=6 lobes). This is one of the most sensitive morphological markers of megaloblastosis.
   • Vitamin B12 (Cobalamin) vs Folate Deficiency:
     • Both deficiencies elevate plasma homocysteine levels (because both are required for methionine synthase activity).
     • Methylmalonic Acid (MMA): Elevated strictly in Vitamin B12 deficiency (cobalamin is a mandatory cofactor for methylmalonyl-CoA mutase, converting methylmalonyl-CoA to succinyl-CoA). MMA is completely normal in isolated folate deficiency, making serum MMA the definitive biochemical test when B12 levels are borderline.
   • Pernicious Anaemia: Autoimmune destruction of gastric parietal cells in the gastric body and fundus, leading to achlorhydria and loss of intrinsic factor (IF) secretion. Associated with HLA-DR3/DR4, autoimmune thyroid disease, and vitiligo. Diagnostic antibodies:
     • Anti-Intrinsic Factor Antibodies: Moderate sensitivity (~50-70%) but exceptionally high specificity (>95-98%); positive serology confirms pernicious anaemia.
     • Anti-Parietal Cell Antibodies: High sensitivity (~90%) but poor specificity (~50%), positive in atrophic gastritis and elderly controls.
   • Subacute Combined Degeneration of the Cord (SCD): Severe neurological complication exclusive to cobalamin deficiency. Defective myelin methylation leads to symmetrical demyelination of:
     1. Dorsal (posterior) columns: Loss of vibration sense and proprioception, positive Romberg's sign, sensory ataxia.
     2. Lateral corticospinal tracts: Upper motor neuron signs including spastic paraparesis, hyperreflexia, and extensor plantar responses (positive Babinski sign).
     3. Peripheral nerves: Symmetrical sensory peripheral neuropathy with absent ankle reflexes (producing paradoxical combination of brisk knee jerks, extensor plantars, and absent ankle jerks).
   • CRITICAL EXAM WARNING: In a patient with megaloblastic anaemia or combined deficiency, folic acid must NEVER be administered without adequate Vitamin B12 replacement. Folic acid drives DNA synthesis and temporarily corrects the peripheral haematological indices while rapidly precipitating or irreversibly worsening catastrophic subacute combined degeneration of the spinal cord!

2. Non-Megaloblastic Macrocytosis:

   • Normal DNA synthesis without hypersegmented neutrophils. Red cells appear uniformly round rather than oval.
   • Common Causes: Chronic alcohol excess (direct bone marrow toxic effect; most common cause of non-anaemic macrocytosis), chronic liver disease (abnormal lipid deposition on erythrocyte membranes), marked reticulocytosis (reticulocytes are 20% larger than mature erythrocytes), hypothyroidism, myelodysplastic syndrome (MDS), and antimetabolite/cytotoxic drugs (hydroxycarbamide, azathioprine, 6-mercaptopurine, and methotrexate).

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2. Comparative Table: Comprehensive Workup of Anaemias by MCV

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