1. When sufficiently concentrated sickle cell hemoglobin containing solutions are exposed to a constant stream of CO2 gas, the hemolysates gel. This gelling phenomenon is indicative of the presence of S hemoglobin and cannot be obtained with any other type of human hemoglobin in the absence of S pigment. The lowest S hemoglobin concentration (Gm. per cent) of a hemolysate at which the gelling phenomenon can still be elicited is designated as its lowest gelling point.

2. A simple apparatus was developed to analyze the gelling phenomenon under standardized conditions. It could be shown that the lowest gelling points of hemolysates prepared from erythrocytes of the sickle cell trait (containing A + S hemoglobins), of the "C variant" (containing C + S hemoglobins), and from sickle cell anemia cells (containing S + F hemoglobins) differ distinctly. Further experiments suggest that the presence of A hemoglobin decreases the minimal amount of S pigment required for gel formation, and that type C hemoglobin reduces this amount even further. F hemoglobin seems to exert no significant influence on the gelling phenomenon. Serum albumin is also capable of decreasing the amount of S hemoglobin required for gelation.

3. A sickled erythrocyte is visualized as an S hemoglobin tactoid or gel, specifically influenced by the companion pigment which interacts with the S compound. Thus, in the sickle cell trait, a positive sickling test is not only caused by the presence of S hemoglobin, but also by its interaction with A hemoglobin. Only in the sickle cell anemia cells does sickling seem to depend solely upon the interaction of the S hemoglobin molecules.

4. The readily demonstrable differences of the lowest gelling points of hemolysates prepared from the various types of sickling red cells form the basis of the diagnostic gelling test which distinguishes sharply between sickle cell anemia and sickle cell trait erythrocytes. By this procedure atypical cases of sickle cell disease, for example, those whose erythrocytes contain C hemoglobin, may also be detected.

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