Spherocytes and Poikilocytosis: Definitions and Clinical Significance
Spherocytes are abnormally round red blood cells that have lost their normal biconcave disc shape and central pallor, while poikilocytosis is the general term for the presence of abnormally shaped red blood cells of various types on a blood smear.
Understanding Spherocytes
Spherocytes are a specific type of poikilocyte characterized by:
- Loss of central pallor due to their spherical rather than biconcave shape
- Reduced surface area-to-volume ratio, making them less deformable
- Decreased flexibility and mechanical stability, leading to premature destruction in the spleen
These cells are classically associated with hereditary spherocytosis, a red cell membrane disorder caused by defects in membrane skeletal proteins (particularly spectrin) that result in loss of membrane cohesion and progressive surface area loss 1. The spherocytes lack the strength, durability, and flexibility to withstand circulatory stresses, leading to hemolytic anemia 2.
Understanding Poikilocytosis
Poikilocytosis is a broader descriptive term encompassing any variation in red blood cell shape. While occasional nonspecific poikilocytes appear in normal blood smears, dominance of one or more specific poikilocyte forms usually indicates a specific anemia or organ system disease 3.
Common Types of Poikilocytes Include:
Round forms:
- Spherocytes (membrane disorders)
- Stomatocytes (acute alcoholism, liver disease)
- Target cells (liver disease, hemoglobinopathies)
Elongated forms:
- Elliptocytes/ovalocytes (hereditary elliptocytosis)
- Dacryocytes/teardrop cells (myelofibrosis, "spent" polycythemia vera)
- Sickle cells (sickle cell disease)
Fragmented forms:
- Schistocytes (microangiopathic hemolytic anemia)
- Echinocytes/burr cells (uremia, liver disease)
- Acanthocytes/spur cells (severe liver disease, abetalipoproteinemia)
Clinical Context and Diagnostic Pitfalls
Critical distinction: In contrast to membrane disorders where spherocytes predominate, conditions like pyruvate kinase (PK) deficiency typically show unremarkable red cell morphology with only nonspecific anisocytosis and poikilocytosis 4. This is a key diagnostic pitfall—the absence of prominent spherocytes does NOT exclude hemolytic anemia.
Important Clinical Associations:
Poikilocytosis with specific disease states:
- Gastrointestinal parasitism, chronic diarrhea, hepatic and renal disease are significantly associated with moderate-to-marked poikilocytosis, often reflecting iron deficiency from chronic blood loss or iron-restricted erythropoiesis 5
- Spiculated cells (echinocytes, acanthocytes) appear more frequently with hepatic and renal disease 5
- Post-splenectomy: Echinocytes may comprise 3-30% of cells in conditions like PK deficiency 4
Age-Related Considerations:
Poikilocytosis is more prevalent in young animals and children, with elliptocytes, dacryocytes, and schistocytes predominating in younger individuals 5. This physiologic variation should not be mistaken for pathology.
Diagnostic Approach
When evaluating poikilocytosis:
- Identify the predominant poikilocyte type—this narrows the differential diagnosis significantly
- Correlate with hemolysis markers (reticulocyte count, LDH, haptoglobin, bilirubin)
- Consider membrane disorders when spherocytes dominate
- Consider enzyme deficiencies when hemolysis exists but morphology is relatively unremarkable
- Evaluate for iron deficiency when moderate-to-marked poikilocytosis is present, particularly with chronic disease 5
The presence of spherocytes specifically should prompt evaluation for hereditary spherocytosis through osmotic fragility testing, flow cytometry, ektacytometry, or genetic testing 1, while nonspecific poikilocytosis requires systematic exclusion of immune-mediated hemolysis, membrane defects, unstable hemoglobins, and metabolic disorders 4.