Normal Pleural Fluid Cell Count
In healthy individuals without underlying medical conditions, normal pleural fluid volume is approximately 0.26 mL/kg body weight (less than 1 mL total), containing approximately 1,716 × 10³ white blood cells per milliliter, with 75% macrophages, 23% lymphocytes, and minimal mesothelial cells (1%), neutrophils (0%), and eosinophils (0%). 1, 2
Volume Characteristics
- Total pleural fluid volume in normal humans is 8.4 ± 4.3 mL (right-sided measurement), which translates to 0.26 ± 0.1 mL/kg body mass 1, 2
- This volume forms a film approximately 10 μm thick between visceral and parietal pleural surfaces 3
- Right- and left-sided pleural fluid volumes are similar, with no significant differences between sides 1
Cellular Composition
White blood cell differential count:
- Macrophages predominate at 75% (interquartile range 16%) 1, 2
- Lymphocytes comprise 23% (interquartile range 18%) 1, 2
- Mesothelial cells are minimally present at 1% (interquartile range 2%) 1, 2
- Neutrophils are essentially absent at 0% (interquartile range 1%) 1, 2
- Eosinophils are essentially absent at 0% (interquartile range 0%) 1, 2
The British Thoracic Society guidelines confirm that normal pleural fluid contains predominantly mesothelial cells, macrophages, and lymphocytes in small numbers 3
Important Clinical Context
Age and sex variations:
- No significant correlation exists between age and pleural fluid characteristics in individuals aged 17-54 years 2
- No significant differences occur between males and females in total or differential cell counts 1
Smoking effect:
- Smokers show a small but statistically significant increase in pleural fluid neutrophils (median 1%, interquartile range 2%, p < 0.015) compared to nonsmokers 1
- This represents the only documented variation in otherwise healthy individuals
Biochemical Parameters in Health
Beyond cell counts, normal pleural fluid contains 3:
- Protein concentrations similar to interstitial fluid
- Higher bicarbonate levels compared to serum
- Lower sodium levels compared to serum
- Similar glucose levels to serum
- Large molecular weight proteins such as lactate dehydrogenase (LDH)
Critical Pitfall to Avoid
Do not confuse normal pleural fluid characteristics with pathologic effusions. When disease processes activate immune responses, these parameters change dramatically 3. For example:
- Lymphocyte predominance ≥50% suggests tuberculosis or malignancy 4, 5, 6
- Neutrophil predominance suggests parapneumonic effusion or empyema 5
- Eosinophilia ≥10% typically indicates nonspecific, nontuberculous, nonmalignant etiology 6
The data establishing normal values were obtained through pleural lavage during thoracoscopic sympathicolysis in otherwise healthy patients with essential hyperhidrosis, using urea as an endogenous dilution marker 1, 2. This methodology provides the most reliable human data available, as prior estimates relied on animal extrapolation 2.