Question 3.3

A blood film from a patient is reported as showing a “dimorphic population of red cells”.
a) What is meant by dimorphic population?
b) Give four causes of this picture.
(25% marks)

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College answer

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Dimorphic population of red cells is the presence of two or more morphologically distinct red cell populations in the same bloodstream. This is a finding usually reported on the basis of a strongly bimodal red cell volume histogram which is produced by automated blood cell analysers (Constantino, 2011). The term is somewhat misleading, as the histogram may have more than two discrete red cell populations sharing the bloodstream. However, usually there's only two cell populations; typically a group from before some sort of illness or treatment, and a group from after that event. Examples include:

  • Microcytic hypochromic cells from before the iron infusion/blood transfusion, and normocytic normochromic cells from after it.
  • Normocytic normochromic cells from before the induction chemotherapy, and microcytic hypochromic cells from after it.

Following from this, numerous possibilities exist to explain the concurrent presence of several morphologically distinct red cell variants, some of which are totally predictable. This representative list is cut-and-pasted from Constantino (2011):

  • Early iron developing microcytic population 
  • Folate/vitamin B12 developing macrocytic population 
  • Post-iron treatment of iron deficiency anemia 
  • Post-iron treatment of iron deficiency with megaloblastic anemia 
  • Post-iron treatment of megaloblastic anemia 
  • Post-iron treatment of megaloblastic anemia with iron deficiency 
  • Post-iron transfusion macrocytic anemia 
  • Post-iron transfusion microcytic anemia 
  • Iron deficiency anemia with either folate or vitamin B12 deficiency 
  • Sideroblastic anemia (myelodysplasia) 
  • Hemolytic anemia (reticulocytosis, spherocytosis, fragmentation, pyropoikilocytosis) 
  • Cold/warm auto agglutination 
  • Erythropoietin-induced erythropoiesis 
  • Delayed transfusion reaction 
  • Homozygous hemoglobinopathies (admixture of many RBC forms) 
  • Myelofibrosis (admixture of extramedullary hematopoiesis) 
  • Constitutional chromosomal translocation t(11;22)(p15.5;q11.21) 


Constantino, Benie T. "The red cell histogram and the dimorphic red cell population." Laboratory Medicine 42.5 (2011): 300-308.