Within the drug trend monitoring arena, there is an increasing emphasis on the need for more timely identification and reporting of new patterns in drug consumption for policy-making purposes. However, there are documented problems with establishing the reliability and validity of results from emerging drug trend monitoring systems (EDTMS), which tend to be multi-indicator and use mixed methods. The aim of this article is to present a standardised and sequential approach to EDTMS development and refinement, i.e. grounded in the key elements of psychometrics, and illustrate its application using an established city level EDTMS. A five-step process is presented and exemplified, incorporating: (1) theoretical conceptualisation of the construct to be measured; (2) score construction; (3) weighting of indicators; (4) exploration of the prospect of categories (subscales); and (5) checking for external validity. The practical application of these validity enhancing stages are demonstrated using the Bergen Earlier Warning System. For non-traditional systems operating in a fast changing environment, an iterative review and refinement process (rather than a standardised system or instrument) has clear benefits, and can be adopted to enhance validity in existing EDTMS, or be used in the development of new models.