Medical apparatus comprising: - at least one machines for extracorporeal blood treatment having; ∘at least a blood treatment unit; ∘an extracorporeal blood line having a withdrawal branch adapted to withdraw blood from a patient access and to bring the withdrawn blood to the blood treatment unit and a return branch adapted to bring the blood from the blood treatment unit to the patient; ∘at least one sensor associated to the extracorporeal line and adapted to provide a measure related to a blood parameter value in the blood circulating in the extracorporeal blood line; - a control unit; - at least one storage memory for storing measures related to the blood parameter value, each measure being made through one of the sensors of said prefixed number of machines and each corresponding to different treatment sessions of patients on said prefixed number of machines; the control unit being configured for performing the following steps: - taking from the storage memory a plurality of measures related to the blood parameter value, each measure being made through one of the sensors of said prefixed number of machines at different patient treatment sessions, said plurality of measures relating to the same patient; - interpolating said plurality of measures made through the sensor for obtaining an interpolated curve defining a blood parameter trend along time; - receiving at least an actual control value of the same blood parameter relating to the same patient, said actual control value being measured at a monitoring time; - determining a correcting factor function of a difference between a prefixed number of measures made through the sensors and the actual control value of the blood parameter, - obtaining an actual value of said blood parameter by varying at least the last measure made through the sensor by means of the correcting factor; characterised in that the actual control value is a laboratory measured blood parameter value and in that the correcting factor is determined as a difference between the laboratory measured blood parameter value and the time corresponding value of the blood parameter in the interpolated curve. - at least one machines for extracorporeal blood treatment having; ∘at least a blood treatment unit; ∘an extracorporeal blood line having a withdrawal branch adapted to withdraw blood from a patient access and to bring the withdrawn blood to the blood treatment unit and a return branch adapted to bring the blood from the blood treatment unit to the patient; ∘at least one sensor associated to the extracorporeal line and adapted to provide a measure related to a blood parameter value in the blood circulating in the extracorporeal blood line; ∘at least a blood treatment unit; ∘an extracorporeal blood line having a withdrawal branch adapted to withdraw blood from a patient access and to bring the withdrawn blood to the blood treatment unit and a return branch adapted to bring the blood from the blood treatment unit to the patient; ∘at least one sensor associated to the extracorporeal line and adapted to provide a measure related to a blood parameter value in the blood circulating in the extracorporeal blood line; - a control unit; - at least one storage memory for storing measures related to the blood parameter value, each measure being made through one of the sensors of said prefixed number of machines and each corresponding to different treatment sessions of patients on said prefixed number of machines; - taking from the storage memory a plurality of measures related to the blood parameter value, each measure being made through one of the sensors of said prefixed number of machines at different patient treatment sessions, said plurality of measures relating to the same patient; - interpolating said plurality of measures made through the sensor for obtaining an interpolated curve defining a blood parameter trend along time; - receiving at least an actual control value of the same blood parameter relating to the same patient, said actual control value being measured at a monitoring time; - determining a correcting factor function of a difference between a prefixed number of measures made through the sensors and the actual control value of the blood parameter, - obtaining an actual value of said blood parameter by varying at least the last measure made through the sensor by means of the correcting factor; characterised in that the actual control value is a laboratory measured blood parameter value and in that the correcting factor is determined as a difference between the laboratory measured blood parameter value and the time corresponding value of the blood parameter in the interpolated curve.