American Board of Internal Medicine (ABIM) Certification Practice Exam

What test is primarily used to diagnose hereditary spherocytosis?

1. Complete blood count

2. Osmotic fragility test

3. Bone marrow biopsy

4. Hemoglobin electrophoresis

The correct answer is: Osmotic fragility test

The osmotic fragility test is the primary diagnostic tool used for hereditary spherocytosis. This condition is characterized by the presence of spherocytes, which are abnormally shaped red blood cells that are more prone to rupture when placed in a hypotonic solution. The osmotic fragility test assesses the ability of red blood cells to withstand changes in osmotic pressure. In this test, blood samples are exposed to varying concentrations of saline, and the degree of hemolysis (destruction of red blood cells) is measured. Normal red blood cells withstand lower concentrations of saline without bursting, whereas spherocytes demonstrate increased fragility and hemolyze at higher saline concentrations. This increased osmotic fragility is a hallmark of hereditary spherocytosis and aids in confirming the diagnosis. While a complete blood count may reveal anemia and reticulocytosis, it does not specifically indicate the presence of hereditary spherocytosis. A bone marrow biopsy is typically used to evaluate hematologic conditions when there are concerns about blood cell production rather than the shape and fragility of the cells themselves. Hemoglobin electrophoresis is beneficial for diagnosing hemoglobinopathies, such as sickle cell disease or thalassemia, but does not provide information regarding