The invention relates to a calculation and control system for determining whether a patient intended to follow several haemodialysis treatment sessions belongs to one group taken from several groups of hypertensive patients, the system comprising: a) means for determining the value of at least one parameter representative of the change in weight of the patient between dialyses for at least two sessions； b) means for determining the value of at least one parameter representative of the plasma conductivity of the patient for at least two sessions； c) means for determining the value of a parameter representative of the ion mass transfer of the treatment for at least two sessions； and d) programmed means for determining whether the patient belongs to a group of hypertensive patients depending on the change in weight of the patient between dialyses and/or on the plasma conductivity and/or on the ion mass transfer.
