Phenolsulfonphthalein (Phenol red) is a pH indicator frequently used in cell biology laboratories. In medicine phenolsulfonphthalein is used as a test to help diagnose problems or disease of the kidneys. This test determines how well the kidneys are working. The test is based on the fact that phenol red is excreted almost entirely in the urine. Phenol red solution is administered intravenously, the urine produced is collected. By measuring the amount of phenol red excreted colorimetrically, kidney function can be determined. Phenolsulfonphthalein has being shown to be a mild estrogenic stimulus. Phenolsulfonphthalein as Phenol red thread test is used to diagnose dry eyes syndrome.