The present invention relates to a method for evaluating a value representing the mass or the concentration of a substance comprised by a tissue or a bodily fluid of a patient, comprising the steps of a) determining a relation between one or more calculated or measured value(s) reflecting the mass or the concentration and a distribution space of the patient or an approximation thereof, and b) assessing whether the relation fulfils a criterion. It relates further to a controller, an apparatus and a device for carrying out the invention, further to digital storage means, a computer program product, and a computer program.