Monocytosis

Last updated Nov. 1, 2025, 4:50 p.m. by ivo

The monocyte reference range will vary depending on the age, gender and pregnancy status of the patient, as well as the laboratory performing the test. In general, the monocyte count is considered elevated when it is > 1 x 10⁹/L.

Causes

An increase in the number of monocytes in the blood most commonly occurs in association with an infectious or inflammatory process and is usually a transient, reactive phenomena.
Chronic inflammatory (e.g., Crohn’s disease, ulcerative colitis, rheumatoid arthritis, SLE) or infective conditions (e.g., chronic osteomyelitis, bronchiectasis, etc).
Recovery phase of a toxic insult to the bone marrow
Post-splenectomy.
Smoking is a common cause
Persistence or progression of a monocytosis, or association with other cytopenias or abnormalities of the full blood count, may raise suspicion of a myeloproliferative or myelodysplastic disorder or other systemic disease or malignancy.

Assessment

Evaluation for infections (EBV, TB, fungal, protozoal, rickettsial)
Signs and symptoms of autoimmune disease or inflammatory conditions
Medications
Previous splenectomy
Blood film to assess for dysplastic features or cytopenias
CRP/ESR

Management

Following an initial finding of an elevated monocyte count, it is reasonable to repeat the blood count in 4-6 weeks to assess progress.
If the monocytosis persists, with no identified reactive cause, and no associated cytopenias, observation, with full blood counts every 3-6 months is appropriate.

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When To Refer

Symptoms

Unxplained fevers > 38° C
Unintentional weight loss (10% of BW)
Night sweats

Physical examination

Hepatosplenomegaly
Lymphadenopathy

Laboratory

Neutrophil count (ANC < 1 x 10⁹/L)
Anaemia (Hb < 100 g/L)
Thrombocytopenia (PLT < 100x10⁹/L)
Immature cells on peripheral blood film (myelocytes, blasts, metamyelocytes)
Leukoerythroblastic / Dysplastic comments on peripheral blood film

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