Saliva Diagnostics – Can It Help Prevent Caries?
http://www.checkdent.com/dental-blog/saliva-tests-and-caries-prevention.html?lang=en
Saliva Tests may improve patients’ monitoring and participation skills in caries-preventing measures.
With the help of saliva diagnostics development, attempts are being made to identify bacteria (streptococcus mutans and lactobacilli) and to take into account saliva flow rates and buffer capacity of saliva in order to identify patients with an increased risk of caries. Furthermore, these tests are supposed to improve patients’ monitoring and participation skills in caries-preventing measures.
Comparisons of caries frequency and bacterial concentrations showed only a small correlation between these two.
This discrepancy is more pronounced in children than adults. Only the absence of the streptococcus mutans correlates strongly to a lack of caries. This means that somebody who has no streptococcus mutans bacteria in the mouth does not have any caries either.
The increased incidence of lactobacilli is obviously an indicator of untreated carious defects. The concentration of lactobacilli in the oral cavity is influenced by the sugar content of food and the effectiveness of oral hygiene. However, lactobacilli are only of minor significance in the development of caries. By contrast, studies show that extremely low saliva flow rates and a simultaneously low buffer capacity correlate with caries frequency.
For this reason, neither an individual saliva test nor the totality of the saliva diagnostics commercially available today can precisely predict caries risk for an individual. This is proven by the often low correlation of the test results with the actual increase in caries. Above all, the border between the normal area and the risk area is unclear. Saliva tests performed only once are associated with a high chance risk.
A more precise prognosis of individual caries risk would necessitate repeated saliva tests, additional clinical examinations (such as the determination of the plaque index) and the ascertainment of individual dietary behavior as well as of fluoride intake.
Thus, saliva tests only serve (if at all) for the motivation, monitoring and education of patients. Lactobacilli correlate with the number of untreated carious defects and are a useful indicator for high sugar consumption. Careful oral hygiene is reflected in lower bacterial concentrations. Therefore, saliva tests can be helpful in motivating and monitoring patient participation in caries prevention.

I've searched for evidence comparing a group with positive saliva test to those with a negative test and measuring caries at end of a year. Cannot find even ONE. However, my mind is not closed, has anyone identified evidence to support the one time use for these costly tests?