Calculation of Hemoglobin from Blood Specific Gravities

The specific gravity of the red blood cells (normally about 1.10) so greatly exceeds that of t.he plasma that the difference between the gravities of whole blood and plasma can serve as a measure of the cell content of the blood. By assuming a constant hemoglobin concentration in the cells, one can estimate also the blood hemoglobin concentration from the gravity values. The calculation is based on two assumed constants; viz., the specific gravity of the red cells and the hemoglobin concentration of the red cells. Neither of these values is in fact constant. In some abnormal types of blood cells, hemoglobin concentration may fall 20 per cent below normal, with cell specific gravity showing a parallel decrease. However, changes in cell hemoglobin concentration and cell specific gravity parallel each other in such a way that their variations almost cancel each other in their effect on the relation of blood hemoglobin concentration to the concentration calculated from the gravities of whole blood and plasma. In consequence the error, even in grossly abnormal blood, is estimated to be only about 0.15 gm. of hemoglobin per 100 ml. of blood greater than the error attributable to the limit of accuracy of the gravity method used. Ashworth and Tigertt (1) first formulated the equation for calculating blood hemoglobin concentration from the specific gravities of whole blood and plasma. The present paper reports tests of the accuracy of gravitycalculated hemoglobin concentrations by comparison with hemoglobin values determined by precise gasometric methods in normal and abnormal human blood. The gravity determinations have been made by the copper sulfate method (2). A nomogram (Fig. 1) is presented for rapid calculation of hemoglobin from the whole blood and plasma gravities.