Mercury is a test that measures the amount of mercury, a toxic heavy metal, in the body. It is most often performed on a blood sample, though urine testing may also be used depending on the type of exposure being evaluated. Mercury can enter the body through certain foods (such as some types of fish), occupational or environmental exposure, and other sources. This test is used to check for exposure to mercury. It may be ordered when someone has symptoms that could be related to mercury exposure or when there is a known or suspected exposure through diet, work, or the environment. It can also be used to monitor levels over time in people being followed for elevated mercury.