Estimating the size of crack cocaine users in France: Methods for an elusive population with high heterogeneity (2020)

Background: Despite several sources corroborating an expanding market and increased visibility and greater diversity in users' profiles, very little is known about the number of crack cocaine users in France.
Method: The estimates rely on a single data source capture-recapture method. Annual data are extracted from treatment centres nationwide. To account for heterogeneity, we use an innovative zero-truncated geometric, regression-based estimator controlling for individual and centre characteristics. We use the well-known Zelterman estimator as a benchmark.
Results: The number of crack cocaine users received in treatment centres increased dramatically, from 3388 in 2010 to 5143 in 2017 (+52%). The estimated number of crack cocaine users is believed to have tripled over the course of the same period (from 9775, 95% CI [8288-11530] to 28983 [24876-33766], respectively), with prevalences below 1‰, similar to other European countries. The coverage rate (observed number/estimated number of users) decreased in a similar fashion, indicating lower utilization. In particular, females and younger users are underestimated by data from treatment centres.
Conclusion: The prevalence of crack cocaine use is fairly low but steadily increasing. The diversity in users' profiles is a challenge to prevention and public health policies that should expand their scope to a more inclusive perspective of what defines crack cocaine users. Our method overcomes several methodological issues (data sources, data linkage, heterogeneity) and can be easily applied to a wide range of settings.
Highlights:
Recent qualitative, in-the-field studies conducted in France have shown an increasing visibility and diversity in the profiles of crack cocaine users.
This study provides the first reliable estimates of crack cocaine users in France using an innovative capture–recapture framework accounting for high heterogeneity.
Similar to other European countries, crack cocaine use in France remains fairly low but is steadily increasing.
Our results show an increase in female and economically more advantaged users, the spreading of crack cocaine toward more advantaged users and new geographic areas.
As these groups are underrepresented in treatment, treatment centres should be provided the means to effectively handle the increasing heterogeneity of crack cocaine users.