The invention relates to a device for characterizing an extracorporeal blood treatment carried out by an extracorporeal treatment apparatus (704, 802) that expels an extracorporeal treatment liquid discharged through a discharge system (706). The device includes a sampling means (102, 104) for sampling, throughout the treatment, a filtrate amount at the outlet of the treatment apparatus (704, 802) and moreover includes a determining means (112, 116, 118, 120) for determining at least one parameter, related to said treatment, by means of analyzing at least one filtrate sample that is collected by the collecting means (102, 104). The device also includes a means (302, 304) for instantaneously sampling the used dialysate at a time t during the dialysis.
