The most useful investigation is the erect PA chest radiograph despite the fact that it tends to under-estimate the size of a pneumothorax by virtue of it being a 2-dimensional image of a 3-dimensional structure.

Historically, pneumothoraces were described in subjective terms, such as ‘small rim’, ‘moderate’ and ‘halfway to the heart border’. In 2023 the British Thoracic Society (BTS) advised that with pneumothorax of sufficient size intervention depends on clinical context but, in general, usually ≥ 2cm laterally or apically on CXR, or any size on CT scan which can be safely accessed with radiological examination.

The rationale for using 2 cm as the cut-off for indicating a large pneumothorax is that this approximates the point at which up to half of the lung volume has been lost.