(a) Determine whether the autoantibody has demonstrable specificity
or whether it may be masking an alloantibody.
1 Specific antibody may be seen with autoimmune hemolytic
anemia or with alloimmunization and coating of transfused donor RBCs (the direct
antiglobulin test may appear mixed-field). It is helpful to know the specificity (lgG or
complement) of the protein coating the RBCs. If lgG is coating the RBCs, antibody can
usually be eluted from the patient's cells; however, if only complement is coating the
patient's cells, antibody will not be eluted. The results must be carefully interpreted.
2 When there is specificity, blood lacking the corresponding
antigen should be selected for transfusion, if possible. Many specific warm
autoantibodies are directed against Rh-Hr antigenic determinants.
3 When the specificity of antibody on the RBCs differs from that in
the serum, it may be necessary to crossmatch with an eluate as well as serum.
4 Patients with autoimmune hemolytic disease may have a
positive direct antiglobulin test, without demonstrable circulating antibody. This is
because all antibodies have been absorbed onto the RBCs and, therefore, tests done
with the patient's serum will appear compatible. To be more certain of compatibility,
eluates prepared from the patient's coated cells may be used for crossmatching.
5 When antibody specificity cannot be determined, blood can be
crossmatched by the titration technique. The patient's serum can then be absorbed
with the cells of the weakest reacting donor and the absorbed serum tested for
additional antibodies. The donor cell may also absorb out an underlying alloagglutinin.
Responsibility for transfusion of blood that is incompatible "in vitro" should be shared by
the blood bank physician, and the patient care physician.
(b) The patient cannot be phenotyped with antisera requiring the
antiglobulin technique, because of the positive direct antiglobulin test. There may also
be spontaneous agglutination with high-protein antiserum. Elution at 45C before
testing may help to remove enough antibody from the cells to allow typing.
e. Some Technical Causes of Apparent Incompatibility. False positive
reactions may be caused by dirty glassware, bacterial contamination, chemical or other
contaminants in reagents (including saline), fibrin clots, and overcentrifugation.