We often "get away" with transfusing unmatched red blood cells because the incidence of unexpected antibodies in patients experiencing medical emergencies is thought to be relatively low (~3–5% is sometimes cited but with little solid evidence).
Antibody incidence may vary according to several factors:
- Genetic disposition
- Patient's underlying disease
- Number of prior transfusions
- Gender (childbearing patients may be exposed to foreign antigens via fetomaternal bleeds as well as transfusion)
- Concordance of antigen phenotypes of patients vs. blood donors in a given locale
In general, antibody incidence increases with the number of transfusions, although most antibody producers will respond within the first 3–4 transfusions. Antibody incidence is very high in transfusion-dependent patients, such as those with sickle cell anemia or thalassemia.
Regardless of likelihood, transfusing uncrossmatched blood to a patient with unexpected antibodies can result in a serious hemolytic transfusion reaction.
