Once considered the gold standard, this test measures the level of lactic acid, also known as lactate, in your blood. Lactic acid is a substance made by muscle tissue and by red blood cells, which carry oxygen from your lungs to other parts of your body. Blood Lactate is shown in millimoles per liter. These tests are typically referred to as “step tests” where the intensity is increased at certain time intervals to determine the 2mmol/l and 4mmol/l markers.