Monocytosis is extremely nonspecific, usually not requiring investigation per se. Monocytosis has been defined as a sustained absolute increase in monocyte count greater than 800/mm3 to 1000/mm3 (Table 1). Transient monocytosis, relative or absolute, is common with recovery from myelosuppression, such as after chemotherapy. Relative or absolute monocytosis may occur in other myelosuppressed states, such as aplastic anemia. This is a favorable factor associated with a lower risk of neutropenic infection, perhaps because some phagocytic capacity is maintained. On the other hand, monocytosis is also very common with hematologic malignancies. In a patient with unexplained cytopenia, monocytosis can be an important clue to an underlying MDS/MPN.
Table1. Changes in Monocyte Number
Infectious Diseases
Mycobacterial infection is a common cause of monocytosis worldwide, related to its propensity for intracellular infection and tissue granuloma formation. Brucellosis and subacute bacterial endocarditis have also been associated with monocytosis. Certain viral infections, including influenza, varicella-zoster, CMV, and dengue, have been reported to cause monocytosis.
Connective Tissue Disorder
Connective tissue disorders, such as SLE and RA, have been associated with monocytosis in the context of chronic inflammation.
Hematopoietic Malignancies Monocytosis is common with MDS/MPN. CMML is defined as persistent peripheral blood absolute monocytosis greater than 1000/mm3 and relative monocytosis in peripheral blood (≥10%), absent Philadelphia chromosome, absent PDGFRA and PDGFRB rearrangements, less than 20% blasts in peripheral blood and BM, and evidence of dysplasia in one or more hematopoietic cell lineages. Juvenile myelomonocytic leukemia, a disease of children that shares pathologic features with CMML, results from defective RAS signaling. Acute myeloid leukemias (AMLs) involving the mono cyteline (acute myelomonocytic and acute monoblastic leukemias) may release substantial amounts of lysozyme (muramidase), which is toxic to renal tubules. Serum lysozyme was used to aid in the diagnosis of these leukemias. Monocytosis can result from other myeloid leukemias, MPNs, and lymphomas, particularly Hodgkin lymphoma.
