This is the measurement of the so-called heterophil antibody. Heterophil antibodies are completely adsorbed by bovine red blood cells but not by guinea pig kidney cells. This feature distinguishes the heterophil antibodies due to IM from other causes. Monospot is the name given to a particular arrangement to test for the presence of heterophil antibody. Monospot is usually positive within four weeks after onset of the clinical symptoms. Titers of antibodies against Epstein Barr viral capsid antigen (VCA) and nuclear antigen (EBNA) are elevated.