Sickle Cell Disease (SCD)

HbS is caused by replacement of a glutamate by valine in the beta-globin chain, which results in formation of hemoglobin polymers with reduced oxygen affinity.

Dx

Sickle cells and target cells are seen on blood smear in SCD, but not in trait.

Metabisulfite screen: causes cells with any amount of HbS to sickle; positive in both disease and trait.

Hb electrophoresis confirms Dx and quantify the different types of Hb

Common manifestations:

1. Bilirubin gallstones
2. Increased infection from autosplenectomy,
3. Osteomyelitis, most commonly from Salmonella
4. Retinopathy
5. Stroke
6. Enlarged heart with hyperdynamic features and a systolic murmur
7. Lower extremity skin ulcers
8. Avascular necrosis of the femoral head

Children often presents with dactylitis

The usual partition of Hb A and Hb S in sickle cell trait is 60:40. Sickle cell-beta(+)thalassemia is subclassified according to the percentage of Hb A present: type 1 has 3-5%; type II has 8-14%, and type III has 18-25%.

Therefore, the maximum percentage of Hb A level in sickle cell beta(+) thalassemia is 25%.