Red Blood Cell Morphology

Size, Shape & Arrangement

RBC Trait	Appearance	Physiology	Associated Conditions	Follow-up Testing
Rouleaux	Stacked formation ("stack of coins") in monolayer Often seen with blue background staining	Increased amount of plasma proteins Excess protein have + charge that reduces RBC membrane charge (zeta potential) Reduced charge allows cells to stack (due to biconcave shape)	Multiple myeloma/plasma cell myeloma Chronic liver disease Lymphomas Acute and chronic inflammatory diseases (especially when plasma proteins like fibrinogen or globulins are increased)	Saline replacement to remove excess protein
Agglutination	Irregular clumping of cells in monolayer	Caused by IgM antibodies (cold agglutinins) Large antibodies can span zeta potential gaps and form lattice with other antibody-coated cells IgMs bind to RBCs at room temperature	Cold autoimmune hemolytic anemia IgM-associated lymphomas Multiple myeloma Paroxysmal cold hemoglobinuria	Pre-warm sample to 37C to dissociate cold IgMs
Microcytosis	Smaller cells (round, biconcave)	More divisions causing smaller cells Hemoglobin errors - cells continue dividing to reach adequate hemoglobin concentrations	Thalassemia (small, uniform size - N RDW) IDA (small, varying size - ↑ RDW) Anemia of chronic inflammation Lead poisoning Hemoglobinopathies Sideroblastic anemia
Macrocytosis	Round macrocytes (size)	Increases in cell membrane size (more cholesterol) Retain shape, but with larger membrane	Moderate ↑ MCV Liver disease Normal neonate Reticulocytosis
	Oval macrocytes (shape)	Fewer divisions causing larger cells - errors in DNA synthesis Cytoplasm increases, but DNA doesn't (asynchrony)	Marked ↑ MCV Megaloblastic anemia (B12/folate deficiency)
Hypochromasia	Cells >1/3 central pallor 1/2-2/3 moderate >3/4 marked Consistent field-to-field, gradual colour change	Decreased or abnormal hemoglobin	IDA Thalassemias Sideroblastic anemia Lead poisoning Some anemia of chronic inflammation	↓ MCH, ↓ MCHC
Increased polychromasia	Larger, irregular bluish cells, without central pallor	Cells at final stage of RBC maturation Have lost nucleus but have not assumed final biconcave shape Increase indicates bone marrow response to push out immature cells (compensatory mechanism)	0.5-2.5% normal in adults Increased in conditions causing RBC damage or destruction	Reticulocyte count Manual supravital staining (methylene blue to visualize RNA) Automated immature reticulocyte fraction (IRF) to stain nucleic acids

Poikilocytosis

NOTE: be careful when distinguishing poikilocytosis from artifacts.

RBC Trait	Appearance	Physiology	Associated Conditions	Follow-up Testing
Sickle Cells (Drepanocytes)	Hemoglobin S Elongated cells with pointed ends; may be curved Dark red/pink, without central pallor	Cells carry mutated globin chains (hemoglobin S) When O2 is low, hemoglobin S polymerizes and forms long fibre 'crystals' that distort cell shape Repeated sickling permanently alters cell	Sickle cell disease or anemia Sickle cell trait	Sickle cell solubility test Hemoglobin electrophoresis
Hemoglobin SC	Hemoglobin SC Mitten/gloved hand shape	Cells carry mutated globin chains for both hemoglobin S and hemoglobin C	Hemoglobin SC disease
Hemoglobin C	Hemoglobin C Gold bar (dense, rounded rectangle)	Cells carry mutated globin chains (hemoglobin C)	Hemoglobin C disease
Oval Macrocytes	Oval macrocytes	Fewer divisions causing larger cells - errors in DNA synthesis Unable to synthesize DNA due to vitamin deficiency Cytoplasm increases, but DNA doesn't duplicate so cell doesn't divide (asynchrony)	Marked ↑ MCV (>80 fL) Megaloblastic anemia (B12/folate deficiency)
Spherocytes	Spherocytes Small, dense, dark, round cells without central pallor	RBCs lose portions of membrane during circulation Intrinsic abnormalities Immune hemolysis Fibrin strands Thermal injury Damage to membrane results in smaller cell size, becoming more spheroid Hemoglobin concentrated in smaller cell	↑ MCHC (>360 g/L) Hereditary spherocytosis Hemolytic anemias Transfused cells (during storage) Severe burns
	Microspherocyte
Target cells (codocytes)	Cells appear with bull's eye in centre	Relative increase in RBC surface membrane Loss of hemoglobin reducing volume Excess membrane cholesterol/phospholipid	Macrocytic Liver disease Normocytic Hemoglobin disorders (sickle cell, HGB C or E) Microcytic IDA Thalessemia
Ovals/Pencils	Ovalocytes
	Elliptocytes (Pencils)
Teardrop Cells
Acanthocytes (Spur cells)
Echinocytes (Burr cells)
Stomatocytes
Schistocytes (fragments) Schistocytes Helmet cells Bite cells Blister cells

RBC Inclusions & Early Cells

Visible by normal staining

RBC Trait	Appearance	Physiology	Associated Conditions	Follow-up Testing
Howell-Jolly bodies		DNA
Pappenheimer bodies		Iron
Basophillic stippling		RNA
NRBCs

Supravital staining

RBC Trait	Stain	Appearance	Physiology	Associated Conditions	Follow-up Testing
Heinz bodies	New methylene blue	Purple-blue inclusions around inner membrane (may be one or multiple)	Normal hemoglobin denatured by oxidative stress Abnormal, unstable hemoglobin precipitates out Removal of Heinz bodies by spleen causes bite cell formation (HBs don't deform when passing through spleen)	Associated with bite or blister cells G6PD deficiency Abnormal or unstable hemoglobin Oxidant drugs or chemicals Some hemoglobinopathies
Hemoglobin H	Brillian Cresyl Blue	Small, evenly dispersed blue-purple granules Golf ball/raspberry appearance	Deletion of alpha-globin genes causes formation of beta-chain tetramers Abnormal hemoglobin H Hemoglobin H susceptible to oxidation, resulting in denaturation - forms precipitate	Hemoglobin H disease (alpha thalassemia)
Reticulocytes	New methylene blue	Groups of blue-coloured inclusions in cell	RNA present in less mature cells precipitates	Various (increased erythrocyte production)

Combinations of Morphology

Dual Population	Usually normal population with abnormal population	Condition being treated (newer, healthy population) Condition plus blood transfusion
Normocytic/normochromic	Normal size and colour of RBC	Normal patient Anemia from reduced RBC numbers (hemorrhage, burns, etc.)
Hypochromic/microcytic	Small cells with large central pallor	Anemia from abnormal hemoglobin production or metabolism (e.g., IDA)

RBC Metabolism

RBC Breakdown

Extravascular Hemolysis (Macrophage-Mediated Hemolysis)

• Occurs in spleen, primarily due to splenic macrophages
• Slower movement through splenic sinusoids allows older or damaged cells to be destroyed
• RBC contents salvaged and metabolized
  • Hemoglobin broken down into globin chains and iron
• Most (90%) of RBCs are destroyed this way

Intravascular Hemolysis (Fragmentation/Mechanical Hemolysis)

• Occurs inside blood vessels
• Mechanical damage to cells causing hemolysis
  • Can also be caused by damage to blood vessels (e.g., clots)
• RBC contents also salvaged
  • Hemoglobin binds to transport proteins (primarily haptoglobin when available, if hemoglobin oxidizes then binds to hemopexin)
  • Taken to liver (hemoglobin) or excreted by kidney
• Less common (10-20%) method of RBC destruction