1. The importance of the saliva flow rate
The flow rate of saliva may influence the concentration of salivary hormones. This is the case if the hormone has a high molecular weight, or if the molecule is polar or ionic, making it difficult to accurately determine hormone levels. On the other hand, the concentration of small and non-polar molecules has not been shown to be dependent on the flow rate. This is a very important aspect in the utility of salivary testing. A classic example is the measurement of DHEA and DHEA-S in saliva. DHEA is a non-polar and small molecule, therefore the free fraction will pass from the vascular system into the salivary system. The salivary DHEA concentration specifically reflects the free fraction found in blood and, consequently, the hormone activity. DHEA-S is highly polar, due to its ionic composition, and cannot pass through the membrane passively. As a result, most of the DHEA-S present in saliva originates either from wounds in the mouth or possible active transporters discussed in reasearch, which would skew the results.
In blood, DHEA-S and DHEA are loosely bound to albumin, while in saliva, only the free active hormone is present. The concentration of DHEA in saliva is around 1:30 of that in serum, while the concentration of DHEA-S in saliva is even lower (1:500 to 1:1000), and is dependent on the saliva flow rate.