Background: The results of human leukocyte antigen flow cytometry crossmatch (FCXM) are widely used when making transplant decisions. However, standardization/harmonization is required because the results vary between laboratories. A harmonized test method was reported to reduce interlaboratory variability. In this study, we evaluated the feasibility of establishing common cut-off values using a harmonized method in multicenter settings.
Methods: Six laboratories participated in the study and conducted FCXM using a harmonized test method. Tests were performed using two donor cells and 25 negative sera samples. As a negative control (NC), laboratory NC (Lab NC) and a common NC (Korea Organ Donation Agency [KODA] NC) were included simultaneously. Median fluorescent intensity (MFI) ratios were calculated for each NC, and means ±2 standard deviation (SD) and ±3SD were estimated for the MFI ratio.
Results: The suggested cut-off values based on the mean ±2SD and ±3SD of the MFI ratio data from 144 non-pronase T cell crossmatch after excluding one positive result were 1.6 and 1.9 using Lab NC and 1.3 and 1.5 using KODA NC, respectively. For pronase B cell crossmatch, values of 1.4 and 1.6 using Lab NC and 1.3 and 1.5 using KODA NC were obtained for the mean ±2SD and ±3SD of the MFI ratio, respectively. Inter-laboratory variations of non-pronase T cell crossmatch and pronase B cell crossmatch were less than 30% when using Lab NC and KODA NC.
Conclusions: Variations in FCXM between laboratories were within the tolerable range when the harmonized method and same negative sera were applied. Therefore, applying common cut-off values with a standardized protocol may be feasible in multicenter settings.

Keywords: Human leukocyte antigen, Flow cytometry, Crossmatch, Harmonized method, Cut-off