Interpretation of saliva drug results
The foremost question in the application of saliva testing to forensic casework is ‘What is the relationship of saliva positive results to blood drug concentrations?’ Drug concentrations in saliva reflect the free, unbound parent drug and lipophilic metabolites that circulate in the blood.
Since these are the forms of the drug that cross the blood–brain barrier and affect performance and behaviour, saliva is a good specimen for detecting patient compliance with medication, drug involvement in driving behaviour, fitness for duty, or impairment of performance for many drugs. However, efforts to use saliva concentrations to predict blood free-drug concentrations, cerebrospinal fluid (CSF) concentrations or degree of performance impairment in individuals have not reached the accuracy associated with blood measurements. Without knowing the instantaneous saliva pH, saliva drug concentrations may not be extrapolated to give blood drug concentrations. When appropriate cut-off concentrations are employed, saliva drug presence may be associated with recent drug use and, in some cases, with being under the influence of the drug.
Saliva concentrations can be exceptionally high when the route of administration is smoking of the drug, such as with marijuana, cocaine, methamfetamine and heroin, sublingual or buccal adsorption, or snorting of the drug. In cases that involve these routes of admin istration, 2 to 4 hours must elapse before the contamination of saliva by the remains of the ingested drug are cleared from the mouth and saliva concentrations reflect plasma levels. Marijuana is the exception. D9-THC found in saliva most likely comes from cannabinoids deposited in the oral mucosa as a result of smoking the drug. However, saliva concentrations of D9-THC follow the same time course as the appearance and decline of physiological indices of marijuana’s pharmacological effects. The rate of clearance of D9-THC from the oral tissues appears to match the binding and clearance of D9-THC from the central nervous system site of action of cannabinoids.
