Serum obtained in the context of a routine outpatient laboratory control (10 ml blood; no additional venipuncture) will be pre-screened for anti-HLA immunoglobulin G (IgG) alloantibodies using LabScreenMixed assays (One Lambda, Canoga Park, CA, USA). For identification of HLA antigen/allele specificities, pre-screen-positive samples will be subjected to single-antigen flow-bead (SAFB) testing (LABscreen Single Antigen assays, One Lambda). SAFB results will be documented as mean fluorescence intensities (MFI), and MFI levels >500 will be considered to be positive. Donor-specificity will be defined according to donor and recipient HLA typing results. Virtual panel-reactive antibody (PRA) levels will be calculated using specific software tools (http://www.eurotransplant.eu).