At the indirect antiglobulin phase of testing, there is no agglutination between patient serum and screening cells. What is the most probable reason?

The situation described involves a lack of agglutination between patient serum and screening cells during the indirect antiglobulin test, which is a critical part of the blood bank testing process. The most plausible explanation for this observation is that the patient has an antibody directed against a low incidence antigen.

Low incidence antigens are present on the red blood cells of only a small percentage of the population. When screening cells are used, they typically contain a wider range of common antigens, making it unlikely that the specific low incidence antigen that the patient’s antibodies are targeting is represented in the screening cell panel. As a result, the absence of agglutination indicates that the antibody in the patient serum is not reacting with any of the antigens present on the screening cells.

This lack of reaction is significant as it suggests the patient's immune response is tailored to recognize a specific antigen that is not widely found amongst the routinely used screening cells, leading to a scenario where the intended agglutination reaction does not occur. Therefore, identifying the patient's antibody as one directed against a low incidence antigen is crucial for further testing and accurate serological assessment.