List lab studies.
CBC: neutrophilic leukocytosis
BMP: ↑ BUN, ↑ creatinine
Blood gas analysis: lactic acidosis (in ischemic perforation)
Describe immediate imaging studies.
Indications: Consider only for patients too unstable to safely undergo CT scanning.
X-ray abdomen 
Findings: free intraperitoneal air (pneumoperitoneum) under the diaphragm and/or between liver and lateral abdominal wall
Point of care ultrasound (POCUS) for which characteristic findings include: 
Enhanced peritoneal stripe sign: a hyperechogenic focal thickening of the peritoneum
Horizontal reverberation artifacts: horizontal stripes resulting from the interface of free air and fascia
List confirmatory studies.
First line: CT abdomen and pelvis with IV contrast (most sensitive)
Indications: acute nonlocalized abdominal pain
Pneumoperitoneum: the presence of air in the peritoneal cavity
An abdominal x-ray showing radiolucent air under the diaphragm and/or the delineation of the bowel wall by radiolucent air is diagnostic.
Can occur after perforation of a hollow abdominal viscus (e.g., perforated peptic ulcer) or after surgery in which air is introduced into the abdominal cavity.
Signs of perforated bowel: loss of bowel wall continuity, localized mesenteric fat stranding
Alternative: formal ultrasound abdomen
Indication: preferred in patients with contraindications to radiation exposure (e.g., pregnancy)
Findings: pneumoperitoneum, localized fluid collection, localized thickening of a bowel segment.