What are the causes of monocytosis (elevated monocyte count)?

From the Guidelines

Monocytosis is typically caused by infections, inflammatory conditions, malignancies, and certain medications, with chronic bacterial infections, inflammatory disorders, and myelodysplastic syndromes being common causes, as noted in studies such as 1 and 1.

Causes of Monocytosis

• Infections: Chronic bacterial infections like tuberculosis and subacute bacterial endocarditis, as well as fungal and protozoal infections, can cause monocytosis.
• Inflammatory conditions: Inflammatory disorders such as inflammatory bowel disease, rheumatoid arthritis, and systemic lupus erythematosus can elevate monocyte counts.
• Malignancies: Myelodysplastic syndromes, acute and chronic myeloid leukemia, lymphomas, and solid tumors may present with monocytosis.
• Medications: Glucocorticoids, certain chemotherapy agents, and colony-stimulating factors can increase monocyte counts.

Diagnostic Criteria

The diagnostic criteria for monocytosis include a monocyte count above 1x10^9/L, as stated in 1, and the presence of dysplasia in one or more cell lines, or an acquired clonal cytogenetic or molecular genetic abnormality, as noted in 1.

Prognosis and Treatment

The prognosis and treatment of monocytosis depend on the underlying cause, with some conditions requiring immediate medical attention, as discussed in 1 and 1.

Recent Guidelines

Recent guidelines, such as those published in 1 and 1, emphasize the importance of careful morphologic review and correlation with clinical features to establish a diagnosis of myelodysplastic syndromes, and provide updated classification systems and diagnostic criteria for these conditions.

From the Research

Monocytosis Causes

• Monocytosis is associated with various conditions, including:
  • Chronic infections such as tuberculosis or endocarditis 2
  • Rheumatic and myeloproliferative disorders 2
  • Atherosclerosis, coronary artery disease, and stroke 2
  • Autoimmune diseases, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA) 3
  • Haematological malignancies, including chronic myelomonocytic leukaemia (CMML) 4
• Monocytosis can also be caused by other factors, including:
  • Respiratory symptoms 2
  • Infections 2
  • Comorbidities, such as congestive heart failure, chronic obstructive pulmonary disease, tumor, diabetes, or renal failure 2
• The differential diagnosis of monocytosis is broad and requires a systematic approach to investigation 5

Disease Associations

• Monocytosis has been linked to various diseases, including:
  • Chronic myelomonocytic leukaemia (CMML) 4
  • Systemic lupus erythematosus (SLE) 3
  • Rheumatoid arthritis (RA) 3
  • Atherosclerosis 2, 6
  • Chronic obstructive pulmonary disease (COPD) 6
  • Lung cancer 6
  • Alzheimer's disease 6

Monocyte Subsets

• Monocytes are a heterogeneous population with phenotypical and functional differences 3, 6
• Monocytes can be divided into three subsets: classical, intermediate, and non-classical 3, 6
• Each subset has distinct surface markers and functions in homeostasis and disease 6