The main finding of our investigation was that the concentration of PTX3 in the synovial fluid was elevated in THA/TKA patients with PJI (compared to those who had aseptic prosthesis revision) and able to predict the infection with high accuracy (i.e., AUC values of 0.85 and 0.95 for EBJIS and MSIS criteria, respectively). The gene discussed is PTX3; the disease is infection.