Usefulness of Sweat Testing for the Detection of Cannabis Smoke

Several reports have demonstrated that sweat is a suitable alternative biological matrix for monitoring recent drug use (1)(2). This is based on the assumption that, in the context of the absorption/distribution/metabolism/excretion (ADME) cycle of drugs, a small but sufficient fraction of a drug is excreted in sweat and can be tested. The passage of lipid-soluble compounds from blood to other fluids or matrices is regulated by the substance’s p K a and by the pH of the other fluids or matices. A modified version of the Henderson–Hasselbach equation, which uses the p K a and the pH, allows theoretical calculation of the fluid-to-plasma concentration ratio (F/P ratio) (3). Drugs are generally incorporated into sweat by passive diffusion because of a concentration gradient in which only the free fraction of drug (unbound to proteins) diffuses through lipid membranes from plasma to sweat. Furthermore, because under normal conditions sweat, with a mean pH of 6.3, is more acidic than blood, basic drugs tend to accumulate in sweat.Two approaches are currently used in testing for drugs in sweat. The first is aimed at detection of recent use of drugs (<24 h) and involves only collection of sweat at a point in time. An immunochromatographic test of the sample then provides a qualitative result (4), or drugs in sweat collected on a cotton wipe can be extracted and subjected to confirmatory analysis (5). This approach is mainly oriented to identify individuals who are under the influence of drugs. The second approach is based on patch technology and allows monitoring of illicit drug use for time windows wider than those provided …