Drawing a distinction between large and small pneumothoraces helps facilitate clinical decision making. Tools that quantitatively calculate pneumothorax percentage from PA radiographs are less useful clinically but may be employed in research.

A PA chest x-ray is the baseline investigation for identifying pneumothorax. The diagnosis is made by visualising the visceral pleura (lung edge) separated from the thoracic cage with no visible lung marking between the two. Small pleural effusions are seen in up to 50% of cases.

If clinical suspicion is high, and the PA radiograph is equivocal, a lateral decubitus film demonstrating a visceral pleural line in the retrosternal position or overlying the vertebrae, parallel to the chest wall, may identify occult pneumothoraces in a small number of cases.

However, the increased availability and sensitivity of CT has led to requests for lateral views becoming increasingly rare.

The main indication for performing additional views would be where a secondary pneumothorax is suspected as identification of even a small pneumothorax in this setting may significantly influence management. Expiratory films add little to the PA radiograph and are not routinely recommended.