The aim of this manuscript is to initiate a constructive discussion about deviations in measured concentrations of uraemic solutes; these deviations, if not perceived or handled appropriately, may lead to incorrect interpretations of the pathophysiological role of uraemic solutes and/or to erroneous therapeutic decisions. To come to an objective approach towards this problem, variability analysis of reported concentrations may be of help. Striking outliers should either be discarded or considered together with other values which are more consistent with the majority of reported data.