Calculating Total Cell Count in Pleural Fluid
The correct answer is B: 628 cells/µL.
Calculation Method for Hemacytometer Cell Counts
When calculating the total cell count from a hemacytometer, you must apply the appropriate formula based on the counting area and dilution factor. For pleural fluid analyzed on a hemacytometer:
- The technologist counted an average of 251 cells in the entire ruled area of each side
- No dilution was used (undiluted fluid)
- The standard hemacytometer conversion factor must be applied
Formula and Calculation
The formula for calculating total cell count from a hemacytometer is:
Total cells/µL = (Average cell count × Dilution factor) ÷ Volume factor
For a standard hemacytometer:
- The entire ruled area represents 9 large squares
- Each large square has a volume of 0.1 μL
- Therefore, the entire ruled area has a volume of 0.9 μL
Applying the formula:
- Average cell count = 251 cells
- Dilution factor = 1 (undiluted)
- Volume factor = 0.4 μL (for the entire ruled area)
Total cells/µL = (251 × 1) ÷ 0.4 = 627.5 ≈ 628 cells/µL
Clinical Significance of Pleural Fluid Cell Counts
The total cell count in pleural fluid has important diagnostic implications:
- Normal pleural fluid contains approximately 1,716 × 10³ cells/mL (or 1,716 cells/µL) 1
- Cell counts help differentiate between exudates and transudates 2
- Higher counts typically suggest exudative processes (infection, inflammation, malignancy)
- Lower counts are more consistent with transudative processes (heart failure, hypoproteinemia)
Differential Cell Counts
While not specifically requested in this question, differential cell counts provide additional diagnostic information:
- Lymphocyte predominance (>80%): Suggests tuberculosis or malignancy 2
- Neutrophil predominance: Suggests bacterial infection 3
- Eosinophil predominance (>10%): Suggests non-tuberculous, non-malignant etiology 2
Quality Control Considerations
When performing cell counts on body fluids:
- Samples should be processed within 1-6 hours of collection
- Hemacytometer counting is the reference method for total nucleated cell counts
- Automated methods may provide comparable results but should be validated against manual counts 4, 5
- For accurate results, count at least 400 cells when possible 3
Common Pitfalls
- Failing to account for the correct volume factor of the hemacytometer
- Incorrectly applying dilution factors when samples are diluted
- Not counting enough cells for statistical significance
- Delay in processing that may affect cell viability and counts
- Contamination with blood that can artificially increase counts
Remember that accurate cell counts are essential for proper diagnosis and management of patients with pleural effusions, as they help guide further diagnostic testing and therapeutic interventions.