BACKGROUND: Non‐ABO blood group (BG) alloantibodies can disappear over time, confounding compatibility testing and predisposing patients to delayed hemolytic transfusion reactions. The goal of this study was to analyze BG antibody disappearance after transfusion‐related alloimmunization in men.

STUDY DESIGN AND METHODS: The transfusion service records of 18,750 military veterans at a Department of Veterans Affairs (VA) medical center were screened to identify male patients with one or more BG alloantibodies and who had at least one type and screen performed after initial antibody detection (n = 304). Antibodies were categorized as to whether they were present at a patient's first antibody screening test (“preexisting”) or after initial negative testing (“hospital‐acquired”).

RESULTS: Overall, the evanescence of hospital‐acquired antibodies (108/222; 48.6%) was significantly higher (p < 0.0001) than that of preexisting antibodies (36/185; 19.5%). Half (54/108) of evanescent, hospital‐acquired alloantibodies disappeared within 6 months of detection, and all disappeared by 10 years. Evanescence of hospital‐acquired antibodies was dependent on duration of follow‐up testing, because antibodies tested 5 or more years after initial development demonstrated the highest evanescence rate (35/55; 64%; p < 0.01). Some evanescent antibodies (9/407; 2.2% of total antibodies) disappeared and reappeared one or more times without known antigenic reexposure (“multiple evanescence”). Among commonly observed alloantibodies, disappearance varied with antigenic specificity.

CONCLUSIONS: Approximately two‐thirds of BG alloantibodies disappeared within 5 years of induction, a rate higher than previously reported in mixed‐sex populations. Evanescence did not appear to be a random, first‐order decay process, as evidenced by the lower evanescence rate of preexisting antibodies.