The Osmotic Fragility Test (Part II)

Another possibility could have been hereditary elliptocytosis. As above, the defect is a structural abnormality of spectrin or a deficiency of the red cell membrane protein (4.1), without anaemia and usually with no splenomegaly and only mild haemolysis. Most patients are asymptomatic, but the number of elliptocytes does not correlate with the severity of haemolysis. Osmotic fragility is usually normal (virginia.edu), but may be similar to that of hereditary spherocytosis, but with less marked changes. 'Idiopathic' acquired haemolytic anaemia is also associated with increased osmotic fragility. However, considering the position of the curve, it is more likely that the disease is associated with decreased osmotic fragility. The osmotic fragility curve of patients suffering from b-thalassaemia major is outside the normal range and its shape is also altered. Its is usually not sigmoidal, but shows a steady slope. The mean corpuscular fragility is decreased in all forms of thalassaemia, except in some a-thalassaemia heterozygotes. The subject being investigated may suffer from some sort of thalassaemia (apart from ß-thalassaemia major and some a-thalassaemia) since the whole curve is 'left shifted'. Since ß-thalassaemia major patients have severe transfusion dependent anaemia and nearly all patients have hepatomegaly and splenomegaly, the diagnosis of this disease could be relatively easily confirmed. The life span of patients with ß-thalassaemia major is short, most dying before adulthood. Sickle cell anaemia also shows the 'shifting' of the curve to the left. The decreased osmotic fragility is another consequence of the presence of HbS. The latter is poorly soluble in low oxygen tension situations, forming a gel and polymerising into fibrilary structures or tactoids. This distorts the red cells causing them to become rigid and sickled. But the sickle cell solubility test is the widely used method for screening for sickle cell anaemia. The sickle cell solubility test relies on the relative insolubility of HbS in concentrated phosphate buffers compared to HbA and other haemoglobin variants. HbS precipitates causing a cloudy solution. This test can be used to confirm any conclusion of sickle cell anaemia.

Conclusion. Although the curve is outside the normal range, suggesting that the patient may be suffering from a haemoglobinopathy, this alone does not provide enough data for tangible definitive conclusions, especially since the deviation from normality is only slight. Experimental conditions might have caused the decrease in fragility: a rise in pH decreases osmotic fragility. This explanation is plausible since the discrepancy is only in the position and not the overall shape of

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