False negative single antigen bead assay: Is it always an effect of prozone?

Bead based flow cytometry and Luminex play a major role in identification of alloantibodies in renal transplant work-up. Strong sensitization events may lead to prozone phenomenon that can affect single antigen bead (SAB) assay and result in false negativity. However, this can also be due to high titer of other blocking antibodies. While methods like, heat inactivation, C1 inhibitor, Ethylene diamine tetra-acetic-acid and Dithio threitol treatment can remove interfering antibodies of complement and IgM, these methods are not optimal if false negativity is due to prozone effect, which is high titer of antibodies alone.

We hereby present a case of a highly sensitized renal transplant recipient with 64% panel reactive antibody positivity (PRA) and a subsequent negative SAB assay. This paradoxical finding hinted at SAB being a false negative result and serial dilutions were used to perform further tests.

Serum dilutions lead to positive flow based panel reactive antibody (PRA) and flow cytometry crossmatch (FCXM), with an increasing trend in FCXM.

In highly sensitized patients serial dilution should be considered during a transplant work-up to avoid missing any underlying antibodies. Serum dilution can be used as first option to circumvent prozone. Also, interference of other antibodies should not be labeled as prozone effect.