AbstractObjectivesDelayed pneumothorax can cause an emergency room visit and be life‐threatening in case of tension pneumothorax after transthoracic needle biopsy. We hypothesized that most delayed pneumothoraces are diagnosed by later enlargement of occult pneumothorax due to the low diagnostic accuracy of a chest X‐ray. Lung ultrasound is a highly accurate tool for detection of pneumothorax. The aim of this study is to evaluate the diagnostic accuracy of lung ultrasound for prediction of delayed pneumothorax on chest X‐ray.MethodsThis prospective pilot study was performed between April 2020 and July 2020 in Chungnam National University Hospital. The participants underwent chest X‐rays and lung ultrasound before, immediately after, and 3 h after transthoracic needle biopsy, respectively. The presence or absence of lung sliding at each anterior BLUE‐point on an ultrasound and pneumothorax on a chest X‐ray was recorded.ResultsPneumothorax occurred in 17 (35.4%) participants, and three of them underwent chest tube replacement. Of the 17 (35.4%) cases of pneumothorax, five participants (10.4%) were diagnosed with delayed pneumothorax. Three out of five participants showed loss of lung sliding on lung ultrasound before the diagnosis of delayed pneumothorax. Therefore, the sensitivity of lung sliding on lung ultrasound for early detection of delayed pneumothorax was 60%. Two undetected cases were asymptomatic, and the pneumothoraces were exceedingly small and recovered spontaneously. Thus, sensitivity for detection of clinically meaningful delayed pneumothorax requiring chest tube replacement was 100% (2/2).ConclusionLung ultrasound can probably predict clinically meaningful delayed pneumothorax after transthoracic needle lung biopsy.