A blood test is an accurate measure of cortisol, and cortisol levels are related to stress levels. Blood cortisol levels reflect both protein-bound cortisol and free cortisol whereas a saliva or urine test only tests free cortisol levels. Cortisol in the blood is at its highest in the morning and lowest later in the day. Blood cortisol levels can also reflect more serious conditions and so it is always wise to consult your doctor if you are concerned about your cortisol levels.