A medical apparatus for extracorporeal blood treatment and a method for determining blood parameter value have been provided. The medical apparatus has a control unit (9) configured for taking from a storage memory (10) a plurality of blood parameter value measures each made through the sensors (8) provided on the bloodline set of each of a machines plurality at different patient treatment sessions； all the blood parameter values relate to the same patient； receiving at least a corresponding laboratory measured value of the same blood parameter relating to the same patient and a laboratory measurement time； determining a correction factor function of a difference between a measured value made though the sensor and a laboratory measured value； 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.
