Bromsulphthalein or bromosulfophthalein (BSP) is a phthalein dye used in liver function tests. Determining the rate of removal of the dye from the blood stream gives a measure of liver function. Bromsulphthalein or bromosulfophthalein (BSP) is a phthalein dye used in liver function tests. Determining the rate of removal of the dye from the blood stream gives a measure of liver function.