• Full blood count
• Clotting screen if there is purpura or jaundice, prolonged oozing from puncture sites, or a low platelet count
• C-reactive protein
• Group and screen
• Blood glucose
• Sodium, potassium, urea and creatinine
• Liver function tests, albumin and calcium
• Serum amylase (raised in pancreatitis, perforated ulcer, mesenteric ischaemia and severe sepsis)
• Other tests to confirm or exclude pancreatitis if indicated (serum lipase; urine dipstick test for trypsinogen-2 (which has a high negative predictive value))
• Arterial gases and pH if hypotensive or oxygen saturation <94% breathing air (metabolic acidosis seen in generalized peritonitis, mesenteric infarction and severe pancreatitis)
• Blood culture if febrile or suspected peritonitis
• Urine: stick test, MC&S and pregnancy test
• ECG if age >50 or known cardiac disease or unexplained upper abdominal pain
• Chest X-ray – looking for free gas under the diaphragm, indicating perforation, and evidence of basal pneumonia
• Abdominal X-ray (supine and erect or lateral decubitus) – looking for evidence of obstruction of large and/or small bowel; ischaemic bowel (dilated and thickened loops of small bowel); cholangitis (gas in biliary tree); radio-dense gallstones; radio-dense urinary tract stones

Selected patients

• In suspected intestinal obstruction, CT scan of the abdomen is the most sensitive test.
• Abdominal ultrasonography is the initial test of choice when there are signs of peritonitis, and can assess for appendicitis, abdominal abscess and pelvic abnormalities without radiation exposure. Ultrasonography is also the first-line investigation for right upper quadrant pain, to exclude gallstones and other biliary pathology.