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Diagnostic Tests: Spherocytes on Peripheral Smear
Overview — What These Are
The peripheral smear shows spherocytes, which are small, dense red blood cells lacking the normal central pallor. Their spherical shape results from loss of membrane surface area, making them less deformable and more prone to destruction, especially in the spleen. Spherocytes are a hallmark of hereditary spherocytosis, but can also appear in immune hemolysis.
Most Accurate Diagnostic Test
The most accurate diagnostic test for hereditary spherocytosis is the osmotic fragility test.
This test demonstrates that spherocytes lyse more easily than normal RBCs when placed in hypotonic solutions due to their decreased membrane surface area.
(Note: An elevated MCHC is supportive but not the most accurate test.)
Genetic Defect
Hereditary spherocytosis is caused by defects in red blood cell membrane proteins, most commonly involving:
Other possible but less common defects include band 3 and protein 4.2, but ankyrin/spectrin is the classic board answer.
When to Choose This Diagnosis/Test
You should think of hereditary spherocytosis—and choose osmotic fragility as the test—when the case involves:
Overview — What These Are
The peripheral smear shows spherocytes, which are small, dense red blood cells lacking the normal central pallor. Their spherical shape results from loss of membrane surface area, making them less deformable and more prone to destruction, especially in the spleen. Spherocytes are a hallmark of hereditary spherocytosis, but can also appear in immune hemolysis.
Most Accurate Diagnostic Test
The most accurate diagnostic test for hereditary spherocytosis is the osmotic fragility test.
This test demonstrates that spherocytes lyse more easily than normal RBCs when placed in hypotonic solutions due to their decreased membrane surface area.
(Note: An elevated MCHC is supportive but not the most accurate test.)
Genetic Defect
Hereditary spherocytosis is caused by defects in red blood cell membrane proteins, most commonly involving:
- Ankyrin gene mutation → leading to
- Spectrin deficiency → which
- Produces membrane instability and premature RBC destruction (extravascular hemolysis)
Other possible but less common defects include band 3 and protein 4.2, but ankyrin/spectrin is the classic board answer.
When to Choose This Diagnosis/Test
You should think of hereditary spherocytosis—and choose osmotic fragility as the test—when the case involves:
- Recurrent hemolysis
- Splenomegaly (often marked)
- Family history of anemia (autosomal dominant pattern in most cases)
- Increased MCHC on CBC (a high-yield clue)
- Spherocytes on peripheral smear
- Signs of hemolysis: jaundice, increased reticulocytes, gallstones (bilirubin stones)
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