Screening whole effluent toxicity tests are cost/effective methods for detecting the presence of toxic concentrations of unknown pollutants, but the application must solve the problem associated with the effect of high and variable concentrations of nutrients in the effluent on the results of algal toxicity tests. This work proposes a cost/effective test, based on three dilution levels measured at a single point time and a discriminant model for establishing if this kind of complex samples, with difficult interpretation of dilution-response curves, should be considered toxic to algae. This procedure identified properly around 85% of the samples considered toxic by expert judgement.