Pharmacology of drugs in saliva
Analysis
Ethanol can be analysed in saliva by the same headspace chromatographic (Jones 1978) or enzymatic methods (Jones 1979a) as used for blood. Dipstick or reagent strip tests for alcohol have been reported (Tu et al. 1992; Pate et al. 1993) but were found to be too unreliable for use in determining blood alcohol content Current enzymatic tests have proved more reliable as quantitative tests and several commercial tests for on-site or point-of-collection testing of saliva alcohol are available. An example of a point-of-collection quantitative test is the QED Saliva Alcohol Test (OraSure, Bethlehem, PA, USA). The saliva or oral fluid is collected by the donor with a cotton swab, which is applied to the test pad (Fig. 1). As the saliva moves along the reagent bar by capillary action, any ethanol present is oxidised by alcohol dehydrogenase to give acetaldehyde, with the simultaneous reduction of nicotinamide–adenine dinucleotide (NAD). This results in a cascade of electron donor–acceptor reactions catalysed by diaphorase, and involving FeCN and a tetrazolium salt, that proceeds to production of a purple-coloured endpoint. The length of the resultant purple-coloured bar on the QED device is directly proportional to the concentration of ethanol in the specimen. The alcohol concentration can be read directly from the height of the coloured reaction bar on a printed scale (mg/dL or mg% ethanol), just as in reading a thermometer (Fig. 1C). To obtain an accurate reading, the capillary must draw saliva all the way to the top of the device. This is signalled by the development of a purple colour within 5 minutes at the QA spot at the top of the ‘thermometer’ (Fig. 1C).
Since the saliva moves along the reagent bar and reacts directly with the indicator chemicals, any oxidant in the saliva can cause a false positive. The most common oxidant found in saliva is ascorbic acid, commonly added to fruit juices, sodas and soft drinks as a preservative. Ascorbic acid is absorbed in the gums and is still found in the mouth in amounts sufficient to give a false positive with the QED Saliva Alcohol test for up to 10 minutes after drinking some soft drinks and sodas. The QED Saliva Alcohol test comes in two ranges, 0–150 (which can be read from 0.01 to 0.15 g/dL) and 0–350 (which can be read from 0.02 to 0.35 g/dL). The first, lower, range is for US Department of Transportation (DOT) and driving under the influence (DUI) applications, and the second is for hospital and overdose applications.
An example of a headspace enzymatic assay is the On-Site Saliva Alcohol Assay for the qualitative detection of alcohol in urine and saliva (Roche Diagnostics, Nutley, NJ, USA; Ansys Technologies Inc., Lake Forest, CA, USA). The On-Site Saliva Alcohol Assay is similar to the QED Saliva Alcohol Test in that saliva is collected by the donor from his or her own mouth with a cotton swab, and the swab is applied to the specimen well. Since alcohols are volatile, alcohol vapours diffuse from the sample pad to the reaction pad, where they react with alcohol dehydrogenase and diaphorase. The hydrogen released is trans ferred to the tetrazolium salt and produces a highly coloured formazan dye. The presence of alcohol is indicated by the appearance of a purple plus sign (+) in the result pad. The On-Site Saliva Alcohol Assay does not have a control spot, so an external control must be run in each testing session on an additional test unit. Since the test detects alcohol vapours from the saliva, the saliva sample does not come into contact with the reagents, so there is no possibility of false positives from oxidising agents, such as ascorbic acid, in the saliva. However, the result is qualitative only. The cut-off is 0.02 g%, so a purple plus sign (+) indicates the presence of alcohol at a concentration greater than 0.02 g%.
Figure 1 The QED Saliva Alcohol Test: (A) collecting saliva; (B) filling the capillary; (C) interpreting the test results.
