Hi Mabel, As I understand it, the 4 drop method is required when normal ionic strength PEG solutions (i.e. 20% PEG in PBS) are used. Since the Hemo bioscience PEG reagent is Low Ionic Strength, only two drops are required for antibody screen/identification testing procedures. Do not hesitate to contact me should you have any questions or require further information. Thanks, Jimmy Lowery, MT(ASCP) Operations Manager Hemo bioscience j.lowery@hemobioscience.com 1-866-332-2835