If a T cell crossmatch is positive by AHG-CDC but negative by flow crossmatch, what could explain this?

A positive T cell crossmatch by AHG-CDC (antihuman globulin complement-dependent cytotoxicity) indicates the presence of antibodies that can cause a reaction against the T cells from a donor. AHG-CDC detects both IgM and IgG antibodies, but it is particularly sensitive to IgM, which can bind to antigens and cause complement activation leading to cell lysis in the presence of complement.

On the other hand, a flow crossmatch typically analyzes the binding of antibodies to target cells using flow cytometry, which is more sensitive for detecting IgG antibodies. The fact that the flow crossmatch is negative suggests that the antibodies present are likely not strong enough, or the specificity does not match, to cause a reaction detectable by this method.

Thus, the presence of IgM antibodies could explain a positive result by AHG-CDC (due to complement-mediated lysis) while resulting in a negative flow crossmatch, because IgM may not bind strongly to the targets in the way that allows flow cytometry to detect a positive result, especially if those IgM antibodies do not trigger a strong response or are not effectively bound in the assay.

In summary, the presence of IgM explains the discrepancy between the two tests