The probability of obtaining compatible blood from related directed donors

Patients with alloantibodies may request directed blood donations from their relatives despite the lack of any scientific data suggesting that blood from these donors is safer. We have derived formulas that are applicable to any ethnic group for calculating the probability that a given class of relatives will have compatible blood for a given patient. These formulas apply to any non-X-linked blood group system (including the ABO and Rh systems) and are simple enough for routine blood bank use in advising patients as to how likely they are to obtain compatible donors. Use of these formulas may be helpful in counseling patients who may have difficulty in obtaining compatible directed donors because of antibodies or an unusual blood type. In general, the presence of an alloantibody should not automatically exclude a patient from utilizing directed donors. Moreover, for patients with antibodies to high-incidence antigens, directed donations from siblings may be the easiest way to obtain compatible blood.