CD40LG and COVID-19: The urea dissociation test of ELISA was carried out with PBS containing 0 mol/liter, 1 mol/liter, 2 mol/liter, 4 mol/liter, 6 mol/liter, and 8 mol/liter urea in 22 RF-IgM-positive serum samples and serum from 14 COVID-19 patients that were positive for SARS-CoV-2 IgM in ELISA before urea dissociation.