What is an Auto Mono on a CBC?
An automated monocyte count (auto mono) on a CBC is the machine-generated measurement of monocytes—a type of white blood cell—expressed as both a percentage of total white blood cells and as an absolute count (cells per microliter or × 10⁹/L). [@1,@2@]
How It's Measured
The automated monocyte count is generated by hematology analyzers that perform an automated white blood cell differential, typically counting 10,000-30,000 cells to provide accurate results [@1@]. Modern analyzers use various technologies including:
- Flow cytometry-based methods that analyze cell size, internal complexity, and light-scattering patterns [@1@]
- Image-based approaches that deposit blood in a monolayer on slides for multispectral analysis, achieving correlation coefficients of r = 0.76 for monocyte counts compared to traditional methods [@6@]
What the Results Include
Your CBC report will show monocytes in two ways [@3@]:
- Percentage: The proportion of monocytes among all white blood cells
- Absolute count: Calculated by multiplying the monocyte percentage by the total white blood cell count [@3@]
Both values should be interpreted against laboratory-specific reference ranges, which vary by age, sex, and the specific analyzer used [@3@].
Clinical Significance
Elevated monocyte counts (monocytosis) can indicate:
- Chronic infections (tuberculosis, bacterial endocarditis) [@4@]
- Autoimmune conditions [@4@]
- Recovery phase from acute infections
- Certain hematologic malignancies—notably, an elevated automated monocyte count >0.9 × 10⁹/L is independently predictive of overall survival in chronic lymphocytic leukemia [@9@]
Important Caveats
The automated differential may be "flagged" by the instrument when abnormalities are detected [@1@]. When this occurs:
- If the flag is NOT on the lymphocyte population, the automated result should be used [@1@]
- If significant abnormalities are flagged, a manual differential counting at least 400 cells may be performed for verification 1
- Flagging efficiency for abnormal white blood cells is approximately 72-78% across major analyzers, meaning more than 20% of samples with ≥1% abnormal cells may not be correctly flagged [@8@]
Specimens must meet quality standards [@2@]:
- Processed within manufacturer-specified timeframes
- Not hemolyzed, frozen, or clotted
- Less than 48 hours old from time of blood draw [@2@]
The automated monocyte count is a routine component of the CBC differential that helps assess immune function, detect infections, and monitor for hematologic disorders, though it should always be interpreted in the clinical context alongside other CBC parameters [@10