1.1 Erythrocyte Anatomy

The mature red blood cell (RBC, erythrocyte) is a non-nucleated, biconcave disc approximately 6–8 µm in diameter and 2 µm thick. This geometry maximises the surface area-to-volume ratio, optimising both gas exchange efficiency and deformability during microcirculation through capillaries narrower than the cell itself.

1.2 Haemoglobin Structure and Function

Haemoglobin is a tetrameric globular protein — four globin chains, each bound to a heme prosthetic group. Adult haemoglobin (HbA) is composed of two α-chains (141 amino acids each) and two β-chains (146 amino acids each): the α₂β₂ tetramer.

1.3 The RBC Lifecycle (Erythropoiesis → Senescence → Clearance)

Erythropoiesis proceeds through a series of morphologically distinct nucleated precursors in the bone marrow over approximately 7 days, culminating in the release of the reticulocyte — an anucleate cell that still contains residual RNA. After 24 hours of peripheral blood maturation, the mature erythrocyte circulates for approximately 120 days before senescent changes trigger phagocytosis.

The four primary RBC indices — MCV, MCH, MCHC, and RDW — are not directly measured. They are calculated from three raw analyte values (haemoglobin, RBC count, and haematocrit) that the analyser measures independently. Understanding this derivation is essential for recognising analyser artefacts and interpreting discordant results.

2.1 Mean Corpuscular Volume (MCV)

2.2 Mean Corpuscular Haemoglobin (MCH)

MCH parallels MCV — hypochromic cells are also smaller. MCH < 27 pg = hypochromia (iron deficiency, thalassaemia); MCH > 33 pg = hyperchromia (hereditary spherocytosis — high MCHC from reduced surface area:volume ratio). MCH is more reproducible than MCHC because it is not divided by haematocrit.

2.3 Mean Corpuscular Haemoglobin Concentration (MCHC)

MCHC is a concentration (Hb density per cell), not a quantity. Its tight reference range (only ~12% variation around the mean) makes it useful as a quality control index — MCHC > 37 g/dL strongly suggests cold agglutinins, haemolysis, or severe lipidaemia as analyser artefacts. True hyperchromia occurs only in hereditary spherocytosis.

2.4 Red Cell Distribution Width (RDW)

3.1 Physical Basis

Wallace H. Coulter’s 1953 US Patent 2,656,508 established the foundational technology for cell counting and sizing. The core insight: a particle (RBC) suspended in conductive electrolyte that passes through a small aperture (<100 µm) between two electrodes displaces a volume of conductive liquid equal to its own volume. This displacement briefly increases electrical resistance — generating a voltage pulse whose height is proportional to cell volume.

4.1 Principles of Optical Analysis

Modern high-end analysers (Sysmex XN series, Beckman Coulter LH 800, Abbott CELL-DYN) employ laser light scattering. A hydrodynamically focused stream carries individual cells through a laser beam intersection, simultaneously generating three signal types:

4.2 VCS Technology (Beckman Coulter)

The VCS system combines Volume (DC impedance), Conductivity (high-frequency electromagnetic field), and Scatter (laser). Conductivity probes nuclear and membrane characteristics independent of cell volume — detecting abnormal cells that have the same volume as normal cells but differ in nuclear density (e.g., abnormal lymphocytes, reactive forms).

4.3 Advantages of Combined Optical + Impedance

5.1 Iron Deficiency Anaemia vs. β-Thalassaemia Trait

Microcytic anaemia (MCV < 80 fL) is the most common haematological presentation worldwide. The two most prevalent causes — IDA and β-thalassaemia trait — produce similar MCV reductions but have entirely different pathophysiologies, treatment approaches, and genetic counselling implications. Distinguishing them is a paradigm for applied computational haematology.

5.2 The Mentzer Index

5.3 Advanced Discriminant Indices

5.4 Case Vignettes

Week 2 Assignment

References & Further Reading