Handling the pretransfusion request for blood in complex cases

For the majority of acute or emergent transfusion requests, pretransfusion testing results are unremarkable and transfusion proceeds with no unanticipated delay. For a small percentage of cases, often in patents that are chronically transfused, the pretransfusion testing yields results that require further testing. Most often, the patient's serum contains an antibody easily identified by routine methods. In some, the determination of blood type or the identification of the serum antibody is a great challenge. These cases often occur in patients chronically exposed to red cell antigens, through transfusion or pregnancy. It is possible that limiting opportunities for potential sensitization could decrease the alloimmunization rate. In pregnancy, if a potentially clinically significant antibody is detected, antigen-negative blood must be transfused, either for the mother or for the baby (intrauterine transfusion). There are some countries that minimize the potential for alloimmunization by matching transfusion given to women with childbearing potential for at least K and often c. Thus, these antibodies are not formed before pregnancy, usually allowing the birth of the first K+ or c+ infant and limiting haemolytic disease of the foetus or newborn (HDFN). There is likely value in antenatal RhIg and definite value in postnatal RhIg in the prevention of anti-D. Both of these modalities may help prevent the initial alloimmunization of the mother to common antigens. When the mother forms clinically significant antibodies to multiple common antigens or to an antigen of high prevalence, provision of compatible red cells is difficult. If detected early enough and if the mother is healthy (even if the haemoglobin is below ‘normal’ donor levels), predelivery autologous donation is possible. If multiple donations are possible, one can be aliquotted into 2 or 3 portions for transfusion to the baby before or after birth, and the others for transfusion to the mother. For intrauterine transfusion, most often due to anti-D HDFN, the blood should be as similar to the mother's red cell phenotype as possible to limit further alloimmunization. The presence of alloantibodies can complicate the provision of red cells for chronic transfusion, for example patients with sickle cell disease or autoimmune haemolytic anaemia (AIHA. In many countries, these patients are extensively phenotyped (13 antigens) or genotyped (30+ alleles), thus knowing the antigens the patient lacks. Limited to extensive pheno- and geno-matching is performed by some countries. Punnett Square analysis is helpful in determining the degree of match available with the typed donor pool. There are rare AIHA cases in which the autoantibody has a relative specificity. Depending on the degree of haemolysis, it may be of value to match for the specificity during the acute crisis. There are cases for which rare blood is exceedingly difficult to obtain. This has been highlighted by presentations and publications of the ISBT Rare Donor Working Party. Antibodies to antigens of very high prevalence require local, regional or national rare donor programmes to obtain red cell products. Obtaining Rhnull, Ko and MkMk are known to be most difficult, but even D- U- red cells can be difficult to find on a continuous basis. The use of regional, national and international collaborative centres with inventories of rare red cells is ultimately what enables these patients to receive the blood they need to survive.