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 (females may get 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 that are given, although most antibody producers will respond within the first 3 - 4 transfusions. Antibody incidence in transfusion-dependent patients, such as those with sickle cell anemia or thalassemia, is very high.
Regardless of likelihood, transfusing uncrossmatched blood to a patient with unexpected antibodies can result in a serious hemolytic transfusion reaction.