Microcytosis

Definition 

Monocyte count above normal range.

Monocytosis is most commonly caused by chronic inflammatory or infective problems – consider TB, SBE, SLE, rheumatoid arthritis, temporal arteritis. Please see Primary Care Management for further detail.

Cigarette smoking is a common cause of mild monocytosis.

Myelodysplasia

The major alternative diagnosis to be considered in patients with monocytosis is myelodysplasia (WHO subtype chronic myelomonocytic leukaemia).  This is a clonal marrow disorder often causing peripheral blood cytopenias and macrocytosis in addition to the monocytosis.  It generally requires a bone marrow test for confirmation of diagnosis.  Treatments are available and may include blood transfusion or other therapies. 

Suggest however that this diagnosis is not discussed in primary care – there are many subtypes of myelodysplasia ranging from a very benign disorder requiring no treatment, to an extremely serious disorder very close to acute myeloid leukaemia.  Much of the literature available on the internet concentrates on the more aggressive subtypes.