How to Calculate Pneumothorax Size Using the Collins Method
The Collins method calculates pneumothorax size percentage using the formula: % Pneumothorax = 4.2 + [4.7 × (A + B + C)], where A, B, and C are interpleural distances measured in centimeters at three specific locations on an upright posteroanterior chest radiograph. 1
Measurement Technique
To apply the Collins method, measure the interpleural distance (the gap between the visceral pleural line and the chest wall) at three standardized locations on an upright PA chest radiograph: 1
- Apical measurement (A): Distance at the lung apex
- Mid-thoracic measurement (B): Distance at the level of the hilum
- Basal measurement (C): Distance at the lower lung zone
Sum these three measurements in centimeters, then apply the formula above. 1
Clinical Validation and Accuracy
The Collins formula was derived from helical CT volumetric measurements and demonstrates excellent correlation (r = 0.98, p < 0.0001) with actual pneumothorax volume. 1 This makes it significantly more accurate than clinical estimation alone, as clinical history and physical examination are not reliable indicators of pneumothorax size. 2
The Collins method is substantially more accurate than the Light index, which consistently underestimates pneumothorax size by an average of 7.3% with wide limits of agreement (ranging from 24% underestimation to 17% overestimation). 3 More recent 3D-printed model studies confirm that the Collins method provides reasonable accuracy for clinical decision-making. 4
Clinical Application for Treatment Decisions
The calculated percentage directly informs management according to British Thoracic Society guidelines: 5, 6
- <15% pneumothorax with minimal symptoms: Observation is appropriate, as 70-80% resolve without persistent air leak 5
- >50% pneumothorax: Generally requires intercostal tube drainage 5
- >2 cm rim (approximately 50% by volume): Classified as "large" and typically requires intervention 2, 6
Important Caveats and Limitations
Always prioritize clinical symptoms over radiographic size—patients with breathlessness require intervention regardless of measured pneumothorax size, as symptoms may indicate tension physiology. 5, 6
The Collins method has several practical limitations: 5
- Less accurate with extensive subcutaneous emphysema obscuring pleural margins
- Cannot differentiate pneumothorax from complex bullous disease (CT scanning required in these cases) 2, 5
- Measurements on expiratory films average 9% larger than inspiratory films, potentially altering treatment classification for 14% of patients 7
For secondary pneumothorax in patients with underlying lung disease, use CT scanning when the plain radiograph is unclear, as misidentifying bullae as pneumothorax can lead to dangerous interventions. 2, 5