For the WCS-based and recombinant IgM EIA tests, the highest rate of cross-reactions were observed in syphilis (n = 20), rheumatoid arthritis (n = 21) and infectious mononucleosis (n = 30) with false positivity rates of 85/20%, 53/27%, and 10/19%, respectively. Here, CD40LG is linked to syphilis.