Increased Osmotic Fragility Test and Spherocytes
A positive (increased) osmotic fragility test is most closely associated with spherocytes.
Understanding Osmotic Fragility
Osmotic fragility testing measures the resistance of red blood cells (RBCs) to hemolysis when exposed to increasingly hypotonic solutions. The test evaluates the membrane integrity and surface-to-volume ratio of RBCs.
- Normal osmotic fragility (curve "B"): RBCs maintain their integrity until reaching a specific hypotonic threshold
- Increased osmotic fragility (curve "C"): RBCs lyse more readily in less hypotonic solutions, indicating decreased membrane stability
Why Spherocytes Show Increased Osmotic Fragility
Spherocytes demonstrate increased osmotic fragility due to their unique structural characteristics:
- Spherical shape with decreased surface-to-volume ratio
- Loss of the biconcave disc morphology of normal RBCs
- Reduced membrane deformability
- Decreased ability to accommodate water influx in hypotonic environments
This structural abnormality makes spherocytes more prone to rupture when exposed to hypotonic solutions, resulting in the characteristic increased osmotic fragility pattern seen in curve "C".
Differential Diagnosis of RBC Morphologies and Osmotic Fragility
| RBC Type | Osmotic Fragility | Explanation |
|---|---|---|
| Spherocytes | Increased | Decreased surface-to-volume ratio limits ability to expand in hypotonic solutions |
| Target cells | Decreased | Excess membrane relative to cell volume provides greater resistance to osmotic stress |
| Sickle cells | Variable/Normal | Abnormal hemoglobin structure rather than primary membrane defect |
| Elliptocytes | Normal/Variable | Elongated shape but generally normal surface-to-volume ratio |
Clinical Significance
Increased osmotic fragility is a hallmark laboratory finding in:
- Hereditary spherocytosis - Most common cause of congenital hemolytic anemia characterized by spherocytes and increased osmotic fragility
- Acquired spherocytosis - Secondary to autoimmune hemolytic anemia or other conditions
Diagnostic Considerations
- Flow cytometric osmotic fragility testing offers a more quantitative and time-effective approach for screening red cell membrane disorders with high sensitivity (100%) and specificity (98%) 1
- The presence of spherocytes on peripheral blood smear should prompt osmotic fragility testing when hereditary spherocytosis is suspected
- Some cases of hereditary spherocytosis may present with normal osmotic fragility initially but show abnormal results after 24-hour incubation at 37°C 2
Pitfalls and Exceptions
- Some microspherocytes, particularly in hemoglobin C-beta thalassemia, may paradoxically demonstrate decreased osmotic fragility due to cellular dehydration 3
- Preincubation of RBCs may not significantly improve the detection of very mild forms of hereditary spherocytosis 4
- A surprisingly high prevalence (approximately 0.9%) of increased osmotic fragility has been found among asymptomatic blood donors, suggesting mild forms may be more common than previously recognized 5
Understanding the relationship between spherocytes and increased osmotic fragility is essential for the accurate diagnosis of red cell membrane disorders and proper management of patients with hemolytic anemias.