Because in the mouth, the Δ9-THC is massively found.
On the other hand, barely no traces of THC-COOH are found, which becomes virtually undetectable after a few tens of minutes after smoking a joint.
To effectively detect cannabis, a saliva test must therefore be calibrated to detect Δ9-THC.
The problem is that most current salivary tests are modified urine tests, and therefore detect THC-COOH.
Therefore, knowing the exact nature of the target molecule is crucial when choosing the saliva test to use!