What is the most suitable investigation to diagnose acute pancreatitis in a patient with severe epigastric pain?

Most Suitable Investigation for Diagnosing Acute Pancreatitis

Serum lipase (Option B) is the most suitable investigation for diagnosing acute pancreatitis in this patient with severe epigastric pain. 1, 2

Primary Diagnostic Test

Lipase should be ordered as the first-line enzyme test because it demonstrates superior diagnostic characteristics compared to amylase:

• Higher sensitivity: 79-89% for lipase versus 72% for amylase 1, 2
• Better specificity: 89-99% for lipase, maintaining excellent specificity even at the upper limit of normal 1, 3
• Larger diagnostic window: Lipase rises within 4-8 hours, peaks at 24 hours, and remains elevated for 8-14 days (compared to amylase's 3-7 days) 1, 4
• Preferred by guidelines: The American Gastroenterological Association, World Society of Emergency Surgery, and UK guidelines all recommend lipase over amylase 5, 1, 2

Diagnostic Threshold

• Use a cutoff of ≥3 times the upper limit of normal (>480 U/L based on the normal range provided) for optimal specificity 1, 2, 6
• Elevations <3 times normal have low specificity and may occur in non-pancreatic conditions 1
• At appropriately elevated cutoff levels (3-4× normal), diagnostic accuracy exceeds 95% 6

Role of Imaging

While lipase is the primary diagnostic test, imaging plays an important complementary role:

Abdominal Ultrasound (Option D)

• Should be performed in all patients to detect gallstones, biliary obstruction, or free peritoneal fluid 1, 2
• Often unhelpful for visualizing the pancreas itself 5
• Serves primarily to identify etiology rather than confirm diagnosis 1

CT Scan (Option C)

• Reserve contrast-enhanced CT for specific situations: 5, 1, 2
  • When clinical and biochemical findings are inconclusive
  • After 72 hours in patients with predicted severe disease (APACHE II >8)
  • When organ failure is present
• Avoid early CT (<72 hours) as it underestimates pancreatic necrosis 1, 2
• CT provides good evidence for presence/absence of pancreatitis when doubt exists, but is not the first-line diagnostic test 5

Why Not Amylase (Option A)?

Amylase has significant limitations:

• Lower sensitivity and specificity compared to lipase 1, 4
• Shorter diagnostic window: normalizes in 3-7 days, potentially missing late presentations 1, 4
• More non-specific elevations: can be elevated in renal disease, appendicitis, cholecystitis, bowel obstruction, head injuries, and hepatic injuries 1
• No added value: Simultaneous measurement of both amylase and lipase provides no diagnostic advantage 7, 4, 8

Clinical Algorithm

For this middle-aged patient with 6 hours of severe epigastric pain:

1. Order serum lipase immediately as the primary diagnostic test 1, 2
2. Interpret using ≥3× upper limit threshold (>480 U/L) for diagnosis 1, 2
3. Obtain abdominal ultrasound to identify gallstones as potential etiology 1, 2
4. Add liver function tests, triglycerides, and calcium to determine etiology 2, 7
5. Reserve CT for 72+ hours if severe disease predicted or diagnostic uncertainty persists 5, 1, 2

Important Caveats

• Clinical features are essential: Diagnosis requires compatible symptoms (upper abdominal pain with epigastric tenderness), not just enzyme elevation alone 1, 2
• Enzyme levels don't predict severity: The degree of lipase elevation is independent of disease severity 1
• Consider pancreatitis even with normal enzymes in patients with unexplained multiorgan failure or SIRS 1, 2
• Renal failure affects interpretation: Lipase cutoffs may be less specific in renal failure 2