Investigation of unexplained reactivity: antibody of unknown specificity (AUS).

When antibody screening results are positive in a patient"s sample, the next step is to identify the specificity of the antibody and plan for transfusion or treatment needs. Most often, the antibody can be identified at the transfusing facility; samples that are not resolved may be referred to an immunohematology reference laboratory. A portion of these referred samples may still not be resolved, and the antibodies in these cases have been termed "antibodies of unidentified or undetermined specificity". In this publication, we selected the term "unknown" for such antibodies. Local medical staff should be involved in the clinical management and transfusion recommendations for these patients. The flow charts described in this article are designed to guide the serologist through steps that may result in a defined antibody specificity, or they may not resolve the specificity, and thus, the antibody remains an "antibody of unknown specificity".