Which test is primarily used to diagnose hereditary spherocytosis?

The osmotic fragility test is primarily used to diagnose hereditary spherocytosis because this condition is characterized by red blood cells that are spherical rather than the typical biconcave shape, leading to increased fragility and susceptibility to lysis in hypotonic solutions. During the osmotic fragility test, red blood cells are exposed to solutions of varying concentrations of saline. In patients with hereditary spherocytosis, the red blood cells will lyse at higher saline concentrations compared to normal cells, indicating their fragility.

The other tests mentioned in the options do not address the specific abnormalities associated with hereditary spherocytosis. For instance, a bone marrow biopsy is more useful for evaluating blood cell production and identifying other hematological conditions rather than diagnosing this specific condition. A hematocrit test measures the proportion of blood volume occupied by red blood cells but does not provide information about their morphology or fragility. The Coombs test is used to detect antibodies against red blood cells and is primarily utilized for diagnosing autoimmune hemolytic anemias, not for hereditary spherocytosis. Therefore, the osmotic fragility test stands out as the definitive diagnostic tool for identifying this hereditary condition.