Ultimately, we want a large number of antibody molecules to be bound to each red cell. To achieve this, reactions require a large serum to red cell ratio. As a minimum in saline phases, 2 drops of serum to 1 drop of 5% red cells should be used to give a serum:cell ratio of 40:1 - ideally, the ratio should be 4 drops serum to 1 drop of 5% cells, i.e., a ratio of 80:1. If too high a concentration of red cells is used, there will be fewer antibodies/red cell, and a false negative can occur. The number of drops is important, as well as using red cell suspensions that are no stronger than 5% (usually 2 - 5%).
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| Antigen-Antibody Concentration |