Antibody detection and evanescence considerations. Upon initial RBC exposure (red arrow), 1 or multiple RBC alloantibodies may be generated. Once an antibody concentration gets above a certain level (dotted line), it can be detected by blood bank methods. Over time, that antibody may evanesce and fall below the level of detection (white arrow). If the patient needs a transfusion at a later time point (blue arrow), the antibody may not be detected. Of note, if an antibody screen was not completed within weeks of the initial transfusion, the antibody or antibodies may not have been detected at all. If the patient was seen at a different hospital in the United States at the time point indicated by the blue arrow, that hospital will most likely be unaware of an antibody or antibodies previously detected by the initial hospital. Figure generated with the assistance of R. George Hauser, MD.