The cell line typically used for cytotoxicity testing is the L929 mouse fibroblast cell. If appropriate for the end use of the product, other cell lines may be employed such as the rabbit cornea cells (SIRC) or embryonic lung cells (WI-38). There are three types of cytotoxicity tests: extract, direct contact and indirect contact. The nature of the sample and its intended use will determine which test is most appropriate. The extract test (minimum essential medium (MEM) elution test) is the most commonly used since it can be applied to a wide variety of raw materials and finished product configurations and extracts all exposed surfaces. The sample is extracted in the MEM used to culture the cells. The extract is then placed into contact with prepared cell cultures and the cells are observed microscopically after a defined incubation period. The cells are evaluated for signs of damage and reduced metabolic function and are graded according to a descriptive and numeric scale.