It's called Monocytopenia. Without any clinical or physical complaints to compare, low monocytes are not really that significant, except that another type of WBC will be slightly higher. Monocytes help “clean up” after bacterial infections, phagocytizing bacteria. If your CBC differential came back only with a low monocyte count and not much else, don’t worry about it. Monocytes has mild contribution in Immunity so low number makes you prone to infection. Overall Nothing to worry about