Regardless of its wide use, FBS has still not gained recognition as an unequivocally defined media element. It is therefore left to discretion of producers to determine the peculiar composition of each every serum product. To fill any voids and to vie for parity across the board, serum free and Chemically Defined Media (CDF) have been devised for use in many cell laboratories.
However, given the high level of sensitivity of cells cultured in vitro, buyers are obliged to test for the compatibility of a specific product with their specific cell line in order to make a choice from the wide assortment of commercially developed fetal bovine serum. This usually entails an effort to adapt cells to new product unit for every turn. For instance, the buyer may test the cells in a mixture containing 50% of both the old and new serum to allow the cell to adapt to the new serum gradually.