Saliva testing has been used in scientific testing for decades and has been shown to be highly accurate. It is the most reliable way to measure free, bioavailable hormone activity — those hormones actually doing their job at the cell level. Standard blood testing does not measure bioavailable hormone levels. Numerous scientific studies have shown a strong correlation between the levels of steroid hormones in the bloodstream and the bio-available levels of steroid hormones in saliva. Peter T. Ellison, Ph.D. of the Department of Human Evolutionary Biology, at Harvard University, Cambridge, MA, has used this method of hormone testing in cross-cultural comparisons of hormone levels among women living in industrialized vs. non-industrialized countries. (Human Reproduction vol.8 no.12 pp.2248-2258, 1993; Human Reproduction Vol.17, No.12 pp. 3251–3253, 2002.) His research in the field is the subject of his 2001 publication, On Fertile Ground: A Natural History of Human Reproduction (Harvard University Press). In addition, saliva hormone testing more accurately reflects tissue uptake and the response of hormones delivered through the skin in creams, gels, or patches than blood tests.