What is most likely responsible for an incompatible antiglobulin crossmatch?

An incompatible antiglobulin crossmatch is primarily attributed to the presence of antibodies that react with antigens on the red blood cells. A positive direct antiglobulin test (DAT) indicates that the donor's red blood cells have been coated with antibodies, typically due to an immune reaction or previous exposure to blood products, which can cause agglutination during the crossmatch process. This reaction suggests that there may be an underlying incompatibility between the donor's red cells and the recipient's antibodies, leading to a positive result in the antiglobulin crossmatch.

In this scenario, if the donor's red cells test positive for the direct antiglobulin test, it means that their surface antigens are already binded by antibodies, which can react with the recipient's serum potentially leading to an incompatible crossmatch.

The other options do not directly support a typical scenario of incompatible crossmatching as effectively as a positive DAT does. For example, recipient red cells having a low-frequency antigen would likely not be the initiating factor for incompatibility unless the recipient had specific antibodies to that antigen. Anti-K antibodies in donor serum could cause a problem if the recipient has the K antigen, but this is less severe than a positive DAT where the donor cells are already coated with