What are the initial investigations for a patient presenting with acute abdomen?

Initial Investigations for Acute Abdomen

The initial investigations for a patient presenting with acute abdomen should include laboratory studies, abdominal CT scan as the primary imaging modality in adults, and ultrasound as the first-line imaging in children and pregnant patients. 1

Laboratory Investigations

• Routine laboratory studies and arterial blood gas analysis should be performed in all patients with suspected acute abdomen 1
• Complete blood count to detect leukocytosis, which is commonly associated with intra-abdominal infections and perforations 1
• Metabolic panel and serum amylase, which are often elevated in cases of perforation 1
• Blood cultures are not routinely recommended in non-immunocompromised adults and children with suspected intra-abdominal infections who have normal/elevated temperature without hypotension, tachypnea, or delirium 1

Imaging Studies Based on Patient Population

For Non-Pregnant Adults:

• Abdominal CT scan is the recommended initial diagnostic imaging modality for suspected acute intra-abdominal abscess and acute diverticulitis 1

  • IV contrast is usually appropriate when CT is obtained and helps visualize abscess walls and subtle bowel abnormalities 1
  • CT is highly accurate, making additional immediate imaging studies usually unnecessary 1

• For suspected perforated peptic ulcer:

  • CT scan is strongly recommended as the primary imaging modality 1
  • If CT is not promptly available, chest/abdominal X-ray should be performed as initial assessment 1
  • When free air is not seen on imaging but clinical suspicion remains high, imaging with water-soluble contrast (oral or via nasogastric tube) is suggested 1

• For suspected acute cholecystitis or cholangitis:

  • Abdominal ultrasound is suggested as the initial diagnostic imaging modality 1
  • CT with IV contrast, MRI/MRCP, or HIDA scan may be obtained if initial imaging is inconclusive 1

For Children:

• Abdominal ultrasound is recommended as the initial diagnostic imaging modality for suspected acute intra-abdominal abscess 1

  • MRI is a reasonable alternative for initial imaging if available 1
  • If initial ultrasound results are negative, equivocal, or non-diagnostic and clinical suspicion persists, either CT or MRI is suggested as subsequent imaging 1

• For suspected acute appendicitis in children:

  • Abdominal ultrasound is suggested as the initial imaging modality 1
  • If initial ultrasound is equivocal/non-diagnostic and clinical suspicion persists, abdominal MRI or CT is recommended as subsequent imaging 1

For Pregnant Patients:

• Ultrasound or MRI are the recommended initial imaging modalities for suspected acute intra-abdominal abscess, acute diverticulitis, acute cholecystitis, or acute cholangitis 1
• For suspected acute appendicitis, abdominal ultrasound is suggested as the initial imaging modality 1

Special Considerations

• The Focused Assessment with Sonography for Trauma (FAST) examination should be used to identify pathological presence of free fluid/blood in traumatic acute abdomen 1
• Serial FAST exams should be performed in response to changes in the patient's condition to visualize the development of previously undetectable free fluids 1
• Ultrasound alone should not be used to identify the cause of a surgical abdomen, as it does not reliably characterize different etiologies 1, 2
• Early administration of analgesia to patients with acute abdominal pain will reduce discomfort without impairing clinically important diagnostic accuracy 3

Common Pitfalls and Caveats

• Relying solely on plain radiographs can lead to missed diagnoses, as they have limited sensitivity for many causes of acute abdomen 1, 2
• Ultrasound is operator-dependent and can yield equivocal results, necessitating follow-up imaging in some cases 1
• CT involves radiation exposure and may require IV contrast, which should be considered especially in children and pregnant patients 1
• MRI is not always readily available, and sedation may be required for young children 1
• Delayed diagnosis and treatment increase mortality rates significantly, with mortality rising for every elapsed hour until specific treatment is provided 4

The structured approach to investigating acute abdomen should be tailored based on the suspected diagnosis, patient population, and available resources, with the goal of rapid and accurate diagnosis to guide appropriate treatment and reduce morbidity and mortality.