What is a Differential Leucocyte Count?
A differential leucocyte (white blood cell) count is a laboratory test that identifies and quantifies the different types of white blood cells in a blood sample, expressed as both percentages and absolute numbers, providing critical diagnostic information for infections, inflammatory conditions, and hematologic disorders. 1, 2
Components of the Differential Count
The differential count identifies and quantifies five main white blood cell types 1:
- Neutrophils: The most abundant white blood cells, typically comprising the majority of circulating leukocytes 2
- Lymphocytes: Include T-cells, B-cells, and NK cells 1
- Monocytes: Precursors to tissue macrophages 1
- Eosinophils: Involved in allergic responses and parasitic infections 1, 2
- Basophils: Least common white blood cells, involved in inflammatory responses 1
Methods of Performing the Count
Automated Counting
- Automated hematology analyzers provide rapid five-part differential counts with analytic variability of 1.9% to 5.3% for lymphocytes 1, 3
- Automated methods count large numbers of cells, reducing statistical variation 4
Manual Counting
- Manual microscopic examination involves counting 100-400 cells on a stained blood smear 1
- Manual counts have higher variability (12.5% to 27% for lymphocytes) but allow detection of morphologic abnormalities 1, 3
- Manual differential counting is preferred when assessing for band forms and other immature cells 2, 3
Flow Cytometry
- Flow cytometry uses fluorescent antibodies to identify cell populations immunologically 4
- Provides extended differentiation including lymphocyte subsets (CD4+ T-cells, CD8+ T-cells, B-cells, NK cells) 1, 4
Reporting Standards
Results should include 5:
- Both percentages and absolute counts for each cell type
- Reference ranges appropriate for the patient's age and sex
- Absolute counts calculated by multiplying the percentage by the total white blood cell count 5
Clinical Significance
Infection Detection
The differential count provides critical diagnostic information for bacterial infections 2, 3:
- Neutrophilia with left shift (band neutrophils >16% or >1,500 cells/mm³) has a likelihood ratio of 4.7-14.5 for bacterial infection 2, 3
- Neutrophil percentage >90% has a likelihood ratio of 7.5 for bacterial infection 2, 3
Other Clinical Applications
- Lymphocytosis >25% suggests granulomatous diseases like sarcoidosis or hypersensitivity pneumonitis 2
- Eosinophilia >1% indicates allergic conditions, parasitic infections, or drug reactions 2
- Detection of immature cells (blasts, myeloid precursors) may indicate hematologic malignancies 4, 6
Important Variability Considerations
Analytic Variability
- Automated counters: 2.2%-7.7% for total WBC count 1, 3
- Manual hemocytometer: 9.3%-17.6% for total WBC count 1, 3
Biologic Variability
Common Pitfalls to Avoid
- Failing to consider clinical context when interpreting results 2
- Not accounting for timing of blood draw in relation to disease process 2
- Relying solely on automated counts when band forms or immature cells are suspected 2, 3
- Overlooking that corticosteroid administration can cause neutrophilia 2
- Missing that overwhelming sepsis may cause leukopenia after initial leukocytosis 2