Sickle cell anaemia is due to homozygous inheritance of abnormal Hb (Hb S-S). In heterozygous condition (Hb S-A), the disease is milder with no symptoms. This condition is called as sickle cell trait.
Sickle cell disease is more severe than sickle cell trait. This disease is not common in India. It occurs mainly in Negro race of Africa. The red cell contains 90 to 100% Hb-S. This disease is usually fatal by the age of 30.
The sickle cells are rarely seen in patient of sickle cell anaemia with peripheral blood smear. The other laboratory findings includes—Normocytic, Normochromic RBC, increased reticulocytes, increased platelets, decrease in osmotic fragility of red cell.
Sickle Cell Trait:
It is found in approximately 10% of American Negroes. The patient is heterozygous containing Hb S-A. The red cells of patient contain 20 to 40% of Hb-S and 60 to 80% of Hb-A. The patient lives normal life with no clinical symptoms. The laboratory findings are normocytic, normochromic RBCs.
The morphology of red cell is seen as normal in peripheral blood smear but these cells sickle under reduced oxygen supply. The degree of sickling however depends on the conc. of Hb-S in the cell. These cells are easily haemolysed because of their abnormal shape, and results in chronic haemolytic anaemia.
Laboratory diagnosis of sickle cell anemia is based on sickling of red cells under deoxygenated condition.
EDTA anti-coagulated venous blood or heparinized blood or capillary blood without anticoagulant can be used.
Whole blood is mixed with sodium metabisulfate (Na2S2Os). It is strong reducing agent that deoxygenates Hb. If the cell contains Hb-S, they become sickle shaped.
Two percent sodium metabisulfate, dropper, pasture pipette, petroleum jelly, slide, etc.
Place a small drop of blood in the center of the glass slide. Add equal drop of Na2 S2 O5 with the help of pasture pipette, mix it carefully with applicator sticks. Place cover-slip. Make sure that no air bubble is present. Using syringe and 19-gauge needle carefully seal the rims of cover-slip with petroleum jelly.
This sealing helps to avoid entry of oxygen inside the cover-slip from the atmosphere, wait for 15 min. Examine under 40 ? or high power objective.
The sickling is visible immediately in case of sickle cell diseased person. In case of positive results of sickle cell trait sickling is seen within 1 hour so the slide should be kept under observation upto 2 hours and then reexamined after 24 hours.
If still red cells are unchanged, it is reported as negative. In case of positive results, the red cells becomes sickle shaped, often with spikes. In some cases, red cells like holly leaf are seen in sickle cell trait.
This test does not distinguish between sickle cell anaemia and sickle cell trait.