Understanding solid-phase HLA antibody assays and the value of MFI

As the practice of medicine becomes more reliant on imaging and laboratory tests, medical decisions will be increasingly based on numbers. Accordingly, following the introduction of solid-phase testing to the HLA testing repertoire, laboratory directors and physicians have employed preset mean fluorescence intensity (MFI) thresholds as the basis for decisions in the management of transplant patients. However, what do MFI values mean? The literature is rife with reports detailing numerous factors that influence antibody assessment including (but not limited to) sensitization history of the patient, level of mismatch between donor and recipient, presence of interfering substances in the serum, whether the antigen on multiplex beads is native or denatured, day-to-day and technologist variability, and the historical performance of an assay in a given institution. How are these variables incorporated into the interpretation of MFI values? Herein, the pitfalls and complexities of single antigen bead (SAB) testing and interpretation are discussed with specific attention to what can and cannot be inferred by MFI.