Semiaccessible domain

It should be noted that in this definition all contribution from contamination present in the accessible domain (isopropanol wash) has been excluded. This is done to enhance the precision of the definition, i.e., to shield Rgsz from the possible influence of the widely varying drug concentrations in the accessible domain. These concentrations can range from zero in freshly shampooed hair to a very large number in freshly contaminated hair. The effective differentiation between drugs present in the accessible and semiaccessible domains is made possible by the dramatic difference in the cleansing power of dry isopropanol (as well as ethanol) and water. This distinction, however, cannot be achieved with methanol, since this resembles water in its wash properties. 

Rew/ on the other hand, is the worst-case estimate of the number of 1/2-h aqueous washes it would have taken to wash out the residual drug found in the hair digest if the residue had been contained in the semiaccessible domain. It is the worst-case estimate because the slope of the wash kinetics operating during the third wash is assumed to remain constant — a highly unlikely situation under nonplateau conditions. Rew values become very large numbers which, in the limit, approach infinity as perfect plateau conditions are attained. 

The critical element for differentiating between endogenous and exogenous drugs is the distribution of drugs between the semiaccessible and inaccessible domains. This distribution is quantitatively defined by the extended safety zone ratio (Resz) ... 

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