Red Blood Cell Morphology Definitions
These are three distinct red blood cell (RBC) abnormalities seen on peripheral blood smear examination that provide important diagnostic clues about underlying hematologic conditions.
Microcytosis
Microcytosis refers to red blood cells that are smaller than normal, defined as a mean corpuscular volume (MCV) less than 80 fL in adults 1, 2. This is a laboratory finding, not a disease itself, that requires systematic evaluation to identify the underlying cause.
Key Clinical Points:
- Most common causes: Iron deficiency anemia and thalassemia trait are the predominant etiologies 2
- Other important causes: Anemia of chronic disease, lead toxicity, sideroblastic anemia, and genetic disorders of iron metabolism or heme synthesis 3, 1
- Age considerations: Use age-specific MCV parameters for patients under 17 years 1
Diagnostic Approach:
The initial workup should measure serum ferritin first 2. Low ferritin indicates iron deficiency, while normal or elevated ferritin with microcytosis should prompt evaluation of total iron-binding capacity, transferrin saturation, serum iron, and possibly hemoglobin electrophoresis 2. Patients with particularly low MCV in the absence of systemic iron deficiency should be evaluated for thalassemia syndromes 1.
Polychromasia
Polychromasia describes the bluish-gray appearance of young red blood cells (reticulocytes) on Wright-stained blood smears, representing the bone marrow's response to anemia or hemolysis 4, 5.
Key Clinical Points:
- Represents young reticulocytes: Specifically corresponds to Heilmeyer groups I, II, and III reticulocytes 4
- Indicates active erythropoiesis: Reflects bone marrow response to erythropoietin stimulation 4
- Common in hemolytic conditions: Frequently seen alongside other features of hemolysis such as elevated reticulocyte count, increased LDH, reduced haptoglobin, and elevated bilirubin 6, 5
Clinical Significance:
Polychromasia on blood smear correlates with reticulocytosis and indicates the marrow is responding appropriately to anemia. In hereditary spherocytosis, polychromasia is commonly observed along with microcytosis and spherocytes 5. The presence of polychromasia helps distinguish regenerative from non-regenerative anemias.
Spherocytes
Spherocytes are abnormally round red blood cells that have lost their central pallor and biconcave shape, most characteristically seen in hereditary spherocytosis but also in immune hemolytic anemias 5.
Key Clinical Points:
- Hereditary spherocytosis (HS): The classic condition featuring spherocytes, characterized by congenital hemolytic anemia with negative Coombs test 5
- Membrane defects: Result from deficiency of membrane skeleton proteins causing loss of membrane surface area relative to cell volume 5
- Clinical triad in HS: Anemia, jaundice, and splenomegaly with dominant inheritance pattern in typical cases 5
Diagnostic Considerations:
Spherocytes alone are not diagnostic—correlation with osmotic fragility testing, autohemolysis test, and family history is essential 5. In hereditary spherocytosis, osmotic fragility is increased (average 0.48 g/dL fresh blood, 0.65 g/dL incubated), and autohemolysis is markedly elevated (average 15.54%) but corrects with glucose addition 5.
Important Caveat:
While spherocytes are the hallmark of hereditary spherocytosis, microcytosis may be more prominent than spherocytosis in many cases, and laboratory findings often reflect microcytosis more than spherocytosis 5. Other morphologic features that may accompany spherocytes include acanthocytes, basophilic stippling, and pincered cells 5.
Clinical Integration: When evaluating a blood smear, the combination of these findings provides diagnostic direction. For example, microcytosis with polychromasia and spherocytes strongly suggests hereditary spherocytosis with active hemolysis 5, while isolated microcytosis without polychromasia points toward iron deficiency or thalassemia trait 2. The reticulocyte count and markers of hemolysis (LDH, haptoglobin, bilirubin) help distinguish between these conditions 6.