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There has been substantial interest in the use of saliva as a diagnostic medium for drugs of abuse because it can be obtained noninvasively. Although drugs of abuse have been investigated in saliva for more than a decade, the role of saliva remains uncertain. A clear picture is difficult to obtain because of variations in (1) the analytical methods used; (2) the dose regimen of subjects, which was either unknown or differed between studies; and (3) the elapsed time between drug intake and sample collection. This communication summarizes the studies on the quantitative determination of different drugs of abuse in saliva to elucidate the current status in this area. Marijuana, cocaine, phencyclidine, opiates, barbiturates, amphetamines, and diazepines (or their metabolites) have all been detected in saliva by various analytical methods, including immunoassay, gas chromatography/mass spectrometry, and thin layer chromatography. Initial studies with cocaine and phencyclidine suggest a correlation between saliva and plasma concentrations of these drugs, indicating a dynamic equilibrium between saliva and blood. Tetrahydrocannabinol, the active component in marijuana, on the other hand, does not appear to be transferred from plasma to saliva. However, tetrahydrocannabinol is sequestered in the buccal cavity during smoking and can be detected in saliva. These findings point to the potential role of saliva in the analysis of many illicit drugs. To clearly identify the role of saliva as a diagnostic medium for drugs of abuse, research efforts should be directed towards (1) performing systematic studies on correlations between saliva, blood, and urine and (2) determining the concentrations of drugs and their metabolites in saliva as a function of dose and time after intake.

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We have investigated the complex formation between an immobilized monoclonal antibody and antigens that differ in size about 50-fold. As a model system, we used an iodinated progesterone derivative and a progesterone-horseradish peroxidase conjugate as tracer and a monoclonal antibody as binding protein. The antibody was immobilized by four different methods: physical adsorption, chemical binding, and binding via protein G in the absence or presence of a protective protein (gelatin). These investigations have shown that the performance of competitive immunoassays is determined by a combination of factors: (a) the relative size of the analyte and the tracer, (b) the antibody density on the solid matrix, (c) the method of immobilization of the antibody, and (d) the binding constants between antibody-analyte and antibody-tracer. All of these interactions have to be considered in designing an optimal immunoassay. The smaller antigen can form a 3- to 35-fold higher maximal complex density than the larger antigen. Dose-response curves are less affected by the size of the tracer than by the binding constant with the antibody. A large enzyme tracer with a relatively low binding constant can, therefore, provide a more sensitive assay. On the other hand, the increase in complex density achieved with a smaller tracer yields a higher signal that in turn can provide a better signal-to-noise ratio in highly sensitive competitive solid-phase immunoassays. We have suggested a model for antibody immobilization that accounts for the interdependence of tracer size, complex formation, and antibody density. The methods described can be used to design and optimize immunoassays of predefined performance characteristics. The results are particularly useful for converting radioimmunoassays to enzyme immunoassays.

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