A test for the presence of protein (albumin) in the urine. A quantity of urine is carefully poured onto the same quantity of pure nitric acid in a test tube. A white ring forms at the junction of the liquids if albumin is present. However, a similar result may be obtained if the urine contains certain drugs or is very concentrated. A dark brown ring indicates the presence of an abnormally high level of potassium indoxyl sulfate in the urine.