The major pathway into saliva for most steroid hormones and other small, neutral molecules is by passive diffusion. The salivary glands are surrounded by dense beds of capillaries, and many blood components pass easily through the capillary walls, bathing the saliva glands. The neutral steroids diffuse readily through the lipo-protein membranes of the secretory cells in the saliva glands and into the saliva. Studies have shown that the speed of entry is rapid, and that stimulation of saliva flow does not affect the concentrations of these neutral steroids in saliva.
Serum proteins like albumin or corticosteroid binding globulin are too large to pass through the membranes of the salivary cells. Since 90-99% of the steroids in blood are bound by these specific or non-specific proteins, only the free, non-bound steroid molecules are able to pass into saliva. For this reason, salivary concentrations of steroids are much lower than those in blood. Blood components can find their way into saliva, however, through the serum-like gingival crevicular fluid, especially when periodontal disease is present, or through micro injuries in the mouth. Because of the much higher levels of steroids in blood, even the presence of very small amounts of blood contamination can falsely elevate steroid measurements in saliva.
Electrically charged steroids such as dehydroepiandrosterone sulfate (DHEA-S) are not able to diffuse through the neutral lipid membranes of the salivary cells. The mode of entry for DHEA-S into saliva is not clear. Formerly, it was thought that it enters saliva only by squeezing through the tight junctions between cells. It is too large to do this readily, however, which was thought to explain the very low percentage of DHEA-S that makes its way into saliva compared to the neutral steroids–typically around 0.1 % of serum levels in pure parotid saliva. More recent work has identified a large family of organic anion transport polypeptides (OATP) that actively transport molecules such as DHEA-S across membranes. It is now thought that such a mode of entry may exist for DHEA-S into the saliva glands as well. Due to the much higher levels of DHEA-S in blood compared to saliva, extra care must be taken to avoid blood contamination in the saliva. Saliva samples may be tested for blood with the Salimetrics Blood Contamination EIA kit, which checks for the presence of Transferrin, a large protein found in high concentrations in blood, but not in uncontaminated saliva.