Sickle Cell Anemia
Definition - Laboratory diagnosis
Definition
It is autosomal disease due to structure gene defect resulting in abnormal Hb (Hb S) which has low affinity to O2 and polymerizes easily when exposed to low O2 tension (hypoxia) resulting in sickling of RBCs .
Hemoglobin S is due to substitution of valine for glutamic acid in Beta chain resulting from change of adenine (A) into thymine (T) in the DNA code .
Hypoxia e.g. high altitude , anesthesia , exercise and cold exposure causing deoxygenation of HbS .
The solubility of deoxygenated HbS is 50 times less soluble than deoxygenated HbA.
Thus deoxygenated hypoxia results in precipitation of HbS molecules inside RBCs causing sickling of RBCs which become more rigid .
Repeated sickling and unisickling causes irreversible damage of red cell membrane and cells become spherocytes and more fragile and are destroyed by RES .
Laboratory diagnosis
- Features of anemia . hemolysis and increased red cell production .
- Blood film sickled cells
- ESR and osmotic fragility are decreased .
- Hb electrophoresis , presence of HbS .