Detection of pneumoperitoneum on chest radiographs: comparison of upright lateral and posteroanterior projections

This study was done to determine whether upright lateral chest radiographs were more sensitive than upright posteroanterior chest radiographs in detecting pneumoperitoneum. We prospectively evaluated the ability of upright posteroanterior and lateral chest radiographs to show subdiaphragmatic air in 100 consecutive patients with known pneumoperitoneum from a variety of causes. The difference in sensitivity between the two was evaluated by means of McNemar's test. The upright lateral chest radiograph showed pneumoperitoneum in 98% of the cases; the upright posteroanterior chest radiograph showed pneumoperitoneum in only 80%. The upright lateral chest radiograph was significantly better at showing pneumoperitoneum than was the upright posteroanterior chest radiograph (p < .01). The upright lateral chest radiograph is more sensitive than the upright posteroanterior chest radiograph in detecting small amounts of pneumoperitoneum. When there is a strong clinical suspicion of a perforated hollow viscus, it may be of benefit to include an erect lateral chest radiograph as part of the acute abdominal series.