The invention provides for a process for preparing a sensitivity control for blood group determination including dissolving an amount of an antigen in water to give an antigen solution of known concentration, contacting the antigen solution with cells to allow insertion of antigen molecules into the cell membranes of the cells to give transformed cells or contacting the antigen solution with cells that have been modified by the insertion of a linker molecule into the membranes of the cells to allow attachment of antigen molecules to the linker molecules to give transformed cells, washing the transformed cells to give a transformed cell solution, and determining the concentration of the transformed cell solution to enable the solution to be used as a sensitivity control for blood group determination.