Low blood mononuclear cell ratio is generally not clinically significant and can be reviewed in a month without special treatment. However, there is a situation that needs to be alerted, patients with hairy cell leukemia will have a decrease in monocytes. Blood mononuclear cell ratio, that is, monocytes accounted for the proportion of all white blood cells, usually monocyte ratio increased more common, viral infection mononuclear ratio increased, the monocyte ratio of normal people is 3%-8%, and its absolute value of (0.12-0.8) × 10^9/L. Hairy cell leukemia presents with trilineage reduction, mononucleosis, splenomegaly, and ciliated cytoplasmic prominence leukemic cells can be found in peripheral blood and bone marrow. Specialist consultation for bone marrow examination is recommended if abnormal blood counts, fever, cough, and bleeding manifestations are found.