When patients arrived at the hospital, they were examined by a member of the surgical house staff. After obtaining a detailed medical history and performing a physical examination, venous blood from each patient was sampled and a plain abdominal radiograph or a chest radiograph was obtained. The clinical diagnosis of peritonitis is defined as a patient having diffuse abdominal tenderness, rebounding pain, and leukocytosis. If peritonitis was diagnosed, the presumed cause was recorded, which was blinded from the ultrasonographer, and the patient then underwent an abdominal ultrasonographic examination performed by a staff surgeon. This surgeon had 6 years of experience in performing abdominal ultrasonographic examinations. Ultrasonography was performed with a handheld 3.75-MHz curved-array transducer (model SSA-340A; Toshiba, Tochigi-Ken, Japan) over the whole abdomen, with screening of the pleural space, hepatorenal recess, paracolic gutter, rectouterine pouch, liver, biliary tract, gallbladder, spleen, pancreas, small intestine, colon, and intra-abdominal fluid collections. The epigastric area was first screened to check the antrum, the first portion of the duodenum, and the pancreas; then screening was shifted to the right hypochondriac region and subcostal area to check the liver, the gallbladder, and the right pleural space. The presence of free air was checked in both regions. Following this, the right paracolic gutter was examined to check the ascending colon, the terminal ileum, and the appendix. Then the rectouterine pouch and the left subcostal area were investigated to check the pelvic condition, the spleen, and the left pleural space. The left paracolic gutter was screened next to check the descending colon. Finally, the central abdomen was examined to check the small intestine.