[When A is not A anymore: problems and pitfalls with blood group typing].

Correct blood group typing is a prerequisite for transfusion. In most cases blood group determination is without problems; however, in individual cases various factors can complicate blood group determination and sometimes lead to confusing findings. For a better understanding the clinician should have basic knowledge of blood typing. Blood group determination usually covers the AB0 blood groups, Rhesus and Kell systems; in addition, a direct Coombs test and an antibody screening test for the detection of irregular antibodies in the recipient are performed. Confusion of patients, blood samples, results or preparations can lead to severe consequences due to incompatible transfusion and must be prevented. In this context, bedside blood type testing before transfusion is of utmost importance. Problems in laboratory analysis as well as patient-related factors, such as the existence of irregular antibodies against red blood cells can complicate the immunohematology diagnostics. Certain medications, such as daratumumab, lead to a significantly increased complexity in laboratory analyses. Massive transfusions can lead to chimerism with more than one population of circulating red blood cells. Hematopoetic stem cell transplantation can also lead to a change in blood groups as well as chimerism. In addition, there are various other rare causes that can result in difficulties in blood group determination, such as rare blood groups or rare disease-associated phenomena. In the case of problems in blood group determination, early and close cooperation with transfusion medicine is essential for the clinician.