Acute systemic toxicity assaying is the most commonly performed, and includes a single exposure with a 72-hour observation period. Repeat or continuous exposure periods are classified as subacute, chronic, and subchronic. The route of exposure should be chosen based on clinical relevance. Possible dose routes include implantation, intraperitoneal, intravenous, oral, and subcutaneous. Exposure to the test article may be in the form of an extract or direct exposure of the test article, as appropriate. Unlike acute systemic toxicity, the sub-acute, sub-chronic, and chronic toxicity studies, include full clinical pathology (clinical chemistry, hematology, and coagulation), necropsy and organ weights, as well as histopathology.