Pathology: The combination of air and fluid in a pleural space.
Pathogenesis: Infection, trauma, iatrogenic and invasive tumor extension
CXR Findings: An air-fluid level is seen with the fluid being in the gravitationally dependent part of the thorax. The hallmark is the air-fluid interface.
Clinical Clue: The commonest cause is iatrogenic, followed by thoracentesis, surgery or lung biopsy.
Air-fluid levels in cavitary or bullous lung lesions (eg. tuberculosis)
Bleeding or infection in a bulla
Necrotic locally invasive tumor with blood
Vasculitides (eg. Wegener’s Granuloma) with blood within.
Chaos: rib margins, horizontal anatomic lines or tissue folds projecting over the lung may fool the beginner. Beware of CHAOS in its many guises.