Anti-Sd^a is strongly suspected if:

The presence of anti-Sd^a is strongly suggested when agglutinates are mixed-field and refractile. Mixed-field reactions occur when there are two distinct populations of red blood cells in the sample: one that reacts with the antibody due to the presence of the corresponding antigen and another that does not. This characteristic is indicative of anti-Sd^a because it frequently shows a mixed-field reaction when interacting with compatible red cells.

The refractile property of the agglutinates further supports the suspicion of this particular antibody, giving a visual indication that there might be a tendency for some red cells to bear the Sd^a antigen while others do not. This mixture can arise from prior antigen exposure, such as from transfusions or pregnancy, which makes the analysis of those agglutinates crucial in identifying the presence of anti-Sd^a.

Other options relate to factors that may affect transfusion reactions or antibody development but do not specifically indicate the presence of anti-Sd^a. For instance, transfusion history or blood group alone doesn't provide specific evidence of anti-Sd^a without accompanying mixed-field agglutination. The number of reactive panel cells, while it can suggest weak or incomplete antibody production, does not as strongly