Metabolomics investigates changes in quantities of metabolites. However, blood plasma is a complex, heterogeneous mixture of components which undergo a variety of possible molecular interactions. In particular, many low molecular weight compounds (including drugs) can exist both ‘free’ in solution, bound to proteins or within organised aggregates of macromolecules. To study the effects of e.g. disease on these interactions we have developed a technique termed ‘interactive metabolomics’.