Morphological abnormalities of red blood cells

This topic is a constant feature of CICM Fellowship SAQs. The questions usually take the shape of "Here's a blood film; it's abnormal. What's wrong with the patient? Give differentials." Probably the most favourite topic is macrocytosis (i.e. "what are the different causes of macrocytosis"). Nucleated red cells, rouleaux formations and inclusion bodies have also made several appearances.
Representative SAQs include the following:

  • Question 3.3 from the first paper of 2021 (dimorphic population)
  • Question 25.2 from the second paper of 2020 (macrocytosis)
  • Question 2 from the first paper of 2019 (B12 deficiency)
  • Question 3.1 from the second paper of 2016 (macrocytosis)
  • Question 7.2 from the first paper of 2016 (polychromasia)
  • Question 12.1 from the first paper of 2013 (sickle cell crisis)
  • Question 12.2 from the first paper of 2013 (macrocytosis)
  • Question 12.3 from the first paper of 2013 (plasmacytosis, nucleated RBCs, rouleaux)
  • Question 12.2 from the second paper of 2012 (Post-splenectomy Howell-Jolly bodies)
  • Question 12.3 from the second paper of 2012 (anaemia of blood loss)
  • Question 24.3 from the second paper of 2010 (Heinz bodies in G6PD)
  • Question 30.3 from the first paper of 2010 (macrocytosis)

Dimorphic population of red cells

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) 


This is defined (at least in our laboratory) as a mean corpuscular volume (MCV) of over 100fl.

There are several common causes:

  • Alcoholism
  • Vitamin B12 deficiency
  • Folate deficiency
  • Myelodysplastic syndromes

There are also a few uncommon causes:

  • Reticulocytosis
  • Nonalcoholic and alcoholic liver disease
  • Hypothyroidism
  • Multiple myeloma
  • Aplastic anemia
  • Acute leukemia
  • Drugs:
    • trimethoprim, triamterine, nitrous oxide, phenytoin, valproate, chemotherapy agents, HIV antiretrovirals and metformin.

One can find a discussion of the many causes of macrocytosis in this article.

Nucleated red blood cells

Nucleated red cells are an immature subtype, and their abundance in a blood sample reflects that either the bone marrow s struggling to keep up with the losses of red cells, or that there are insufficient resources to complete the normal maturation process. Thus, any nutritional hematinic deficiency and any stimulus for erythropoisesis can cause their appearance.

A review article from Laboratory Medicine lists several causes of nucleated red cells in the peripheral blood:

  • Asplenia
  • Anaemia
  • Hypoxia
  • Bone marrow invasion by malignancy
  • Extramedulary haemopoiesis
  • Uremia
  • Sepsis
  • Liver disease
  • Diabetic ketoacidosis
  • Inflammatory bowel disease
  • Renal transplant
  • Thermal injury
  • Chemotherapy

This article from which this list is derived appears to be important, because the CICM question writers have drawn upon it in the past. In fact, their official answer is a word-for-word facsimile of the table on page 225, "Mechanisms and Conditions Associated With Normoblastemia".

Rouleaux formation of red blood cells

This is a form of reversible RBC aggregation, and it seems to be a cause of some altered blood rheology. The rouleaux are stacks of red blood cells which form due to macromolecule bridging between their surface molecules. In 1926 Eric Ponder published on the subject, and his paper contains beautifully drawn diagrams of his experimental design. In short, anything which might increase your ESR will cause rouleaux formation, and thus the differentials include a broad range of conditions:

  • Infection of any sort
  • Inflammatory disease of any sort
  • Hyperviscosity syndromes
  • Multiple myeloma (or any sort of hyergammaglobulinaemia)
  • Malignancy of any sort
  • Dehydration
  • Diabetes
  • Chronic liver disease (albumin has a counter-aggregatory effect on RBCs)

Target cells (codocytes)

These are bell-shaped cells in vivo; when processed for microscopy they assume the characteristic "mexican hat" shape. The target cell has too much membrane for a given small amount of hemoglobin content; it is essentially the "reverse" of a spherocyte (which has too little membrane, and is therefore spehrical). The typical reason for this abnormality is some sort of hemoglobinopathy, or at least a failure to clear the normal (usually small) proportion of abnormal red blood cells.

You will see lots of target cells in

  • Thalassemia
  • Hepatic disease with jaundice
  • Hemoglobin C disorders
  • Post-splenectomy

Fewer target cells are seen in

  • sickle cell anemia
  • iron deficiency
  • lead intoxication
  • Deficiency of enzyme lecithin cholesterol acyl transferase


Literally "lots of colour", this is a disorder of erythrocyte maturation whereby numerous abnormally coloured (blue-grey) red cells are released into the circulation prematurely. The term applies only to the giemsa stain, in which all such immature forms stain blue-grey, whereas the mature red cells either stain orange or not at all. The blue-grey colour is due to the affinity of the giemsa stain for the phosphate groups of RNA, and reveals the fact that these red cells still have genetic material inside them.

Polychromasia and reticulocytosis are not synonymous. However, most reticulocytes are polychromophilic, and most polychromophilic erythroid cells are reticulocytes. The difference between them is the presence of RNA clumps in the cytplasm, which appear as a mesh-like pattern in the reticulocyte. In contrast, the entire cytoplasm of the polychromophilic cell will be abnormally stained.

Polychromasia  can be interpreted as either a non-specific marker of bone marrow stress, or as a marker of impaired erythrocyte quality control (like Howell-Jolly bodies). In polychromasia, the RBC count will be high indicating that numerous immature forms have been released from the marrow.

Possible causes of polychromasia include:

  • Response to red cell loss by normal marrow:
    • Haemorrhage
    • Haemolysis
  • Recovery of normal marrow function
    • Iron infusion
    • Vitamin B12 replacement
    • Erythropoietin injection
    • Recovery following chemotherapy
  • Failure of bone marrow to sustain normal function
    • Myelofibrosis
    • Malignant marrow infiltration
  • Failure of RBC quality control
    • Splenectomy

Laboratory features of B12 deficiency

This is from Snow (1999) and UpToDate:

  • Macrocytosis - the MCV tends to increase before anaemia develops. Only a truly massive MCV is sensitive for B12 deficiency (over 130 fL).
  • Hypersegmented neutrophils also precede anaemia - this is the presence of at least one neutrophil with at least 6 lobes or more
  • Anaemia is usually hypochromic and macrocytic
  • Leucopenia/thrombocytopenia and low reticulocyte count reflect poor haematopoiesis
  • Evidence of haemolysis may be present (eg. RBC fragments)


Kyle, R. A., and S. V. Rajkumar. "Criteria for diagnosis, staging, risk stratification and response assessment of multiple myeloma." Leukemia 23.1 (2009): 3-9.


Aherne, W. A. "The differentiation of myelomatosis from other causes of bone marrow plasmacytosis." Journal of clinical pathology 11.4 (1958): 326-329.

Constantino, Benie T., and Bessie Cogionis. "Nucleated RBCs—significance in the peripheral blood film." Lab Medicine 31.4 (2000): 223-229.

SCHMIDT, JOHN J., HAROLD J. ROBINSON, and CHARLES S. PENNYPACKER. "Peripheral plasmacytosis in serum sickness." Annals of internal medicine 59.4 (1963): 542-546.

Bäumler, H., et al. "Basic phenomena of red blood cell rouleaux formation."Biorheology 36.5 (1999): 439-442.

Wagner, Christian, Patrick Steffen, and Saša Svetina. "Aggregation of red blood cells: From rouleaux to clot formation." Comptes Rendus Physique 14.6 (2013): 459-469.

REPLOGLE, ROBERT L., HERBERT J. MEISELMAN, and EDWARD W. MERRILL. "SPECIAL ARTICLE Clinical Implications of Blood Rheology Studies." Circulation 36.1 (1967): 148-160.

Ponder, Eric. "On sedimentation and rouleaux formation-I." Experimental Physiology 15.3 (1925): 235-252.

Aslinia, Florence, Joseph J. Mazza, and Steven H. Yale. "Megaloblastic anemia and other causes of macrocytosis." Clinical medicine & research 4.3 (2006): 236-241.

Walker, H. Kenneth, et al. "Peripheral blood smear." (1990). in Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition.

Bessis, Marcel. "Codocytes and Target Cells." Corpuscles. Springer Berlin Heidelberg, 1974. 59-64.

Jones, Kathy W. "Evaluation of Cell Morphology and Introduction to Platelet and White Blood Cell Morphology." I do not know which textbook this is form, but it is a chapter which is available for free online ... for now.

Snow, Christopher F. "Laboratory diagnosis of vitamin B12 and folate deficiency: a guide for the primary care physician." Archives of internal medicine 159.12 (1999): 1289-1298.

Bull, BRIAN S., J. Breton-Gorius, and E. Beutler. "Morphology of the erythron."New York, McGraw Hill (2001): 271-288. - this is an online re-posting of a chapter of Williams' Haematology, but without the figures.

Corazza, G. R., et al. "Howell‐Jolly body counting as a measure of splenic function. A reassessment." Clinical & Laboratory Haematology 12.3 (1990): 269-275.

Bain, Barbara J. "Diagnosis from the blood smear." New England Journal of Medicine 353.5 (2005): 498-507.

Sakka, Vissaria, et al. "An update on the etiology and diagnostic evaluation of a leukemoid reaction." European journal of internal medicine 17.6 (2006): 394-398.

ul Haque, Anwar, and Noor ul Aan. "Leukemoid Reaction: Unusual Causes." International Journal of Pathology 8.1 (2010): 39-40.

HARLEY, JOHN D., and ALVIN M. MAUER. "Studies on the formation of Heinz bodies. II. The nature and significance of Heinz bodies." Blood 17.4 (1961): 418-433.

JANDL, JAMES H. "The Heinz body hemolytic anemias." Annals of internal medicine 58.4 (1963): 702-709.

Davidson, E. "The significance of blue polychromasia." Journal of clinical pathology 12.4 (1959): 322.

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