Abstract:Objective To analyze the characteristics of common clinical laboratory tests in children with infectious mononucleosis（IM） to provide referrence for early diagnosis of IM. Methods A total of 237 children with IM hospitalized in Xi'an Children's Hospital between January 2019 and June 2021 were selected as case group, and 150 healthy children for physical examination during the same period were served as control group. The peripheral blood absolute monocyte count, lymphocyte count, neutrophil to lymphocyte ratio（NLR）, monocyte to lymphocyte ratio（MLR） and erythrocyte distribution width（RDW） were detected and compared between two groups. The case group was divided into subgroups according to age and the proportion of heterotypic lymphocytes, and the above-mentioned indicators were also compared between sub groups. Results Compared with control group, the numbers of monocytes, lymphocytes and RDW increased significantly, and NLR and MLR decreased in case group（P＜0.01）. According to the age, 237 cases were divided into three groups(<3 years old, 62 cases; aged 3-6 years old,123 cases; >6 years old, 52 cases),there was no significant difference in the levels of MLR, NLR and atypical lymphocytes (%) among the three groups (P>0.05). According to the different percentage values of atypical lymphocytes, 237 cases were divided into 3 groups (atypical lymphocytes<10% 70 cases, 10%-20% 115 cases, >20% 52 cases), the levels of lymphocyte count, CD3*