In the method proposed by Rawles, a sample of plasma is diluted 1: 1 with an equal volume of deionized water.
percent plasma or serum water =
= (osmolality of diluted specimen) / ((osmolality of undiluted specimen) - (osmolality of diluted specimen)) * 100
In the modified method by Faye and Payne, a sample of plasma is diluted with X:Y with (volume sample) : (volume deionized water).
percent plasma or serum water =
= (osmolality of diluted specimen) / ((osmolality of undiluted specimen) - (osmolality of diluted specimen)) * ((volume of water) / (volume of specimen)) * 100
They preferred using the ratio of 1 part water to 2 parts specimen, so that the equation became:
percent plasma or serum water =
= (osmolality of diluted specimen) / ((osmolality of undiluted specimen) - (osmolality of diluted specimen)) * (1 / 2) * 100
