Can the interval between antibody identifications be increased for alloimmunized patients?

New alloantibody formation is unpredictable in patients who have been previously alloimmunized. Pretransfusion testing is designed to detect these antibodies while antibody identification (ABI) techniques are designed to identify the specificity of the antibody. Pretransfusion testing intervals are prescribed by regulatory and accrediting agencies, intervals for ABI in alloimmunized patients are not. Our institution evaluated the safety of increasing the interval from every 72 hours to 14 days. The current 72-hour interval was chosen at our institution to align with AABB Standard 5.14.3.2, which requires a pretransfusion specimen drawn within 3 days of the scheduled transfusion for potentially immunized patients.

Over 2 years, all ABI entries in the laboratory information system were screened. All cases of alloimmunized patients with an additional antibody specificity that developed within 14 days of a previous ABI were reviewed and confirmed by four transfusion medicine physicians.

Initially, 8948 entries were screened. Thirty patients were identified to have formed 33 newly identified clinically significant alloantibodies within 14 days. After further categorization, only 13 antibodies (0.15% of all ABIs, 0.47% of alloimmunized patients examined) were deemed to be newly formed clinically significant antibodies that would have led to a change in transfusion practice.

Retrospective analysis of ABI results over a 2-year period revealed that 0.47% of previously alloimmunized patients that have samples for pretransfusion testing develop a new clinically significant alloantibody in 14 days or less. While there would be significant resource advantages to increasing the duration between repeat ABI, it does not outweigh the risk of a potential hemolytic transfusion reaction.