system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
Best investigation for air in peritoneal cavity is
assistant
The correct answer is  free gas under the diaphragm is best shown on an erect chest x-ray or x ray abdomen. If the patient is too ill for an erect film, a lateral decubitus film is done.