First and forement, devices / techniques must screen for THC-delta-9 in oral fluid NOT THC-COOH and/or delta-11, etc. The latter is a metabolite found only in urine at any level that can be commonly detected, the former is the active ingreadient of marijuna and found in oral fluid.
Detection of THC-delta-9 in oral fluid has repeatedly been demonstrated to be possible from connsumption... up to 24 hours post consumption.
The following is one of several references available on this topic:
Relationship of (9)-tetrahydrocannabinol concentrations in oral fluid and plasma after controlled administration of smoked cannabis byHuestis MA, Cone EJ.Intramural Research Program,National Institute on Drug Abuse, National Institutes of Health,Baltimore, Maryland 21224. J Anal Toxicol. 2004 Sep;28(6):394-9
Understanding the relationship of (9)-tetrahydrocannabinol (THC) concentrations in oral fluid and plasma is important in interpretation of oral fluid test results. Current evidence suggests that THC is deposited in the oral cavity during cannabis smoking. This "depot" represents the primary or sole source of THC found when oral fluid is collected and analyzed. In this research, oral fluid and plasma specimens were collected from six subjects following smoking of cannabis cigarettes containing 1.75% and 3.55% THC. There was at least one week between each cannabis administration. Plasma specimens were analyzed by gas chromatography-mass spectrometry (GC-MS) and paired oral fluid specimens were analyzed by radioimmunoassay (RIA). In addition, one individual's oral fluid specimens were also analyzed by GC-MS. These data are unique in that they represent simultaneous or near simultaneous collection of oral fluid and plasma specimens in subjects following controlled cannabis dosing. The first oral fluid specimen, collected from one subject at 0.2 h following initiation of smoking, contained a THC concentration of 5800 ng/mL (GC-MS). The similarity in oral fluid and plasma THC concentrations following the dissipation of the initial "contamination" indicates the likelihood of a physiological link between these specimens. Recent studies have shown that sublingual or transmucosal administration of pure THC results in direct absorption of intact THC into the bloodstream, thereby bypassing the gastrointestinal tract. The current study demonstrates that THC is deposited in the oral cavity and remains for up to 24 h following cannabis smoking. The decline in THC oral fluid concentration over this time suggests that there may be absorption of THC into blood as previously shown with pure THC. Passive cannabis exposure studies appear to indicate that positive oral fluid tests for THC can occur shortly after cannabis smoke exposure, but results were negative within 1 h. Consequently, when very recent passive exposure to cannabis smoke can be ruled out, it is concluded that a positive oral fluid test provides credible evidence of active cannabis use.