The pitfalls of prevalence estimation: the case of regular and dependent methamphetamine use in Australia

Objectives: The prevalence of regular and dependent methamphetamine use in Australia in 2013/14 was recently estimated indirectly using multipliers at 268,000 regular (defined as >monthly use in the past 6 months) and 160,000 dependent (defined as >3 on the Severity of Dependence Scale) methamphetamine consumers, respectively. We replicated this approach using more contemporary multipliers and alternate data sources to explore identified limitations with this approach. Design: Two multipliers were derived from surveys of methamphetamine consumption in Sydney (NSW) in 2003/04 (the original multiplier) and Melbourne (VIC) in 2010 (a more contemporary multiplier), and applied to the time series of treatment utilisation figures for NSW and VIC, along with ABS-estimated resident population figures. Results: Using the more contemporary VIC multiplier, we estimated numbers of regular (67,250, 95% CI = 50,312-91,635) and dependent (43,232, 95% CI = 34,477-57,947) methamphetamine users for Australia in 2013/14. This is approximately one-third of the figures reported previously. Our national estimates using the 2003/04 NSW multiplier applied to NSW treatment data (175,452, 95% CI = 124,514-256,575 for regular, and 103,761, 95% CI = 73,576-154,699 dependent, methamphetamine users respectively) are approximately two-thirds of the figures reported previously. Conclusions: We urge caution in using treatment multipliers to estimate the prevalence of illicit drug use, particularly during periods of dramatic changes in treatment system capacity, user behaviour, drug purity and policing practices.