First the fluid is either transudate or exudate. Vaginal lubrication is mostly provided by plasma seepage known as transudate from the vaginal walls. Rivalta test may be used to differentiate an exudate from a transudate. Definitions of the terms "transudate" and "exudate" are the source of much confusion. Hydrothorax is a type of pleural effusion in which transudate accumulates in the pleural cavity. This engorgement of blood vessels increases the volume of transudate from the vaginal walls. If the drop dissipates, the test is negative, indicating a transudate. For instance, an ultrafiltrate of blood plasma is transudate.