An apparatus for extracorporeal treatment of blood comprising: a preparation line having one end configured for being connected to an inlet of a secondary chamber of a treatment unit, a semi-permeable membrane separating said secondary chamber from a primary chamber of the same treatment unit; a spent dialysate line having one end configured for being connected to an outlet of said secondary chamber; a control unit configured for commanding execution of the following steps: - causing a fresh treatment liquid to flow in the preparation line towards the secondary chamber at a flow rate (Qd in), the treatment liquid including lactate; - causing a used treatment liquid to flow in the spent dialysate line at a flow rate (Qd out); characterized in that the control unit is configured for commanding execution of the following further steps: - receiving one or more measured values of a parameter related to the concentration of lactate (Cd out) in the used treatment liquid flowing in the spent dialysate line; - receiving one or more values of a parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid flowing in the preparation line; - computing at least one value of a parameter (D, KT) indicative of the effectiveness of the extracorporeal blood treatment based on: • said one or more measured values of the parameter related to the concentration of lactate (Cd out) of the used treatment liquid; • said one or more values of the parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid; • at least one of: said flow rate (Qd in) of fresh treatment liquid and said flow rate (Qd out) of used treatment liquid. a preparation line having one end configured for being connected to an inlet of a secondary chamber of a treatment unit, a semi-permeable membrane separating said secondary chamber from a primary chamber of the same treatment unit; a spent dialysate line having one end configured for being connected to an outlet of said secondary chamber; a control unit configured for commanding execution of the following steps: - causing a fresh treatment liquid to flow in the preparation line towards the secondary chamber at a flow rate (Qd in), the treatment liquid including lactate; - causing a used treatment liquid to flow in the spent dialysate line at a flow rate (Qd out); - causing a fresh treatment liquid to flow in the preparation line towards the secondary chamber at a flow rate (Qd in), the treatment liquid including lactate; - causing a used treatment liquid to flow in the spent dialysate line at a flow rate (Qd out); characterized in that the control unit is configured for commanding execution of the following further steps: - receiving one or more measured values of a parameter related to the concentration of lactate (Cd out) in the used treatment liquid flowing in the spent dialysate line; - receiving one or more values of a parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid flowing in the preparation line; - computing at least one value of a parameter (D, KT) indicative of the effectiveness of the extracorporeal blood treatment based on: • said one or more measured values of the parameter related to the concentration of lactate (Cd out) of the used treatment liquid; • said one or more values of the parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid; • at least one of: said flow rate (Qd in) of fresh treatment liquid and said flow rate (Qd out) of used treatment liquid. - receiving one or more measured values of a parameter related to the concentration of lactate (Cd out) in the used treatment liquid flowing in the spent dialysate line; - receiving one or more values of a parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid flowing in the preparation line; - computing at least one value of a parameter (D, KT) indicative of the effectiveness of the extracorporeal blood treatment based on: • said one or more measured values of the parameter related to the concentration of lactate (Cd out) of the used treatment liquid; • said one or more values of the parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid; • at least one of: said flow rate (Qd in) of fresh treatment liquid and said flow rate (Qd out) of used treatment liquid. • said one or more measured values of the parameter related to the concentration of lactate (Cd out) of the used treatment liquid; • said one or more values of the parameter related to the concentration of lactate (Cdin) of the fresh treatment liquid; • at least one of: said flow rate (Qd in) of fresh treatment liquid and said flow rate (Qd out) of used treatment liquid.