Monocytes are the largest white blood cells and have phagocytic and defensive roles in the body. The reference interval for the normal value of monocytes is 3%-10% in percentage and (0.1-0.6) x 10^9/L in absolute value. Low monocytes are defined as being below the reference interval for normal values. In general, the clinical significance of low monocytes is not great because the amount of monocytes in the body is very small, and needs to be analyzed in conjunction with other leukocyte test results. First of all, if only monocytes are low and other white blood cell components such as neutrophils, lymphocytes, eosinophils and basophils are normal and there is no discomfort, there is no cause for concern. If there is discomfort, further exclusion of hairy cell leukemia can be done by examining peripheral blood and bone marrow smears for cells with hair-like projections on the cell membrane. Secondly, if the monocyte ratio is low with the presence of other high white blood cell values, such as high neutrophils, lymphocytes, eosinophils, and basophils, a bacterial infectious disease may be present. If the monocyte ratio is high and the white blood cell count is low, such as neutrophils, lymphocytes, eosinophils, and basophils, it may be caused by immune deficiency, viral infectious diseases, and some diseases of the blood system, such as monocytic leukemia, multiple myeloma, etc. In addition, daily attention should be paid to more rest, better nutrition, and moderate daily outdoor exercise to help strengthen the body’s resistance and help prevent the occurrence of diseases.