What is Monocytosis?

Monocytosis refers to a monocyte count > 0.8 ×10⁹/L. It can be reactive to infection, inflammation, or signify haematological malignancy if persistent.

Common Causes

Chronic infections (e.g. tuberculosis, endocarditis)
Autoimmune diseases (e.g. systemic lupus erythematosus, rheumatoid arthritis)
Recovery phase of acute infections
Myelodysplastic syndromes (e.g. chronic myelomonocytic leukaemia - CMML)
Malignancy-associated inflammation
Splenectomy (reactive monocytosis)

Stepwise Clinical Approach

Step 1: Confirm and Repeat

Confirm persistent monocytosis on a repeat sample 2-4 weeks later to exclude transient rise from infection.

When repeating, also request:

CRP and ESR: to assess for chronic inflammation
Blood film: to assess for blasts or dysplasia
Autoimmune screen (e.g. ANA, ENA): if clinical suspicion of connective tissue disease
CXR: to assess for chronic infection (e.g. tuberculosis)
Renal and liver function tests: to screen for systemic disease

These investigations help distinguish between reactive causes and early haematological disease.

Step 2: Identify Potential Reactive Causes

Recent or ongoing infection?
Symptoms or history of autoimmune disease?
History of splenectomy?

If a reactive cause is found, treat the underlying condition and monitor recovery.

Step 3: Interpretation Based on Results

Mild monocytosis (< 1.5 ×10⁹/L): often reactive
Moderate to severe monocytosis (> 1.5 ×10⁹/L) or persistent: consider myelodysplastic syndromes or CMML
Abnormal blood film: suggestive of haematological malignancy

Step 4: Monitor or Refer

Reactive cause identified: monitor clinically and repeat FBC after 3-6 months
Persistent unexplained monocytosis: refer to haematology for further evaluation

Step 5: Red Flags Requiring Urgent Action

Monocytosis with blasts or dysplastic changes on blood film
Unexplained constitutional symptoms (e.g. weight loss, persistent fever)
Splenomegaly or hepatomegaly

Urgent referral to haematology is indicated if any red flags are present.