Question: how long to fast before cortisol blood test?
Answer: March 18, 2015. This article waslast modified on March 6, 2020. You may be required to rest before sample collection. For a salivary cortisol test, you may be instructed to refrain from eating, drinking, or brushing your teeth for a period of time (may be some time between 15 to 30 minutes) prior to the test.