An extracorporeal blood treatment apparatus comprising: - a supply line having an inlet end connectable to a source of treatment liquid and an outlet end connectable to an inlet port of a blood treatment device; - a waste line having an inlet end connectable to an outlet port of the blood treatment device and an outlet end connectable to a discharge of used treatment liquid; - an ultrafilter inserted in the supply line and having a semipermeable membrane dividing the ultrafilter into a first chamber and a second chamber, the ultrafilter presenting: ○ a first port connecting a first tract of the supply line to the first chamber, ○ a second port connecting the second chamber to a second tract of the supply line; - an air inlet line connected to the first chamber of the ultrafilter or to the first tract of the supply line; - at least one waste pump (34, 38) on the waste line; - at least one pressure sensor; and - a controller connected to the waste pump (34, 38) and to the at least one pressure sensor, characterized by the fact - the at least one pressure sensor is configured for detecting pressure in one of: ○ the second chamber of the ultrafilter, ○ the second tract of the supply line, ○ the waste line; - the controller is configured to carry out, an integrity test procedure comprising the following steps: ○ causing filling of the first chamber of the ultrafilter with air, optionally by at least commanding opening of an air valve or operation of an air pump located on the air inlet line, ○ after filling the first chamber with air, making pressure more negative or creating a negative pressure in the second chamber of the ultrafilter by operating the waste pump (34, 38), ○ verifying if, while the waste pump (34, 38) is running, the pressure sensed by the at least one pressure sensor reaches a set negative pressure threshold (Pt), ○ determining that the ultrafilter semipermeable membrane has a multi-fiber break if pressure sensed by the at least one pressure sensor during said verification step does not reach said set negative pressure threshold (Pt) within a set time interval, pressure being considered relative to atmospheric pressure present in an ambient where the apparatus operates. - a supply line having an inlet end connectable to a source of treatment liquid and an outlet end connectable to an inlet port of a blood treatment device; - a waste line having an inlet end connectable to an outlet port of the blood treatment device and an outlet end connectable to a discharge of used treatment liquid; - an ultrafilter inserted in the supply line and having a semipermeable membrane dividing the ultrafilter into a first chamber and a second chamber, the ultrafilter presenting: ○ a first port connecting a first tract of the supply line to the first chamber, ○ a second port connecting the second chamber to a second tract of the supply line; ○ a first port connecting a first tract of the supply line to the first chamber, ○ a second port connecting the second chamber to a second tract of the supply line; - an air inlet line connected to the first chamber of the ultrafilter or to the first tract of the supply line; - at least one waste pump (34, 38) on the waste line; - at least one pressure sensor; and - a controller connected to the waste pump (34, 38) and to the at least one pressure sensor, - the at least one pressure sensor is configured for detecting pressure in one of: ○ the second chamber of the ultrafilter, ○ the second tract of the supply line, ○ the waste line; ○ the second chamber of the ultrafilter, ○ the second tract of the supply line, ○ the waste line; - the controller is configured to carry out, an integrity test procedure comprising the following steps: ○ causing filling of the first chamber of the ultrafilter with air, optionally by at least commanding opening of an air valve or operation of an air pump located on the air inlet line, ○ after filling the first chamber with air, making pressure more negative or creating a negative pressure in the second chamber of the ultrafilter by operating the waste pump (34, 38), ○ verifying if, while the waste pump (34, 38) is running, the pressure sensed by the at least one pressure sensor reaches a set negative pressure threshold (Pt), ○ determining that the ultrafilter semipermeable membrane has a multi-fiber break if pressure sensed by the at least one pressure sensor during said verification step does not reach said set negative pressure threshold (Pt) within a set time interval, ○ causing filling of the first chamber of the ultrafilter with air, optionally by at least commanding opening of an air valve or operation of an air pump located on the air inlet line, ○ after filling the first chamber with air, making pressure more negative or creating a negative pressure in the second chamber of the ultrafilter by operating the waste pump (34, 38), ○ verifying if, while the waste pump (34, 38) is running, the pressure sensed by the at least one pressure sensor reaches a set negative pressure threshold (Pt), ○ determining that the ultrafilter semipermeable membrane has a multi-fiber break if pressure sensed by the at least one pressure sensor during said verification step does not reach said set negative pressure threshold (Pt) within a set time interval,