The blood sample for crossmatching should be within three days prior to the transfusion. Cross-matching refers to the use of blood serum of the patient who receives blood transfusion and the red blood cell suspension of the person who provides the blood, the blood donor’s serum and the red blood cell suspension of the patient who receives the blood transfusion, and the two are mixed for the agglutination test, and the cross-matching test is performed to make sure that there is no difference in blood type and obvious antigen-antibody reaction between the two bloods to ensure that no serious coagulation reaction will occur. Generally speaking, before transfusion, blood group identification and cross-matching procedures and examinations must be carried out, and the blood specimen for the recipient’s blood-matching test must be within three days before the transfusion, because when the blood is out of three days, the physiological and biochemical indicators of the blood may be abnormal, which may cause the blood to become unstable. Therefore, blood specimens from the recipient within three days should be used for cross-matching.