Treating cells with a cytotoxic compound can result in a variety of cell fates:

• Necrosis, in which they lose membrane integrity and die rapidly as a result of cell lysis
• Cytostatic effect: the cells can stop actively growing and dividing (a decrease in cell viability)
• Apoptosis: the cells can activate a genetic program of controlled cell death.

The in vitro tests for cytotoxicity assess the response of cells in culture to direct contact with devices or to their extracts. Three categories of cytotoxicity tests are listed: extract test, direct contact test, indirect contact (filter diffusion) test.

The choice of one or more of these categories depends upon the nature of the sample to be evaluated, the potential site of use and the nature of the use. Extracts of test devices and materials are tested by exposure to the cell culture system. The presence of cytotoxic leachates is indicated by loss of cell viability.

To test cytotoxicity by direct contact, the agar diffusion or overlay assay is conducted by placing the test device or a representative portion directly on a mammalian cell layer that is protected from mechanical damage by a layer of agar.

Cytotoxic leachates diffuse into the cell layer via the agar, and toxicity is indicated by a loss of viable cells around the test device. The direct contact assay involves the placement of the test material directly on the cell culture medium, without the agar layer.

The filter diffusion test involves the exposure of cells grown on filters to test and control materials that are applied to the filters on the side opposite the cells.

The numerous methods used and endpoints measured in cytotoxicity determination can be grouped into the following categories of evaluation:

• assessments of cell damage by morphological means
• measurements of cell damage
• measurements of cell growth
• measurements of specific aspects of cellular metabolism