Another rare manifestation of mononucleosis is erythema multiforme. Some studies indicate about 80–90% of patients with acute Epstein-Barr virus infection treated with such antibiotics develop a red, diffuse rash.
Diagnosis of acute infectious mononucleosis should also take into consideration acute cytomegalovirus infection and Toxoplasma gondii infections. These diseases are clinically very similar by their signs and symptoms. Because their management is much the same, it is not always helpful, or possible, to distinguish between EBV mononucleosis and cytomegalovirus infection.
Early in the course of the mono, blood tests may show an increase in one type of white blood cell (lymphocyte). Some of these increased lymphocytes have an unusual or “atypical” appearance when viewed under a microscope, which suggests mono.