Bone marrow evaluation of monocytosis

An absolute peripheral blood (PB) monocytosis is defined as greater than 1000 monocytes/μl. The differential diagnosis of an absolute peripheral monocytosis includes a reactive monocytosis and a neoplastic process that may be associated with various haematological neoplasms, that is chronic myelomonocytic leukaemia, myeloid and lymphoid neoplasms with eosinophilia and rearrangement of PDGFRB, juvenile myelomonocytic leukaemia, chronic myeloid leukaemia, as well as acute myeloid leukaemias with a prominent monocytic component. The diagnostic approach to a PB monocytosis, including appropriate evaluation of cytomorphological features and flow cytometric analysis of the PB, associated bone marrow findings, as well as the cytogenetic and molecular findings, will be discussed.