The variable manifestations of infectious mononucleosis rarely cause clinicians to suspect primary Epstein-Barr virus or cytomegalovirus infection; consequently, costly diagnostic tests and unnecessary treatments are undertaken. Seventeen cases of clinically atypical and 11 cases of clinically typical infectious mononucleosis were diagnosed through screening for atypical and apoptotic lymphocytes in the peripheral blood samples by means of an automated hematologic analyzer. Atypical and typical cases did not differ significantly with respect to peripheral white blood cell counts; percentages of lymphocytes, atypical lymphocytes, CD4(+) lymphocytes, human leukocyte antigen--DR positivity in CD3 lymphocytes, or apoptotic cells in blood smear after incubation; or levels of aspartate aminotransferase, alanine aminotransferase, and lactate dehydrogenase. Only the percentage of CD8(+) lymphocytes was significantly higher in patients with typical infectious mononucleosis than it was in patients with atypical infectious mononucleosis. Because certain atypical cases of infectious mononucleosis display laboratory abnormalities that are characteristic of typical infectious mononucleosis, enhanced awareness can help in the diagnosis.

Human interleukin 10 (hIL-10), a product of monocytes, T cells, and B cells, shares extensive structural and functional similarity with viral IL-10 (vIL-10), a product of Epstein-Barr virus (EBV) replication. With two ELISAs, one that recognizes both hIL-10 and vIL-10 and the other specific for vIL-10, IL-10 was measured in serum or plasma and in saliva from 50 patients with acute EBV-induced infectious mononucleosis and from 19 normal subjects. In serum or plasma, 60% of the patients had measurable hIL-10 and/or vIL-10 and 18% had measurable vIL-10. In saliva, 20% of the patients had detectable hIL-10 and/or vIL-10 and none had detectable vIL-10. In contrast, hIL-10 and/or vIL-10 was undetectable in all 19 normal serum or plasma samples (P < .001 vs. patient samples). Among normal saliva samples, 21% had detectable hIL-10 and/or vIL-10 but none had detectable vIL-10. Thus, most patients with acute EBV-induced infectious mononucleosis transiently have abnormally high levels of circulating IL-10.