A drop of blood is allowed to fall in copper sulphate solution of specific gravity 1.053 from a height of 1 cm. Specific gravity of 1.053 is equivalent to hemoglobin concentration of 12.5 grams/dl. The drop of blood gets covered with copper proteinate and remains discrete for 15-20 seconds. If the drop sinks within this time, its specific gravity is higher than that of copper sulphate solution (i.e. hemoglobin is >12.5 grams/dl) and hemoglobin level is acceptable for donation. If it floats, hemoglobin level is unacceptable. However, specific gravity of whole blood is also affected by total leukocyte count and concentration of plasma proteins. In the presence of leukocytosis (e.g. as in chronic myeloid leukemia) or hypergamma-globulinemia (e.g. multiple myeloma), hemoglobin value will be misleadingly high.