Commentary on Strickland et al. (2020): Do commodity purchase tasks provide clinically meaningful indices of motivation to consume drugs?

Abstract

Drug purchase tasks have the potential to measure clinically meaningful between- and intra-individual differences in strength of motivation to use drugs. These differences may guide decisions about degree of risk and need for treatment. Alcohol and drug purchase tasks are brief self-report measures that are modeled after laboratory drug self-administration paradigms used to quantify the strength of drugs as reinforcers, also known as drug demand. Individuals complete 10–20 hypothetical questions that ask how many standard units of a drug they would purchase and consume across a range of prices. Their responses are then used to generate three relatively distinct indices of strength of desire or motivation to use drugs that are central to theoretical and diagnostic models of addiction, but are not routinely collected as part of clinical assessments. First, the amount of the drug the individual would purchase/use if the drug was free (referred to as demand intensity) is collected; secondly, the greatest drug expenditure across the task items is collected (referred to as Omax, or output maximum). For example, purchasing five drinks priced at $10 would result in a $50 expenditure, which might be an individual's peak expenditure value. Thirdly, the rate of decrease in consumption as a function of increases in price (i.e. price sensitivity or elasticity of demand) is collected. A number of studies have demonstrated that alcohol and cigarette purchase tasks provide reliable and valid assessments of individual differences in these demand parameters. These differences then predict change in level of use over time and are acutely responsive to laboratory manipulations 1, 2 and clinical interventions 3-5. Strickland et al. 6 extend this literature with their timely and rigorous meta-analytical summary of the smaller but growing body of literature examining illicit drug purchase tasks. A relative challenge with drug purchase tasks is the lack of standardization of drug doses and varieties compared to alcohol standard drinks. This challenge may impact the accuracy of the specific quantities purchased more so than the validity of demand parameters as indicators of elevated risk for relapse or high-risk consumption. Strickland et al.’s results are generally consistent with the alcohol purchase task literature 7 and suggest that individual differences in demand show consistent medium effect size associations with cannabis and cocaine use and problems. One important conclusion of this meta-analysis, also consistent with the alcohol purchase task literature, is that demand intensity and Omax may be more clinically relevant indices of strength of motivation or problem severity than demand elasticity. An important research direction is to determine the extent to which drug purchase tasks have the potential to provide information beyond that offered by standard clinical measures of actual recent consumption in the same way that alcohol purchase tasks have. Given that there are myriad contextual factors that influence one's recent drinking and drug use (e.g. availability of drugs and alcohol for those who are underage or in an environment that prohibits use), purchase tasks may provide informative data on the likelihood of future consumption that could be useful in determining change in degree of clinical risk and need for intervention services. For example, an individual may report a desire to consume large amounts of an opiate if it were available or to drink at a level that could lead to a blackout, even if neither behavior occurred recently due to external constraints. Willingness to spend large amounts of money on drugs also indicates a level of motivation for use that may portend stable or increasing drug use over time, even if recent drug use levels are relatively low. Conversely, the presence of relatively low drug demand for an individual with high levels of recent drug use may suggest an impending reduction in drug use due to a shift in motivation or environmental context. Future research is needed to evaluate the utility of drug purchase tasks as dynamic indices of motivation that are responsive to both value enhancing stimuli (negative affect, drug and social context cues) and value reducing stimuli (treatment, next-day responsibilities, cognitive/visceral manipulations such as episodic future thinking or mindfulness 8). Alcohol demand can reduce immediately following an intervention and the extent of the reduction predicts subsequent changes in use 9, suggesting the potential clinical utility of these very brief measures for guiding drug use treatment planning and stepped-care decisions. Given that the availability and price of illicit drugs may fluctuate more than alcohol, and that self-reports of actual illicit drug use may expose individuals to legal or occupational sanctions, hypothetical drug purchasing scenarios may have unique utility in gauging strength of desire to use drugs. None.