A blood test is a lab analysis of things that may be found in your blood. You may have blood tests to keep track of how well you are managing a condition such as diabetes or high cholesterol. You may also have them for routine checkups or when you are ill. Blood tests are very common. The complete blood count (CBC) is one of the most common blood tests. It is often done as part of a routine checkup. This test measures many different parts of your blood, including red blood cells, white blood cells, and platelets.