Saliva measures hormone that actually made it into tissue, because hormones pass through saliva gland tissue before getting into saliva. Blood measures hormones that eventually get to the tissue.
Saliva hormone testing is most useful for uncovering hormone imbalance. For example: discovering a woman has insufficient progesterone to balance high estradiol levels or; that high cortisol levels are interfering with the action of progesterone and testosterone. Once a hormone imbalance has been uncovered, treatment can be instituted.
Saliva collection is painless and more convenient for the patient, particularly when multiple samples are required. Multiple saliva samples can be collected by the patient during the menstrual cycle, and this enables a more in-depth look at hormonal balance than is available with a one-time blood measurement.