Once size has been established, shape and chromia carry most of the diagnostic weight. Each named abnormality is produced by a specific mechanism, and the mechanism is what links the finding to a cause.
Target cells (codocytes) are red cells with a central haemoglobin dot surrounded by a pale ring and a second rim of haemoglobin. They form when the surface area of the membrane is increased relative to the cell volume, which happens in thalassemia, in liver disease, after splenectomy, and in haemoglobin C disease. In thalassemia trait the excess membrane is the direct result of unbalanced globin chain synthesis, so target cells appear alongside a normal or near-normal RDW.
Schistocytes are fragmented red cells — helmet shapes, triangles, and small irregular pieces. They are produced by mechanical shearing as cells pass through fibrin strands or damaged endothelium, so they point to microangiopathic haemolysis such as thrombotic thrombocytopenic purpura, haemolytic uraemic syndrome, or disseminated intravascular coagulation, and to mechanical valves. Spherocytes are round cells with no central pallor, produced when the membrane is lost in pieces; they are seen in hereditary spherocytosis and in immune haemolysis. Bite cells are red cells with a single semicircular defect, produced when the spleen removes a Heinz body from the membrane; they point to oxidative haemolysis from G6PD deficiency or oxidant drugs.
Hypochromia — central pallor wider than about one third of the cell diameter — reflects reduced haemoglobin content per cell. It is prominent in iron deficiency and in thalassemia, and it is the finding that, together with the shape pattern, separates the two.