NORMOCYTIC ANEMIAS
HEREDITARY SPHEROCYTOSIS (HS)
Hereditary spherocytosis is a familial hemolytic disorder characterized by anemia, intermittent
jaundice, splenomegaly, and responsiveness to splenectomy. However, there is marked
heterogeneity of the clinical features, ranging from an asymptomatic condition to a fulminant
hemolytic anemia. The morphologic hallmark of HS is the microspherocyte, which is caused
by loss of membrane surface area and is characterized by an abnormal osmotic fragility in
vitro. Hereditary spherocytosis is the most common of the hereditary hemolytic anemias
among people of Northern European descent. The mode of inheritance in 75% of individuals
is autosomal dominant, while the other 25% have an autosomal recessive presentation. The
defect in the disorder is a deficiency of the key membrane protein, spectrin, and, to a lesser
degree, a deficiency of membrane protein ankyrin and band 3 deficiency, and protein 4.2
defects.
The normal red cell is capable of stretching 117% of its surface volume only if spectrin and
ankyrin are in the proper amount and fully functioning. The red cell membrane in patients
with hereditary spherocytosis is stretchable, but it is less elastic and can only expand 3%
before it ruptures.
Spherocytic red cells also exhibit problems with membrane diffusion. They are less able to
tolerate changes in their osmotic environment before they swell and lyse.
Laboratory diagnosis
Typically the patient with hereditary spherocytosis will manifest with anemia, jaundice, and
splenomegaly.
The laboratory diagnosis of hereditary spherocytosis is relatively easy in an individual with
elevated MCHC, RDW, and the presence of spherocytes.
The confirmatory tests, though, are labor intensive and usually not offered as part of a regular
laboratory menu of test items.
The osmotic fragility test, incubated and unincubated, is the test of choice to confirm a
diagnosis of HS. Red blood cells from patients suspected of having HS are subjected to varying
salt solutions ranging from isotonic saline (0.85% NaCl) to distilled water (0.0% NaCl).
Page | 1
, Under isotonic conditions, normal red cells reach equilibrium and have little hemolysis. As the
solutions become more hypotonic (less salt and more water), hemolysis occurs and is observed
at the initial appearance of hemolysis and after complete hemolysis. The level of complete
hemolysis is usually the only data reported on the patient sample.
Normal red cells initially hemolyze at 0.45% NaCl. Red cells from patients with HS have a
decreased surface to volume ratio and an increased osmotic fragility. They are less able to
tolerate an influx of water and therefore lyse at 0.65%. An increased osmotic fragility curve
will be seen in conditions other than HS such as autoimmune hemolytic anemia or any of the
acquired hemolytic processes. Conditions such as thalassemia and iron deficiency anemia will
show a reduced osmotic fragility (hemolysis at 0.20%) owing to the high number of target
cells, red cell morphology with a capacity to intake a high influx of water before hemolysis.
HEREDITARY ELLIPTOCYTOSIS (HE)
Hereditary elliptocytosis (HE) is a highly variable red cell membrane disorder with many
clinical subtypes. Its occurrence is 1:4000 in the population, affecting all racial and ethnic
groups. The inheritance is usually autosomal dominant. The membrane defect is a disordered
or deficient spectrin and proteins commonly associated with the alpha and beta spectrin
regions. A decreased thermal stability occurs in each of the clinical subtypes. There are four sub
types of hereditary elliptocytosis: common HE, Hereditary pyropoikilocytosis, Southeast Asian
ovalocytosis/ Southeast Asian ovalocytosis and spherocytic hereditary elliptocytosis
At least 15% of the cells appear elliptical, and often as many as 50 to 90% of red cells are
oval, whereas smears from normal subjects contain fewer than 15% elliptocytes.
HEREDITARY STOMATOCYTOSIS AND XEROCYTOSIS
Hereditary stomatocytosis is a rare hemolytic disorder in which the red cells have an intrinsic
defect related to sodium and potassium permeability. The defect, which is autosomal
dominant, is identified as a deficiency in the membrane protein, stomatin, thought to regulate
ions across the red cell channel. Because of this transport lesion, the intracellular sodium
content increases, leading to increased water content and a mild decrease in intracellular
potassium. The red cell swells and take on a morphology that appears as if the red cells have
slits or bars in the center. Peripheral smears show 10% to 30% stomatocytes with an elevated
MCV and decreased MCHC.
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HEREDITARY SPHEROCYTOSIS (HS)
Hereditary spherocytosis is a familial hemolytic disorder characterized by anemia, intermittent
jaundice, splenomegaly, and responsiveness to splenectomy. However, there is marked
heterogeneity of the clinical features, ranging from an asymptomatic condition to a fulminant
hemolytic anemia. The morphologic hallmark of HS is the microspherocyte, which is caused
by loss of membrane surface area and is characterized by an abnormal osmotic fragility in
vitro. Hereditary spherocytosis is the most common of the hereditary hemolytic anemias
among people of Northern European descent. The mode of inheritance in 75% of individuals
is autosomal dominant, while the other 25% have an autosomal recessive presentation. The
defect in the disorder is a deficiency of the key membrane protein, spectrin, and, to a lesser
degree, a deficiency of membrane protein ankyrin and band 3 deficiency, and protein 4.2
defects.
The normal red cell is capable of stretching 117% of its surface volume only if spectrin and
ankyrin are in the proper amount and fully functioning. The red cell membrane in patients
with hereditary spherocytosis is stretchable, but it is less elastic and can only expand 3%
before it ruptures.
Spherocytic red cells also exhibit problems with membrane diffusion. They are less able to
tolerate changes in their osmotic environment before they swell and lyse.
Laboratory diagnosis
Typically the patient with hereditary spherocytosis will manifest with anemia, jaundice, and
splenomegaly.
The laboratory diagnosis of hereditary spherocytosis is relatively easy in an individual with
elevated MCHC, RDW, and the presence of spherocytes.
The confirmatory tests, though, are labor intensive and usually not offered as part of a regular
laboratory menu of test items.
The osmotic fragility test, incubated and unincubated, is the test of choice to confirm a
diagnosis of HS. Red blood cells from patients suspected of having HS are subjected to varying
salt solutions ranging from isotonic saline (0.85% NaCl) to distilled water (0.0% NaCl).
Page | 1
, Under isotonic conditions, normal red cells reach equilibrium and have little hemolysis. As the
solutions become more hypotonic (less salt and more water), hemolysis occurs and is observed
at the initial appearance of hemolysis and after complete hemolysis. The level of complete
hemolysis is usually the only data reported on the patient sample.
Normal red cells initially hemolyze at 0.45% NaCl. Red cells from patients with HS have a
decreased surface to volume ratio and an increased osmotic fragility. They are less able to
tolerate an influx of water and therefore lyse at 0.65%. An increased osmotic fragility curve
will be seen in conditions other than HS such as autoimmune hemolytic anemia or any of the
acquired hemolytic processes. Conditions such as thalassemia and iron deficiency anemia will
show a reduced osmotic fragility (hemolysis at 0.20%) owing to the high number of target
cells, red cell morphology with a capacity to intake a high influx of water before hemolysis.
HEREDITARY ELLIPTOCYTOSIS (HE)
Hereditary elliptocytosis (HE) is a highly variable red cell membrane disorder with many
clinical subtypes. Its occurrence is 1:4000 in the population, affecting all racial and ethnic
groups. The inheritance is usually autosomal dominant. The membrane defect is a disordered
or deficient spectrin and proteins commonly associated with the alpha and beta spectrin
regions. A decreased thermal stability occurs in each of the clinical subtypes. There are four sub
types of hereditary elliptocytosis: common HE, Hereditary pyropoikilocytosis, Southeast Asian
ovalocytosis/ Southeast Asian ovalocytosis and spherocytic hereditary elliptocytosis
At least 15% of the cells appear elliptical, and often as many as 50 to 90% of red cells are
oval, whereas smears from normal subjects contain fewer than 15% elliptocytes.
HEREDITARY STOMATOCYTOSIS AND XEROCYTOSIS
Hereditary stomatocytosis is a rare hemolytic disorder in which the red cells have an intrinsic
defect related to sodium and potassium permeability. The defect, which is autosomal
dominant, is identified as a deficiency in the membrane protein, stomatin, thought to regulate
ions across the red cell channel. Because of this transport lesion, the intracellular sodium
content increases, leading to increased water content and a mild decrease in intracellular
potassium. The red cell swells and take on a morphology that appears as if the red cells have
slits or bars in the center. Peripheral smears show 10% to 30% stomatocytes with an elevated
MCV and decreased MCHC.
Page | 2
