A transfusion safety device, intended to carry out a check prior to a transfusion of the blood group of packed red blood cells (PRBCs), contained in a bag and the blood group of the blood of an individual receiving a transfusion and determine the compatibility thereof to enable or prevent said transfusion, the device comprising: - a blood transfusion unit which in turn comprises: - a bag line, intended to be linked to the bag containing the PRBCs, - an individual's line, intended to be linked to an individual receiving the transfusion, - a connection to link the bag line to the recipient's line, and - a flow regulator arranged in the bag line, which controls the passage of the flow of PRBCs circulating between the bag line and the individual's line, - a first blood group checking device intended to establish the blood group of the PRBCs contained in the bag, - a second blood group checking device intended to establish the blood group of the individual receiving the transfusion, located at the end of a second bypass that starts from the individual's line, and - a control unit, intended to simultaneously read the results obtained in the first and second blood group checking devices, to establish the compatibility or incompatibility of both blood groups based on the reading of the results obtained in the first and second blood group checking devices, and to actuate the flow regulator in order to enable or prevent the passage therethrough of the flow of PRBCs, being the device characterised in that: - the first blood group checking device is located at the end of a first bypass that starts from the bag line, and in that - each one of the first and second blood group checking devices further comprises: - a first reaction chamber, wherein an anti-A reagent for the agglutination of A group antibodies is housed, linked to a first reading chamber, - a second reaction chamber, wherein an anti-B reagent for the agglutination of B group antibodies is housed, linked to a second reading chamber, - a third reaction chamber, linked to a third reading chamber for a control, and - a plurality of visual information elements linked to each one of the reading chambers (20,22,24). - a blood transfusion unit which in turn comprises: - a bag line, intended to be linked to the bag containing the PRBCs, - an individual's line, intended to be linked to an individual receiving the transfusion, - a connection to link the bag line to the recipient's line, and - a flow regulator arranged in the bag line, which controls the passage of the flow of PRBCs circulating between the bag line and the individual's line, - a bag line, intended to be linked to the bag containing the PRBCs, - an individual's line, intended to be linked to an individual receiving the transfusion, - a connection to link the bag line to the recipient's line, and - a flow regulator arranged in the bag line, which controls the passage of the flow of PRBCs circulating between the bag line and the individual's line, - a first blood group checking device intended to establish the blood group of the PRBCs contained in the bag, - a second blood group checking device intended to establish the blood group of the individual receiving the transfusion, located at the end of a second bypass that starts from the individual's line, and - a control unit, intended to simultaneously read the results obtained in the first and second blood group checking devices, to establish the compatibility or incompatibility of both blood groups based on the reading of the results obtained in the first and second blood group checking devices, and to actuate the flow regulator in order to enable or prevent the passage therethrough of the flow of PRBCs, - the first blood group checking device is located at the end of a first bypass that starts from the bag line, and in that - each one of the first and second blood group checking devices further comprises: - a first reaction chamber, wherein an anti-A reagent for the agglutination of A group antibodies is housed, linked to a first reading chamber, - a second reaction chamber, wherein an anti-B reagent for the agglutination of B group antibodies is housed, linked to a second reading chamber, - a third reaction chamber, linked to a third reading chamber for a control, and - a plurality of visual information elements linked to each one of the reading chambers (20,22,24). - a first reaction chamber, wherein an anti-A reagent for the agglutination of A group antibodies is housed, linked to a first reading chamber, - a second reaction chamber, wherein an anti-B reagent for the agglutination of B group antibodies is housed, linked to a second reading chamber, - a third reaction chamber, linked to a third reading chamber for a control, and - a plurality of visual information elements linked to each one of the reading chambers (20,22,24).