Evaluation of Uncertainties Associated with the Determination of Community Drug Use through the Measurement of Sewage Drug Biomarkers

The aim of this study was to integrally address the uncertainty
associated with all the steps used to estimate community drug consumption
through the chemical analysis of sewage biomarkers of illicit drugs.
Uncertainty has been evaluated for sampling, chemical analysis, stability
of drug biomarkers in sewage, back-calculation of drug use (specific
case of cocaine), and estimation of population size in a catchment
using data collected from a recent Europe-wide investigation and from
the available literature. The quality of sampling protocols and analytical
measurements has been evaluated by analyzing standardized questionnaires
collected from 19 sewage treatments plants (STPs) and the results
of an interlaboratory study (ILS), respectively. Extensive reviews
of the available literature have been used to evaluate stability of
drug biomarkers in sewage and the uncertainty related to back-calculation
of cocaine use. Different methods for estimating population size in
a catchment have been compared and the variability among the collected
data was very high (7–55%). A reasonable strategy to reduce
uncertainty was therefore to choose the most reliable estimation case
by case. In the other cases, the highest uncertainties are related
to the analysis of sewage drug biomarkers (uncertainty as relative
standard deviation; RSD: 6–26% from ILS) and to the back-calculation
of cocaine use (uncertainty; RSD: 26%). Uncertainty can be kept below
10% in the remaining steps, if specific requirements outlined in this
work are considered. For each step, a best practice protocol has been
suggested and discussed to reduce and keep to a minimum the uncertainty
of the entire procedure and to improve the reliability of the estimates
of drug use.