Imaging = CT Abd/pelvis (preferred) or 3-view abdomen XR
Ultrasound may reveal certain etiologies
Other work-up based on clinical suspicion, and may include:
CBC, metabolic panel, coags, lipase, UA, stool studies
Diagnostic paracentesis to evaluate for SBP (PMN ≥ 250 cells/mm³)
Ultrasound may reveal certain etiologies
CBC, metabolic panel, coags, lipase, UA, stool studies
Diagnostic paracentesis to evaluate for SBP (PMN ≥ 250 cells/mm³)
Generally a clinical diagnosis
Peritonitis
Abdominal aortic aneurysm
Spontaneous bacterial peritonitis
Aortocaval fistula
Appendicitis 
Acute gastroenteritis
Appendicitis
Aortoenteric fisulta