In the ICU blood tests are usually done at least once a day. They measure such things as how the kidneys are working, cardiac markers (to make sure the heart is healthy) and levels of potassium (K+) and calcium (Ca++) as well as other elements. These are some of the indicators of how the body is working and can show the intensive care specialist how well a patient’s body is coping with their illness. Intra-arterial and intravenous lines (tubes placed in arteries and veins) are often used to monitor the body and, once established, allow rapid, reliable and pain-free access  for repeated blood tests.  Some conditions will require multiple repeated blood testing every few hours.