Antibodies against denatured HLA class II molecules detected in luminex-single antigen assay

False-positive anti-HLA reactions may occur in Luminex-single antigen (SA) beads assays, and it is important to recognize them to correctly interpret the test. The purpose of this report is to describe a peculiar pattern of reactivity, characterized by positivity with beads coated with HLA-DRB1∗09:01, DRB3∗01:01, DRB3∗02:02, DRB3∗03:01, DPB1∗02:01, DPB1∗20:01 and DPB1∗28:01, that was observed in 141 of 8121 serum samples tested in our laboratory with three different lots of the same kit (LABScreen® SA, One Lambda). These 141 serum samples came from 56 different patients on the kidney transplant waiting list, corresponding to 1% of the patients. Of these, 10 males had never been transfused or transplanted. About 66% of the patients had positive reactions against self-antigen HLA-DRB3 alleles. No reactions against native HLA-DRB1∗09:01 were observed in flow cytometry crossmatch and in absorption/elution experiments, leading to the conclusion that the reactivity was due to antibodies against epitopes present in denatured forms of HLA-class II antigens. The occurrence of this reactivity pattern was associated with female gender and systemic lupus erythematosus (SLE).