Pseudo-monocytosis (falsely elevated monocyte count) on a 5-part haematology analyzer typically occurs when other white blood cell (WBC) types, abnormal cells, or cellular debris are misclassified as monocytes due to similar size, density, or staining properties. This is a recognized limitation of automated analysers, especially when dealing with complex haematological samples.
- Presence of Abnormal/Immature Cells
- Hairy Cell Leukaemia (HCL):
- A major cause of pseudo monocytosis, where "hairy cells" (lymphocytes with cytoplasmic projections) are misidentified as monocytes by optical fluorescence or scatter methods, often masking the true, underlying monocytopenia.
- Blasts or Atypical Lymphocytes:
- Immature cells, such as blasts in acute leukaemia or reactive lymphocytes in viral infections (e.g., infectious mononucleosis, COVID-19), may be incorrectly classified as monocytes due to their large size and high cytoplasmic content.
- Dysplastic Macropolycytes:
- In myelodysplastic syndromes, large, hypogranular, or malformed neutrophils can be misclassified as monocytes.
- Analytical Interferences (Instrument-Dependent)
- Peroxidase-Negative Eosinophils:
- In Siemens (formerly Technicon/Bayer) analysers that use peroxidase staining, some eosinophils with low peroxidase activity can be misidentified as monocytes.
- Resistant Red Blood Cells (RBCs):
- If the lysis step is inadequate, unlysed or resistant RBCs (e.g., in patients with hemoglobinopathies or high bilirubin levels) may be counted in the WBC differential, specifically misclassifying them in the monocyte region.
- Fragmented RBCs or Platelet Clumps:
- Large platelet clumps or debris from red cell fragmentation can be falsely counted in the monocyte region of the scattergram.
- Procedural Factors
- Sample Age:
- Using old blood samples can lead to cell degradation, causing atypical lymphocytes to appear like monocytes, or increasing debris.
- Antibody Interference:
- Rarely, specific patient antibodies can cause cell agglutination, confusing the optical or impedance, and volume (VCS) technologies.
Key Takeaway: Any unexpected monocytosis without a corresponding clinical, infectious, or inflammatory reason should be verified with a manual peripheral blood smear review to rule out the above-mentioned causes.
