Screening of alloantibodies: stability of false positive results as a quality indicator

Screening of alloantibodies is required before each transfusion. As part of our blood bank quality assurance, we have developed a quality indicator to monitor these false positive antibody results. We have studied 25.162 samples: sera were first screened by automated column agglutination technology (CAT). Positive results were found in 1.365 of the 25.162 samples. False positive results, ie positive screening test followed by a negative identification, were found in 271 (20%) cases. In the 116 patients remaining (43%) no factor could be evidenced. Interestingly, the percentage of patients with false positive antibody screening was stable month after month. In our experience, this percentage is very stable, it may be used as an indicator of quality laboratory and its unusual variation allows to suspect alterations of the reagents (hemolysis, loss of specificity, sensitivity).