A disposable cassette assembly for an extracorporeal fluid treatment medical apparatus of the type having at least a peristaltic pump, the cassette assembly comprising: - a rigid casing defining at least a first fluid flow chamber, said rigid casing also having an inlet to said first chamber and an outlet from said first chamber, - a first flexible tube having a first and a second end (17, 18) secured to the rigid casing to form a portion of a loop designed to receive a rotor of the medical treatment apparatus peristaltic pump, the first end of the first flexible tube being connected to said first chamber outlet, - a second flexible tube having a first end connected to the inlet of said first chamber, - a third flexible tube having a first end secured to the rigid casing and in fluid communication with the second end of said first flexible tube, characterized in that the cassette assembly further comprises: - a rigid tubular piece having: a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube; said first and second end connectors (25, 26) having an external prismatic surface and an internal cylindrical surface, the external prismatic surface of at least one of said end connectors bearing a radially protruding element which is designed to cooperate in use with a corresponding mating recess provided on a holder of the medical apparatus, the radially protruding element being in one piece with the rigid tubular piece and comprising side walls emerging from an external surface of the end connector and a terminal wall extending transversally to said side walls. - a rigid casing defining at least a first fluid flow chamber, said rigid casing also having an inlet to said first chamber and an outlet from said first chamber, - a first flexible tube having a first and a second end (17, 18) secured to the rigid casing to form a portion of a loop designed to receive a rotor of the medical treatment apparatus peristaltic pump, the first end of the first flexible tube being connected to said first chamber outlet, - a second flexible tube having a first end connected to the inlet of said first chamber, - a third flexible tube having a first end secured to the rigid casing and in fluid communication with the second end of said first flexible tube, characterized in that the cassette assembly further comprises: - a rigid tubular piece having: a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube; said first and second end connectors (25, 26) having an external prismatic surface and an internal cylindrical surface, the external prismatic surface of at least one of said end connectors bearing a radially protruding element which is designed to cooperate in use with a corresponding mating recess provided on a holder of the medical apparatus, the radially protruding element being in one piece with the rigid tubular piece and comprising side walls emerging from an external surface of the end connector and a terminal wall extending transversally to said side walls. a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube; said first and second end connectors (25, 26) having an external prismatic surface and an internal cylindrical surface, the external prismatic surface of at least one of said end connectors bearing a radially protruding element which is designed to cooperate in use with a corresponding mating recess provided on a holder of the medical apparatus, the radially protruding element being in one piece with the rigid tubular piece and comprising side walls emerging from an external surface of the end connector and a terminal wall extending transversally to said side walls.