The black rag test is a very old method of finding threadworms . It is used to gently wipe itchy or infected areas that are suspected of being wormy. See my pictures in Image Gallery of the threadworms in black cotton rags along with some skin infections I still had a year into treatment , many others have used this with much success . I believe someone named Brandi just used it and she seemed to have caught some threadworms(strongylida family of nematodes).