What is monocytosis and what are its causes?

Monocytosis: Definition and Causes

Monocytosis is defined as an absolute monocyte count greater than 0.8 × 10^9/L in peripheral blood and can be caused by various infections, inflammatory conditions, and hematologic malignancies. 1

Definition and Normal Values

• Monocytosis is defined as an elevated absolute monocyte count above 0.8 × 10^9/L in the peripheral blood
• Some guidelines use a threshold of >1 × 10^9/L, particularly when considering chronic myelomonocytic leukemia (CMML) 1
• Persistent monocytosis is defined as elevation lasting at least 3 months

Diagnostic Approach

Initial Evaluation

1. Complete blood count (CBC) with differential
2. Peripheral blood smear examination
3. Assessment for other abnormalities (anemia, thrombocytopenia, other white cell abnormalities)
4. Basic laboratory testing:
   • Inflammatory markers (ESR, CRP)
   • Liver function tests
   • Renal function tests

Advanced Testing (if initial evaluation suggests hematologic malignancy)

1. Bone marrow aspiration and biopsy
2. Cytogenetic analysis
3. Immunophenotyping
4. Molecular testing

Causes of Monocytosis

Infectious Causes

• Bacterial infections:
  • Tuberculosis
  • Subacute bacterial endocarditis
  • Brucellosis
• Viral infections:
  • HIV
  • Cytomegalovirus (CMV)
  • Epstein-Barr virus (EBV)
• Parasitic infections:
  • Malaria
  • Leishmaniasis

Inflammatory and Autoimmune Conditions

• Inflammatory bowel disease
• Rheumatoid arthritis
• Systemic lupus erythematosus
• Sarcoidosis
• Still's disease

Hematologic Malignancies

• Chronic myelomonocytic leukemia (CMML)
• Acute myeloid leukemia with monocytic differentiation
• Myelodysplastic syndromes
• Myeloproliferative neoplasms

Other Causes

• Post-splenectomy
• Recovery from bone marrow suppression
• Certain medications
• Chronic liver disease

Clinical Significance and Prognostic Value

Monocytosis can be a significant finding with prognostic implications:

• Increased monocyte count predicts adverse outcomes in patients admitted to emergency departments 2
• Persistent unexplained monocytosis warrants follow-up to rule out underlying hematologic malignancy 1
• Sustained monocytosis (at least two requisitions in 3 months) significantly increases the risk of CMML, although the absolute risk remains low (0.1%) 3
• In patients with fever and neutropenia, monocyte recovery may be a positive prognostic sign 1

Diagnostic Criteria for CMML (World Health Organization)

• Persistent peripheral blood monocytosis >1 × 10^9/L
• No Philadelphia chromosome or BCR-ABL1 fusion gene
• Less than 20% blasts in peripheral blood and bone marrow
• At least one of the following:
  • Dysplasia in one or more cell lines
  • Acquired clonal cytogenetic or molecular genetic abnormality
  • Persistence of monocytosis for at least 3 months with no other cause 1

Management Approach

1. For transient monocytosis with identified cause:

   • Treat the underlying condition
   • Follow-up CBC to confirm normalization

2. For persistent unexplained monocytosis:

   • Regular CBC monitoring every 2-4 weeks initially
   • Hematology referral if persistent beyond 3 months
   • Repeat evaluation if other cytopenias develop or clinical status changes

Key Pitfalls to Avoid

• Don't ignore persistent monocytosis: Even mild but persistent monocytosis may indicate an underlying hematologic malignancy
• Don't attribute monocytosis to infection without confirmation: While infections commonly cause monocytosis, persistent elevation requires exclusion of neoplastic causes
• Don't overlook monocytosis in the presence of other cytopenias: The combination may suggest myelodysplastic syndrome or other hematologic disorders
• Don't confuse reactive monocytosis with neoplastic causes: Flow cytometry can help distinguish between them by identifying aberrant monocyte phenotypes

Remember that monocytosis itself is not a disease but a sign of an underlying condition that requires identification and appropriate management 1.